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Battery, Volts, Amps, Fuses, etc.--Page 1

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rule

Subject: EE 101--Basic Electricity Concepts     Top

From: William Sommerwerck

It looks like I'm going to have explain some basic electricity.

Think of a rechargeable battery as a sort of huge wastebasket into which you toss ball bearings. If you hold the wastebasket above the ground and let the ball bearings fall (or run) out, they can do work. Like demolishing an ant hill or making holes in concrete.

The height of the wastebasket above the ground represents voltage (electrical potential energy). The number of ball bearings in the wastebasket represents the capacity of the battery. Let's forget about voltage and think about capacity.

When we charge a battery, we are in effect moving electrons from a low energy state to a high energy state (just as moving ball bearings from the ground to the wastebasket raises their potential energy). The larger the battery, the more electrons it can accept, and the higher its capacity.

When we connect a load to a battery, the electrons move from a higher energy state to a lower state, doing useful work. The battery poops out when we run out of higher-energy electrons.

One coulomb of charge passing a point each second is one ampere of current. If a battery has a capacity of 50 ampere-hours, it can store 50 * 3600 coulombs of charge (180000 coulombs) at the higher energy level.

If we draw one ampere, the battery should last for about 50 hours. If we draw 10 amperes, the battery is good for only five hours. And if we draw 100 amps, the battery will barely last a half hour. This should give you a rough idea of what the "capacity" of a battery is. Please don't post messages saying that this is not a complete or exact explanation. I know it isn't. But it is basically correct.

rule

Subject: LX200 Battery Power Supply Cautions     Top

From: John Hopper <JohnLX200a_taol.com>

Batteries are often hooked in parallel, discharged together, and charged together without incident. Batteries also sometimes get hot, boil, melt, or explode when being hooked in parallel for charging.

While there are dangers inherent in CONNECTING batteries in parallel, and other (typically smaller) dangers simply KEEPING them in parallel, charging doesn't do much to add to those dangers. The main dangers are that a drained condition or internal short in one battery causes it to become the target of current dumps from the rest of them, and/or a spark when connecting them explodes the battery gases.

I don't think you will ever see a pack of (diesel, boat, golf cart, locomotive, etc.) batteries being disconnected and charged in a different configuration than they were discharged in. THEY STAY HOOKED UP ONE WAY.

Even without any defect, spark, or short:
A serious danger occurs when CONNECTING the three 6V batteries in parallel after discharging them in series. This is mainly due to the possibility that they are discharged to different voltages and one or two will apply an extremely high charging current to the other one or two. This is one reason that batteries explode sometimes when you jump start a dead car battery. It really does happen, and would happen more if people used heavier, shorter jumper cables!!

Discharging batteries in series and charging them in parallel IS A BAD IDEA, although if everything goes "nominally" as NASA would say, and they are discharged from equal voltages down to other equal voltages, there may be no problem. This is far from guaranteed, though, and some MAPUGers have been known to do downright dangerous things like tapping off 6V or 12V to run a dew heater, CCD camera, or whatever, virtually guaranteeing unequally discharged batteries. In that case, they MUST be charged separately or you're taking a real risk every time you hook them up in parallel for charging.

If you must go with an 18V battery pack, you could get an 18V charger if you don't tap intermediate voltages. If you do tap intermediate voltages, charge them separately (or at least each group between taps if you know what you're doing.)

rule

Subject: Battery Information URLs     Top

From: Don Tabbutt <dona_ttabbutt.com> Date: Dec, 2003

For good information on batteries, try the West Marine web site. Mariners, especially long distance sailors, have battery demands that we landlubbers rarely think about. West Marine is at:
<http://www.westmarine.com/> Should open a new browser window over this one.

From: Gregg Ruppel <ruppelgla_tSLU.EDU>

Check out this web site for everything you ever wanted to know about batteries but were afraid to ask!

<http://www.uuhome.de/william.darden/>
Note: should open a new browser window over this one.

rule

Subject: Battery Connectors   Top

From: Doc G

At long last I have found some battery connectors that are of better quality than any I have used before. They are made by Switchcraft and are superior in all ways to those from Radio Shack. And, to most of those on existing equipment.

Their catalog numbers are L712A, L722A for the plugs and 716K, S716K for the sockets. The former are with 2.5 mm pins and the latter with 2 mm pins. The plugs are particularly nice. They have a larger than normal grip which allows the use of larger gauge wire and they have a screw down locking ring so they are held firmly into the socket.

They are available from Newark and probably many other suppliers. They are not cheap, but they are very good. ($4 per part) Those who are having trouble with these connectors might well replace troublesome versions with the higher quality ones from Switchcraft.

rule

Subject: 12-volt Battery Charger Recommendation   Top

From: Gene Roeschlei <GRoeschleia_taol.com> Date: Jul 2004

I have one of the 'smart chargers' Vectron Model Number VEC 092 WM by Vectron. Details at:

<http://mastercatalog04.westmarine.com/0624.asp>. I did buy mine at West Marine but you can probably get a better price at a number of outlets. This is undoubtedly the best battery charger I have ever had or used or heard of. I have the one that goes to 35A.

West Marine catalog has a lot of good information on lead acid batteries in their catalog. Be sure to read it.

Most common battery chargers (with essentially a transformer and bridge rectifier) only charge the battery to 75% or so. And this I think is why a lot of people complain of not getting the calculated amp-hrs from their battery. This charger has three separate charging parts to the charge cycle (all automatic of course) and will get the battery as fully charged as I have ever seen. I use two 6 volt golf cart batteries in series and an Exeltech XP1100 inverter with great results. Pure sine wave output better than the power line. This charger also de-sulfates batteries and I have restored several batteries which I would have in the past just discarded.

rule

Subject: Deep Cycle Battery - What is the Real Capacity? --part 1 of 2   Top

From: Roger Hamlett <ttelmaha_tntlworld.com> Date: Mar 2004

> I am clear on the fact that batteries should not be run down
> to the ground, so I want to distribute my loads well. I am
> going to get another battery. Hopefully a 115 AH battery.
> Here is what I am thinking of doing. Can someone help me
> figure out how long I can run these things before I get
> either of the batteries below 30%:
> What I have to run is:
> - Laptop: Dell Inspiron. The power supply says it consumes 4.5A.

It says that the supply can _deliver_ a maximum of 4.5A (probably at about 18v). This is different from the consumption. The supply only has to deliver this when the laptop batteries are flat, and it is having to charge them at the same time as the laptop is being used. You can keep the actual consumption of the laptop lower, using the options to switch the processor speed down, keep the display dimmer, etc. etc.. Ideally, get a 12v supply for the laptop. These are available from Dell themselves, and (cheaper) from a lot of third party companies (given that your unit has the higher current supply - there are two versions of Dell supply, according to the Inspiron model involved, make sure that the supply you get is for the higher current unit). This gets rid of the double inefficiency of converting to 120v, and then back down to 18v, and means that the whole system in this area is running at the lower (safer if conditions are damp) voltage.

> - SBIG ST-7: From the SBIG website for the ST-7 it says: 5 VDC at
> 1.5 amps, 12 VDC at 0.5 amp desktop power supply included. So
> which one do I use?

Again ideally, get the 12v supply. The same comment applies as for the laptop. The figures given are the _outputs_ from the supply, which is having to deliver a maximum of 19.5W (5*1.5 + 12*.5 + 12*.5).

> - Thousand Oaks digital dew controller: Unfortunately, their site
> has very little info on current consumption and the unit has
> nothing printed on its back. I'd be running a 10" strap, a Telrad
> heater, and a 2" strap for EPs.

The unit itself, draws practically nothing. This is why it does not have a figure on it. It is the _straps_ that draw the power, and the amount varies according to how high the heaters are set. Typically, a 10" strap, will draw perhaps 3A on full, while the eyepiece strap (and the Telrad heater), will only draw about 0.2A at the same setting.

> Here's the site: <http://www.thousandoaksoptical.com/>
> - LX200: As advised, I'll turn slewing speed down to maybe 4?

Perhaps 2... I suspect the reason for the laboured sound, is that the _peak_ current delivery of the 1812 module, is less than the mains supply. High speed is of no advantage really (if you are going to another part of the sky, the scope taking a few seconds longer is not a great loss).

> That's it. I have a Coleman 800W inverter that does up to 7 amps,
> and I'll probably get another Coleman 400W that does up to 3.5 amps.

You do not need more inverter power here. If you are buying extra stuff, go for low voltage supplies instead, and get away from running high voltages in the conditions round the scope, and the inefficiencies involved.

> I was thinking of running the laptop and SBIG on one battery (to keep
> them separate from the scope) and since they are the ones that consume
> the most, run them on the big battery (115 ah) and the scope and
> digital dew system on the other battery (the 75 ah).

> Do you guys agree with this? Should I get the smaller coleman
> inverter or another big one? They are both on sale right now. $29.99
> for the small one and $50 for the big one.
> Will this be the best use of these batteries? How do I calculate how
> long I should run them before getting to 30% charge?

Work like this:
Take the quoted battery power, and _halve_ it. (this allows for the battery ageing, and the fact that you will be using it at low temperatures), while also avoiding fully discharging the battery. So for the 115Ah battery, treat it as if it can deliver 57.5Ah. Now work out the total power needed:

Laptop say perhaps (working at 3/4 the 'peak') 3A, at 18v = 54W. ST7 = 20W

Heaters = (take 3/4 power) 32W

Scope (look at an average between the 'peak,' and 'continuous' power, at perhaps 0.7A at 18v) = 13W

Assuming you have switched to using all '12v' inverters, instead of 'double' converting, then add these together, and add an 'inefficiency' for the inverters at 20%.

Total = (54+20+32+13)*1.2 = 143W
Current (at 12v) = 143/12 = 11.9A.

The battery can then give about 5 hours.
In fact the figure for the laptop, heaters, and scope, are all higher than 'real.' My own Inspiron, only draws about 2A, provided the internal battery is charged, while the heaters typically operate nearer to perhaps 30% power. I see more like 10 hours operation, with a similar setup, on this sort of battery. The figure represents a 'worst case,' for a really cold night. However if you work through the 'double inverters' (using the Coleman to go up to 120v, then down again to the voltages required by the individual units), you would need to think in terms of: Total = 143*1.2 (to allow for the inefficiency of the second inverter) = 170W

Notice that this is still well short of what your 800W inverter can handle.
Battery current= 170/12 = 14.2A

The 'worst case life,' then falls to only 4 hours!... The same comment then applies that the figures are worse than real, but does begin to bring home how much power is actually involved.

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Subject: Deep Cycle Battery - What is the Real Capacity? --part 2 of 2   Top

From: Bill Keicher <wekeichera_tcomcast.net>

The ampere hour rating is for a voltage of 12V. So it really is a measure of how much energy is stored in the battery.

75 ampere hours x 12V = 900 Watt hours

Including losses in the inverter which might be 20%, the energy available at 120V is:
  80% x 900 Watt hours = 720 Watt hours

So at a voltage of 120V, the "Ampere hours" are:
  720 Watt hours / 120V = 6 Ampere hours

The telescope, computer, etc. will require about the same amount of energy no matter how you change the voltage (aside from inverter losses), so it is best to think in watt hours available from the battery as John has done.

rule

Subject: Parallel Battery Charging   Top

From: John Hopper <JohnLX200a_taol.com>

Gene Horr writes:
<< But the current level/voltage charge curve should be the same for batteries of the same design but different capacities. And if they are in parallel this will "force" them to always be at the same voltage level, which would prevent overcharging. >>

In my opinion, this is the key point which people were missing, that once the batteries are successfully hooked in parallel and have reached equal voltages with no current flowing between them, they are effectively one battery which can be charged and discharged as one.

Furthermore, the charging currents will sort themselves out, and will be lower than if the same charger were used on either of the two batteries separately. All the worries about a "high charging-current draw" by one battery somehow forcing current into the other wasn't correct, as the presence of the second battery REDUCES the charging rate on each.

The only more important point is that INITIALLY CONNECTING two batteries in parallel is DANGEROUS. All the normal cautions which apply to jump-starting a dead car battery should be applied, and it's STILL dangerous! A charged battery and a dead battery are at significantly different voltages, and due to almost negligible internal resistance, the currents can be extremely high when you connect them.

This is one reason why I prefer to use cheap, light jumper cables over expensive, heavy ones. A cheapo 8-gauge pair won't have the same current-carrying and voltage-forcing effect as a "professional" 2-gauge pair. The smaller wire will support a larger voltage drop across it (due to its higher resistance) at a given current than a heavy wire. Hence this will reduce the voltage seen by the weaker battery, which in turn lowers the current. You're dumping more energy into heating up the smaller wire than with the thicker wire.

A suitable resistor to put in series with the jumper wire between the last two terminals to be connected, is a 12v automotive headlight. Typically they're around 2 ohms, 6 amps, 72 watts. Power = (Voltage squared / Resistance) = (Current * Resistance). If you can see some light, and then it slowly dims until you can't see any more light coming from the filament, you know the voltage has been almost equalized and there isn't much current flowing to the (formerly) more dead battery. Then I'd wait a couple more minutes, remove the headlight and jump them together with less fear of a problem. Just make sure the light didn't go out instantly due to being put across 24 volts if you connected + to - !!

Rather than actually try hooking up batteries with different states of charge, I'd charge each battery separately, check that they took the charge OK, then follow the procedure above. Maybe with a couple of 50-foot 14-gauge wires and making the final connection at the headlight, 50 feet from the batteries ;-)

Just remember, if you get the terminals mixed up, connect a dead battery to a strong one, make a spark where hydrogen gas has been venting from the battery, charge too fast for the hydrogen to vent properly, or otherwise don't know what you're doing, it really is dangerous.

The sulfuric acid in batteries can (and often, does) do bad things to delicate organic matter like eyeballs, lungs, and skin. It's not the absolute nastiest acid in existence, but it's close enough that you've got to respect it and not take chances.

I've been playing with sulfuric, nitric, and worse acids since the age of 12, and have jump-started a lot of cars under all kinds of circumstances, etc. I've never had a problem, but I do know other seemingly competent people who have had batteries explode in their faces. I'm not sure how much of it was luck vs. knowing what I was doing, but I'm certain it wasn't exclusively one or the other.

rule

Subject: Measuring Remaining Battery Charge--1 of 6   Top

From: Radu Corlan <rcorlana_tprofis.ro>

Guy Schwartz wrote:
> I use a Sears marine battery to power my scope remotely. What can
> I use to check/measure the remaining charge of the battery. I know
> that voltage is not the only thing.

In general, you can't determine the state of charge of a battery by measuring something on it -- it depends too much on the charge history and general state of the battery.

When you start discharging a fully charged battery, the voltage goes down fast at first, to little above 12V. Then it goes down more or less linearly to below 11V -- this is the portion where you get most of the charge. After that, the voltage begins to drop at an accelerating rate (and you should stop discharging the battery!). If you knew the "turning points" of the discharge curve, you could then determine the amount of charge, However, there's no way to reliably determine those, other than monitoring the discharge of your battery in roughly the same conditions (discharge rate and ambient temperature) as when you use them.

Large UPS designers (I used to be one) would kill for a formula or something to determine the charge level; it just doesn't exist.

All you can know for sure about a battery is when it's fully charged (when it draws less than about 1A at boost voltage (14.4V) or was kept a long time (>8 hours) at float voltage (13.8V) ) and when it's discharged (voltage less than about 10V).

I found that the best way to determine charge is to integrate what you take out of the battery, and compare that with the total capacity (which you determine by doing a complete discharge). If you make the test discharge and the actual discharge at similar rates (the battery capacity goes down when you increase the discharge rate) you can get pretty accurate results with this method. However. if you change the ambient temperature a lot, all bets are off!

The best advice I can give is to monitor the voltage during the night -- you will get a feeling for what it means in time; To do better is a lot of work, and not always possible.

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Subject: Measuring Remaining Battery Charge --part 2     Top

From: Doc G

I have avoided this topic for years now. I appreciate Radu's response. For those who may have doubts, about his response, I will be at least one person, who has also worked with batteries in industrial applications, to verify what he is saying. Measuring what energy remains in a battery is exceedingly tricky. It can be done under the limiting conditions he describes. I usually suggest charging the battery and then trickle charging to hold the charge until you are ready for use. Then monitor the voltage while you use it and stop at between 10 and 10.5 volts.

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Subject: Measuring Remaining Battery Charge --part 3    Top

From: Doug Azwell <deazwea_tnetzero.net>

I agree with your explanation. The most significant factor in battery capacity is the temperature, especially with lead-acid batteries.

The driving force for battery activity is the chemical reaction that provides the charge. In lead-acid batteries, this is the reaction of sulfuric acid oxidizing lead in the neutral state to a charge of +2. This reaction's activity constant changes by ~20% for every change in temperature of 10 deg C.

This is why the capacity of a lead acid battery is significantly reduced on those cold nights. Just ask anyone that lives in Canada about that, they will have plenty to tell you about car batteries on cold nights.

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Subject: Measuring Remaining Battery Charge --part 4

From: Don Tabbutt <dona_ttabbutt.com>

Try: <http://www.westmarine.com/electrical.html> There is much about batteries there.

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Subject: Measuring Remaining Battery Charge --part 5   Top

From: Roger Thompson <rthompsna_tnb.sympatico.ca>

Measuring the remaining charge in a battery is very difficult thing most of the time, you can't just use a voltmeter. What you have to do is measure the voltage drop and current under load. This will give you some ideal as to where on the batteries discharge curve the battery is currently at. Unfortunately, the discharge curve is seldom ever supplied with consumer batteries, and it can vary considerably depending on conditions, discharge current, age, temperature, etc. at best you have to make a judgment call as to the remaining capacity in the battery. Your best bet is to insure that the battery is always maintained in the condition specified by the manufacture. The batteries should always come with some time of charging schedule and discharge ratings, follow them for longest battery life. You can try to log the usage and over time you should get a feel for the condition of the battery after use.

Be very careful about using the battery with unknown capacity, since a lower than normal terminal voltage can put any inverters/converters you may be using, under severe stress, and can cause their failure.

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Subject: Measuring Remaining Battery Charge --part 6 of 6  

From: Don Tabbutt

With you having said all that, still all you need is a voltmeter. When a deep cycle (marine type) battery reaches 10.5 volts under load, whatever that load may be, it is discharged. This is the voltage level that is the standard in the industry.

To determine amp-hour capacity and reserve capacity, which are different tests, the battery is discharged with specific loads to 10.5 volts. These tests are what determines the content of the label stuck on the battery for consumers.

So stick a voltmeter on it, discharge it to 10.5 volts, and then recharge it. You should recharge it at 10 to 15 percent of the battery's amp-hour rating. Thus a 100 amp-hour battery should be recharged at 10 to 15 amps.

By the way, lead-acid batteries have no "memory" like NiCads, and can be recharged from any reasonable discharge level. Just don't discharge it to below 10.5 volts...you risk sulfating the battery and inhibiting its ability to recharge.

rule

Subject: Reverse-Power Protection     Top

From: Paul Goelz <pgoelza_teaglequest.com>

> I wouldn't give a passing grade to a student who designed
>something for field operation and didn't protect it against obvious
>environmental hazards, of which the most obvious is reversed power. (This
>can be done several ways without a 0.6-volt silicon voltage drop... a
>Schottky rectifier costs only 0.3 volt... there's a MOSFET circuit invented
>by Robert A. Pease that costs essentially nothing.) But it is very common
>for digital devices to be built without that kind of protection.

>When the warranty on my LX200 expires, I'm going to add protection --
>probably a Schottky rectifier.

It's even simpler to add a reverse biased diode ACROSS the power input (i.e.., band connected to the positive input). A 3A diode and appropriate fuse on the power input (there is one there already, I think?) will give you total protection and costs zero volts drop. It's sole purpose in life is to blow the input fuse on reverse power application. And since silicon devices usually fail shorted, you are protected even if you blow up the diode before the fuse opens.

I am going to add this sort of protection to my 2045D. The last time I had it out, I managed (in the dark) to plug my polarized battery connector in backwards and blew up the input transistor. Thankfully, that was all that blew up! And it's even worse on the 2045D, because they use a battery holder with a "9V style" snap connector where if you attempt to snap the connector onto the battery pack, there is nothing to prevent you from attempting to mate it backwards. The snaps will not mate but they will certainly touch. If the cable is plugged in to the scope… POOF! As we say in the electric airplane community "You will let the magic smoke out". (There is magic smoke in all electrical devices. If you let it out, the device won't work any more).

rule

Subject: 12v Gel Cell Battery Charging Tips   Top

From: Ric Ecker <rleckera_tjuno.com>

Gel cells need to be fully charged up when not in use to give a long service life, I have been doing this for years now with great success, learning about batteries from battery manufacturers. plus keeping a few different UPS (battery back-ups) for systems that can't go down. I use a 12vdc battery charger that puts out 12 amps, plus it will trickle charge, and I use a 1 ohm resistor 20 watts or larger in series. I can leave this arrangement on the battery at all times keeping the battery charged @ 14.2vdc. This will allow the charger to be hookup to the battery all the time and you don't have to remember to disconnect it after a couple of weeks. The resistor acts like a switch when the battery is fully charged so you can't over charge the battery.

rule

Subject: Restoring a Gel-Cell   Top

John McVey <jmcveya_thpbslq.boi.hp.com>

When the cell voltage drops too low because the battery is discharged or allowed to self-discharge, a insulative chemical can form on the electrodes within the battery. This causes a high impedance state which makes it difficult to charge. It is, however, possible to charge the cell by applying a higher than normal DC charging voltage which is carefully current limited. Many nice laboratory power supplies are capable of this. You just have to seek one out.

Proceed as follows; this has always worked for me:

Set the output voltage of the power supply to 30 volts or so (even 20 will probably work, it will just take longer). Limit the output current to 50ma or so. Now charge the battery until the battery is taking the full 50ma of current. This may take hours or perhaps a full day. Watch the battery, it may start to get warm because you are reversing the chemical reaction that formed the insulative layer in the first place.

Once the battery is taking 50ma or so you can reduce the power supply voltage to probably 15V and limit the current to the Ampere-Hour capacity divided by 5 or 10. Charge the cell until its voltage has stabilized around 14.5V (I am assuming you have a 12V cell). Don't worry too much about overcharging lead acid gel-cells. They typically like trickle charging.

rule

Subject: Operating the LX200 Classic on 15V    Top

From: Robert Preston

A while ago I installed a JRC 7815A three-terminal 15 volt regulator (Digi-Key cat. no. NJM7815A-ND, $0.55 each) on the output of my Meade 120VAC/"18"VDC power supply (It actually had a 20VDC output under load).

I then mentioned on MAPUG-Astronomy that slewing was slightly uneven when using the 15V regulated supply, so that the pitch of the slew-whine would rise and fall noticeably during the course of a high-speed slew.

It now turns out that my regulated supply was putting out an average 13V, not 15V, and when I fixed the circuit so it really puts out 15V, the slewing sounds just fine and I'm pleased with the 15V operation (and hope that it will make the motors, regulators, and motor driver circuits longer-lasting).

The final circuit also includes three 6-amp diodes (Radio Shack) in series between the meade supply and the regulator input so the input to the regulator is 17.5 volts and it doesn't run so hot. The regulator and diodes still are hot to the touch after an hour of operation, but I think well within design specs. The regulator is on a little Radio Shack heat-sink for TO-220 ICs, and I drilled a large number of 1/8 inch holes in the Meade power supply case to give better ventilation. The three fat diodes are attached to the inside top of the case under the air slots so their heat goes right out of the case. Thanks go to Bill Miller and other mapuggers for the diodes and holes ideas.

For those who care about details, I'll add the following explanation: When I initially installed the regulator, I omitted what I thought was an optional 0.1 microfarad disc-ceramic capacitor across either the input or the output (can't recall which, now) since it seemed that the huge electro- lytic capacitors in the supply and on the front panel of the lx200 made those little discs unnecessary. When I initially checked it, that circuit did give 15V output, so I thought it was working fine. Later on, I added a bunch of red LEDs as safety lights on the top of a stepstool/toolbox and had them plugged into the regulated supply. Imagine my surprise one night when I saw the two dozen LEDs blink in intensity while running on my fancy regulated power supply! The voltage of the supply was erratic, but mainly 13V, not 15V. Adding the capacitor (now there's a disc cap across BOTH the input and output of the 7815A) finally fixed the problem.

Update: By the way, the 15V 1-amp regulator added to my Meade supply shut down from overheating, despite its little heatsink and the 3 series diodes and my drilling enough ventilation holes in the Meade plastic case to make it look like it was shotgunned. So I got a larger, metal case from Radio Shack and mounted the 7815 regulator chip directly against the aluminum case. Now it seems to be happy as a clam, with the entire metal case acting as a heatsink. (I discarded the top part of the Meade plastic case but installed the bottom part inside the new case as a holder for the Meade transformer and rectifier circuit).

rule

Subject: Reducing 18v down to 15v   Top

From Stan Thomas <thomasa_tcosmic.physics.utah.edu>

If I understood the original question you asked how to reduce the 18 VDC power supply voltage down to 15-16 VDC. The problem with using a resistor is that the voltage drop across the resistor depends on the current. A simple way to achieve a constant voltage drop is to use a series of diodes instead. Each diode will have a voltage drop of ~0.7 VDC. Therefore, to drop from 18 VDC to 15-16 VDC all you need to do is put 3 or 4 diodes in series between the positive (+18 VDC) supply and the telescope. The diode should be rated for more than the maximum current the telescope will draw.

Just in case you are not familiar with diodes I should mention that they are marked with a dark band painted near one end of the diode. The side with the dark band must be connected to the telescope side (right side in the diagram below) and the other side connected to the +18 VDC side (left side in the diagram below).

Supply (+18 VDC)

+18 V -- diode 1 -- diode2 -- diode3 -- diode4 -- telescope +

GND -------------------------------------------------- telescope -

Telescope ( +18 VDC minus four times 0.7 VDC = +15.2 VDC)

rule

Subject: Adjusting Meade 18V Supply   Top

From: Dave Sage <dave.sagea_tcanrem.com> June, 1996

If anybody feels that they really need to adjust the output voltage of the 18/12 converter, it is really a simple matter (detailed below). I don't really think it should be necessary, but then I havn't experienced any problems with my scope either, and my converter puts out 18.2Vdc un-loaded. For those of you who have converters putting out more than that then you should probable have a look at fixing it. The voltage should not change significantly when the converter is loaded. The chip inside should be able to put out 5 amps, although it may not have an adequate heatsink to do that. I would suggest you open your unit up and check all solder connections. The circuitry design is sound but converters are built by a third party and are really very poorly constructed. I found two un-soldered joints in mine and the chip legs bent improperly so that the part was not making contact with the heatsink at all. I am not surprised that some converters have self-destructed.

In any case the output voltage is set by a simple voltage divider which takes the 18Vdc output and divides it down to 1.24 volts to be compared by the reference voltage in the chip. The 1.24 volts appears on pin 2 of the chip with respect to the negative of the supply.

Open the box and hold it sideways so that the 12vdc input is on the right, the 18Vdc output is on the left and the IC and heatsink is along the top edge and more or less in the top right quadrant of the box. You will notice three blue resistors side by side horizontally to the bottom left of the chip. The top two are the two which form the voltage divider. The third (bottom of the three) is of no concern. The top one should have color bands brOWN, RED, YELLOW, brOWN, brown (1240 ohms) left to right or reversed.

The center resistor of the three should have colours brOWN, BLUE, WHITE, RED, brown (16900 ohms)

For those that can figure out a resistor divider you will see that those two resistors across 18 volts will have 1.24V at the junction.

In order to change the voltage to say, 15V should only require changing the top resistor from 1240 ohms to 1500 ohms (brown,green,red). This should give approx. 15 volts.The designer has been careful to use precision resistors in the circuit to get as close as possible to 18 volts and the output voltage changes quite a bit if the resistors are a bit off so a standard 1.5k resistor out of you junk box may only give you marginal success. You may want to try 1.3k or 1.4k first to see what happens.

BE VERY CAREFUL CHANGING RESISTORS and be sure to measure the output before you connect the scope. Observe spotless solder techniques. If pin 2 gets grounded the output of the converter will be VERY high.

I realize that a variable resistor could be put in place of the resistor but there are too many ways for an amateur to mess up this connection and have the ability to make the output go to 30 or 40 volts or more. Also the pot could go bad and the voltage could go that high without you knowing.

rule

Subject: Operating the LX200 Classic on 12 volt Battery Pack   Top

Ed Stewart, <stargazera_tskymtn.com>

In watching the discussions on 12 volt vs 18 volt and dec motor failure, I noticed that one recent post tied the two discussions together. He wondered if those running on 12v experienced less dec failures and noted there was less dec motor sparking when slewing speed was dropped to a value of 6. From a past thread on experiences in powering at 12 vs 18 volt, there was considerable concern about doing anything other than what Meade described in the manual, i.e., use 18v.

When another commented that he had been using 12v for a year with no problems (although it seems he did not have high speed slew capabilities?), I decided to use the 12v gel cell that powers the DobDriver II on my 12.5". This unit was purchased at Target dept. store in the camping dept. and has the label "Power Source" on it. Has a 12v and a selectable 12-9-6-3 volt recepticals, a built-in light and a 6-amphr rating with fuses on all circuits. Cost $30 and weights about 5 lbs. I ran simulated observing sessions with 3 power-ups, 20+ slews, and continuous tracking. After 5 hrs. total, the battery pack indicated low voltage, but the scope still performed normally including high speed slewing (set at level 6). I shutdown and within 5 seconds tested the voltage--it was 11 volts!

As to dec failure, just listening to the mechanical sound at full speed vs a low setting of 5 or 6 indicates to me less stress, plus the noted reduction in sparking, probably would result in less failures (?). I will continue to use 12v and will report periodically on my findings.

I have since used the battery pack for several years with normal performance. Should the need for longer observing or additional accessories require more capacity, then I will move to a small marine battery or wheelchair gel cell. Some advice on gel cells: first, be sure to have a 1 amp or smaller fuse in the circuit; second, do *not* use an automotive battery charger. Instead, use a trickle charger (one probably will come with the battery pack). Should a resistance build up to charging, use a very low rated charger (100 milamps) over several days to reverse the chemistry.

rule

Subject: Operating the Classic on 12v Vs. 18v   Top

From: Doc G

I need to correct a comment I made on this issue. I stated that the keypad sets the speed of the drives by sending counts to the motor circuit. This is correct. I also stated that the slew rate thus did not vary with voltage (when set from the keypad). However, it is the case, as someone suggested, that the scope will move more or less rapidly with voltage changes, when the maximum slew rates are selected. Or when the default rate is selected (this is the maximum rate). The reason is that the motors are slewing as fast as they can for a given voltage under those situations. This means that the drive amplifiers are putting out the maximum current possible under very fast slew conditions and they cannot keep up when the scope is on a lower voltage.

Never-the-less, the actual motion of the motor is still kept track of by the computer by its counting the encoder pulses. This so the computer can tell the motor when to stop. If this were not the case, a slew overload would cause the scope to loose track of where it is, and it does not. The apparent mode of operation does depend on how mechanically tight the drive is. So the observed conditions might vary depending on the mechanical condition of the scope. Again, the LX electrical and mechanical mechanisms prove more complex than they at first seem. This matter has been a controversial topic for years. The facts, as I understand them, are that almost all of the LX scopes work well with 12 volts. However there are exceptions. Scopes that are tight mechanically or have significant stiction require a much greater torque and thus a much greater motor current to get them started as you have described. This requires higher voltage.

A solution to this problem has been for Meade to recommend a higher voltage, 18 volts, in recent years. This is in many ways an unfortunate solution to what is basically a mechanical problem. This is why. With the higher voltage and a case of mechanical stiction the motors can overheat or the transistor drivers overheat and burn out. With the lower voltage, you can more easily get erratic running of the motors as you have observed. In order to solve the overload problem with the higher voltage, Meade has added current limiting resistors to the circuit. You will find these described in the archives on recent board replacements. This is a partial solution but not a perfect solution. It is a fix (add on)(after thought). With the present design it is hard to get off of the horns of this dilemma. The best solution, I think, is to see to it that the mechanical system of the telescope is smooth and free running. This can be done by insuring that the bearings are aligned, the scope is balanced and it is well lubricated. Unfortunately, all of these problems go back to the original design, which is, in my opinion, fine for the smaller versions of this scope but somewhat problematical for the larger scopes.

I really regret to report this analysis because it means that some LXs will give users problems. So it appears, that if you have a tight scope, it is essential and worth the time to get it in the best possible mechanical condition. I have personally done several things to my two LXs (a 10 f/6.3 and a 12 f/10). These are: to replace the declination bearings with roller bearings, carefully align the axes, lubricate the bearings with the best grease, carefully adjust the motor/worm mechanism to give just the right tension on the worm wheel, balance the scope carefully and use the 18 volt supply. I put in a considerable amount of time and constant care to keep the scope running smoothly.

rule

Subject: Voltage Regulators--Short Primer      Top

From: Doc G

This is a short primer on power supplies which may be of some interest to those concerned about electronic equipment which has both digital and analog sections, and particularly which drive transducers such as motors. This may or may not apply to any particular piece of equipment since there are some alternate designs possible. (but not usually used because of problems with cost.) I am not privy to the details of specific designs which may be of concern to MAPUGers at this time.

In general the power supply will have both +12 and +5 volt regulators for the voltage busses. The 5 volt bus is for the digital parts of the circuit and the 12 volt bus is for the analog and higher level output and auxiliary switching electronics. There may be -12 and -5 volt supplies as well, as in the case of a PC, but these are not necessary for less complex digital/analog designs.

Additionally, the electronics may have semiconductor drivers for the output circuits that can withstand higher voltages. This is particularly true if the outputs drive inductive loads like motors of most types. (either steppers or DC)

The 12 and 5 volt supplies are usually regulated with standard three terminal regulators like the 7812 or 7805 which are integrated packages of high quality and low cost.

In order to regulate 12 volts, the regulator must have a voltage in excess of 12 volts by a few volts. Typically 3 to 5 volts. There are some low drop regulators which will work with as little as 0.5 volts differential but they are not usually used in older designs. Thus we see that there is no problem at all regulating the 5 volt digital power supply since the source is always substantially higher than the 5 volts. But a 12 volt electronic regulator will typically start to run out of regulation capacity with less than a 3 volt drop. It will certainly run out of regulation when the source voltage is equal to the regulated voltage.

Thus it is prudent to advise that equipment with 12 volt regulators be operated with sources of at least 15 to 18 volts. This straight forward design limit is the reason that manufacturers recommend voltages for the source at least a few volts higher than that of the highest voltage regulated bus in the equipment. Of course if the source voltage is too high, there is more and more power lost in the regulator itself. Thus it would be foolish to use a 24 volt source for a 12 volt regulator. In that case, the regulator would dissipate a power equal to that dissipated in the equipment. This situation would double the heat sinking necessary for the equipment, a costly design mistake.

At the same time if the source is the power mains, the voltage from the source will vary with the line voltage which might be well over plus and minus 10% depending upon the policies of your power company. So a safely factor must be built into the design. If the source is a battery, it should maintain a voltage about 3 volts above the regulated bus voltage. For a 12 volt regulated bus that would be 15 volts or slightly more. A convenient voltage is 18 volts. A battery voltage as low as 13 volts might result in the failure to maintain good regulation. This does not necessarily mean that the electronic equipment would fail immediately. But, if the regulation is an important part of the design it might cause erratic operation.

An additional factor in designs which use inductive loads in the output is that inductive loads, especially switched loads, often require voltages higher than the voltage used for the main circuitry. When that is the case, it is necessary to connect the high side of the load to a higher voltage power supply and to use devices in the output stage that will handel both the higher voltages and the spikes that result from switching transients. This is easily done in proper circuit design. If the drive voltage on the motor is too little, the current will be too little and the motor will either stall or miss counts since it is the current which provides the mechanical torque. Neither condition is desirable.

Thus it is often standard design for reliability purposes to have a source voltage higher than the voltage of the regulated supply and to have enough voltage to drive the motors. The higher source voltage is of course not regulated nor does it have to be. The above information is a very simple overview of power supply design. It is based on 40 years of design experience in electronic and computer applications and instrumentation.

rule

Subject: Fried 18V Converter   Top

From: Richard Tabbutt <76367.3371a_tcompuserve.com>

The Meade 12VDC to 18VDC converter is comprised of a simple switching regulator circuit, of which the LT1170 chip is the "heart". This chip is definitely not a rectifier. Instead, it contains the switching transistor (hence the heat sink) and regulator circuit. It controls (switches on and off) the current through the inductor located just below its leads, which then charges (through a diode rectifier) the large capacitor located at the output. The ouput voltage is sensed by the LT1170 and controlled by varying the inductor switching current. Voila! 12V in becomes 18V out.

If your converter is new, you should send it back. Otherwise, if you feel you can handle the desoldering and soldering involved, try Radio Shack for the parts you need. They advertise that they can obtain almost any IC if you can provide a part number. Also, I'd replace the output diode rectifier, as failure of this part could have caused the failure of the LT1170.

rule

Subject: LX200 18v AC Adapter Problem --part 1 of 2 Top

From: Brian Bond <brian.bonda_tblueyonder.co.uk> Date: Nov 2003

----- Original Message -----
From: Christian Molnar <cmolnara_tiship.com>
I just finished a power box, marine deep cycle battery inside a metal
toolbox with 5 lighter outlets on top and leads for my inverter.
I don't know whether this is just my paranoia but, since I just did this, I
noticed it. When I run my LX200 on outlet power (from the house) it is very
fast and there are only 3 LEDs turned on the indicator most of the time.
When I use this power box, you can tell it's moving slower and it is just
very different. There's more between 4 and 5 led turned on constantly and
it peaks every time it slews. The slews are more sluggish and the motors
don't sound like they do when the scope is plugged in.
Is this normal? I can't see how this battery wouldn't be powerful enough.
It's like a 75 amp hour boat battery.
----- End of Original Message -----

There is a problem with one of the components in the Meade 12 - 18 volt convertor, where the off load voltage is 18 but when plugged into the LX200, under load conditions, drops to 12 volts. The flyback inductor controlling the voltage change develops a shorted turn over time (heat) and prevents the circuit performing as it should. Normally there is nothing wrong with the chip. My internet site <www.brianbond.eu.com> has details of rewinding the inductor to clear the problem. This has always been the cure.

I've copied the relevant section below:

'Must handle at least 3 amps (Meade = 94 turns of 24SWG Enamel wire on original bobbin for 330uH, saturated in Polyurethane - original fault showed low conversion once the chip had been changed - due to inductor turn short circuit)'

--------------------------------------------------------

Subject: LX200 18v AC Adapter Problem --part 2 of 2 Top

From: Damon Raphael <w7mda_tgci-net.com>

----- Original Message -----
From: Bob Manley <RManley234a_taol.com>
I have now confirmed that the thermal cutoff is in series with the transformer primary. I have also confirmed that it has not blown. All fuses are in place, of the proper sizes, and not blown. I believe that your diagnosis is the correct one- that there is an internal short in the secondary winding. The resistance of the secondary seems to be too low- only about 0.4 ohms, after the resistance of the ohmmeter leads is subtracted from the total. This AC adapter has been outside in my unheated and well ventilated observatory for four or five years, and I suspect that some corrosion of the core or breakdown of the magnet wire enamel has taken place. It now appears to be unrepairable.
----- End of Original Message -----

I think that your assessment of the cause of failure is correct. If the transformer was exposed to moisture, that would accelerate the insulation deterioration. In a transformer secondary short, you would usually expect that the failure of an external component would have initiated the insulation breakdown; that is, heating secondary to excessive current draw through the secondary winding. If the other components are OK, then moisture is probably the culprit. If the primary had shorted, it would have blown the thermal cutoff device. You tweaked my curiosity with your post because I have had some problems with my LX200 which were power source related (as have others in the group). My AC Adaptor is branded as "SCEPTRE" made in China with a nominal output of 18VDC rated at 2 Amps. This is the original unit which was supplied with my 8" f/10 LX200 purchased new in 1996. I have had no problems with the unit and am always careful to protect it from the elements. I live in Tucson, Arizona where moisture is usually not a problem.

I tested it tonight with my Fluke 77 DVM. The unloaded DC Voltage across the output is 21.3 VDC. I located the manufacturer/supplier of the 18v converter at <http://www.gpelectronics.com/AC-DC.htm>, and it is still available from them. They have a .pdf spec sheet online but no schematic. They do list it as an unregulated AC to DC power supply.

I opened the case and examined the contents. Of course your unit may not be similar to mine. I was curious to see whether it used switching components or a voltage regulator of an kind. Voltage regulators are near the top of the checklist for common component failures, one rung below electrolytic capacitors. I see the transformer, 4 1N5400 common silicon diodes, an electrolytic capacitor 6800 mF, what is probably a resistor and a fuse. I did not remove the circuit board to see how the diodes are hooked up but it looks like a simple voltage doubler circuit with a single capacitative filter stage.

There are no zener diodes, voltage regulator transistors or ICs and certainly, this is not a switching power supply (which would add several levels of complexity and potential problems). In a more perfect world, you would be able to call Sceptre and order a replacement transformer for a few bucks and the problem would be solved. You can give them a buzz and try it but they may not speak English, may not provide replacement parts, charge more for the transformer than a new whole unit or refuse to sell to you. Maybe all of the above or tell you to call Meade for service. I then downloaded the motherboard schematic from Doc's site and checked to see what happens to the power when it goes into the telescope. I see that several capacitative filter stages are added and there is a voltage regulator chip which converts 18 VDC to 5 VDC for the solid state devices. The way the LX200 is designed, the filter and voltage regulator parts of what would properly be called an "AC to DC Power Supply" are on the motherboard. The power transformer and rectifier parts are external to the scope. I was curious to see if a failure of a component inside the scope could cause the transformer in the power supply to fail. I think not if the fuse in the Sceptre unit and the fuse in the DC connector cable are proper. The series output of the motherboard section of the power supply is fused to protect the motherboard and plugged in devices should there be a failure in the power supply.

It would have been nice if there was current limiting circuitry to prevent damage from low voltage situations. I wonder if current limiting has been engineered into the newer GPS versions.

rule

Subject: 1812 Voltage Converters   Top

From: Ken Fields <KNSTARSa_taol.com>

I fried two of the 12 to 18 volt Meade converters. I have switched to using 12v to 110v converters from Fries for about $40 or $50. I use a big 120 amp. marine battery which handles the draw of that converter well and then I can use the regular 110 to 18 volt converters that come with my laptop and other equipment. Since I switched to that kind of power source I have had no problems. I have it all hooked into a switch panel in the back of my truck so that I can turn on or off any line including a portable worklite.

Also see: <http://obs.nineplanets.org/meade/1812/1812.html>

rule

Subject: Meade 12v to 18v Converter Alternative   Top

From: Ted Van Sickle, Date: Nov, 2001

Radio Shack has a very nice DC to DC converter that connects to automotive battery and by a switch selection provides an output voltage of about 8 to 30 volts DC. It has a capacity of 30 watts which is just fine for an LX200. I have used it with a garden tractor battery and it has run the scope for more than 30 hours with no sign of trouble or battery discharge. I don't believe that the voltage is too important in this type of equipment, an LX200, but for those that are concerned with the voltage, this system is perfect because you can set the voltage to whatever value you want in one volt steps. Also, I think that it cost $59.95 or something near that price.

rule

Subject: 12v to 18v Converter Source

From: Gary Giddings <garyga_twwwsite.com> Date: Nov 2001

Radio Shack has a very nice DC to DC converter, #273-1826, that connects to an automotive battery. It has a capacity of 30w which is just fine for an LX200, 11-16v in and selectable 9-24v out (+-5%).

rule

Subject: Inverters & Batteries   Top

From: Paul Goelz <pgoelza_teaglequest.com>

>First of all, the mount. Unless you are slewing all over the
>place the mount draws very little power. I don't have any specs

My LX200 draws in the neighborhood of 400 mA tracking or slewing, with brief excursions to maybe 1A at the beginning or end of a slew. This is at 12VDC. At 18V, I would guess it would be maybe 20-30% lower.

>Two inverters available at Walmart (cheaper) or Radio Shack (expensive)
>allow you to power your equipment without having to do anything
>more difficult than plugging it in.

Note the output of the inexpensive inverters is a two step square wave that has an RMS value of 120VAC. However it is not a sine wave and may not run all types of equipment (especially ones with inductive components) satisfactorily.

So be careful when running expensive equipment from an inverter. I think a couple sensible precautions might be: Don't turn the inverter on into a load... turn the inverter on and then connect the load. When you switch on the load, make sure it is running correctly Many inverters have an output current limit that can prevent the output voltage from coming all the way up to 120 volts when trying to start into a load with a high inrush requirement. This can result in a steady state condition where the output stabilizes at a lower than expected (and very unstable) voltage.

Avoid powering expensive and/or difficult to repair loads that contain power transformers unless you are sure it is OK (like you have tried it before and nothing went wrong). Look for a true sine wave output inverter if you want to be safe. I have not seen one but I understand they exist, at a higher cost. My experiences with inverters have been less than satisfactory, and I would rather power my field devices directly off 12VDC if at all possible.

rule

Subject: DC Power Source & Inverter     Top

From: Bill Arnett <billa_tnineplanets.org> Date: Sept., 2000

Jchoroszucha wrote:
> I recently bought a used LX200.I did not receive a DC adapter with the
> purchase.My scope requires 18 volts DC. The Meade #1812 costs $100. I
> have a marine battery sitting around and was thinking of buying a 300 or 400
> watt inverter hooking it up to the marine battery. And plugging in my scope
> to the inverter since I already have a AC adapter with the scope. Any
> suggestions on this from any one if this will work? If it will draw the
> battery down fairly quick? I plan on getting some software down the road
> probably TheSky by Bis. I also don't have a DC adapter for the lap top so
> I thought the inverter would work for this to. The battery is pretty large I
> use it for a trolling motor I think it it has 700 crank amps. Don't remember
> and the inverter costs $50. Would welcome any comments or suggestions.

First of all, "crank amps" is irrelevant. That's a measure of the maximum short term output current which is important for starter motors but not here; your LX200 plus a laptop, dewheaters and CCD cameras is unlikely go draw as much as 1/100th of that much current.

What is important is the amp-hour rating of the battery. That (given that it's 12v) is a measure of its energy storage capacity. And that's what determines how long it will run your setup. A "big" marine battery is usually 100 amp-hours or more. That is enough to run an LX200 for weeks and a full imaging setup for several days.

Using an inverter instead of the 1812 is fine. It's a little less efficient but if you have a "big" battery you don't care. And it does make it convenient to run other AC devices. I've run this way many times with no problems. (And I've had lots of problems with the 1812, all of which were ultimately my fault but ...)

If you do get an inverter get one that puts out a "pure sine wave" output. The LX200 will probably work fine with a "modified square wave" but a laptop or a CCD camera may not.

rule

Subject: Inverter Issues & Questions --part 1 of 5     Top

From: Dave Graham <hardwarea_tdatasolaz.com> Date: Mar 2002

stephejlwrote:
> First: Large batteries are Dangerous.

Yes, and bad things can happen fast. Some thoughts on making life easier and safer... Check out the battery accessories at a marine supply house or boat store. Some items on my shopping list:

  1. A polyethylene battery box. They have slip-on lids and usually a plastic web strap to hold the lid on. The lids are made with channels that allow the cables to exit the box. They can help contain the acid if the battery is tipped beyond normal, and the spatter that can result from over charging. They have handles molded into the case (sometimes a plastic rope loop and handle) which make the battery much easier to lift and carry.
  2. Battery terminals which attach to the posts like automotive clamps, but instead of going directly to a cable, They have a stud with a wing nut for a ring terminal. Put ring terminals on the end of the cables, and you can attach them without tools.
  3. (More likely from an electrical supply house) A polarized connector rated for 50 amps, wired to very short cables going to the battery. Keeps you from accidentally connecting the battery backwards, and eliminates the temptation to walk around with long cables dangling from the battery. Put the female side of the connector toward the battery, especially if the connector doesn't have protected male plugs. Eliminates the possibility of an accidental short circuit. Might want two (or more, depending on how many charging options you desire) of the male connectors so one can be wired to your equipment distribution rig, and the other(s) rigged for charging.
  4. An in-line fuse between the battery and the connector. A 40 amp fuse will keep the wiring from a meltdown or fire (or worse, a battery explosion, if you *do* somehow have a catastrophic short circuit. Downstream, wire an in-line fuse sized for each individual device as close to the distribution point as possible so it protects the wire as well as the electronics.
  5. #8 fine-stranded wire (or heavier if you like) for the cabling between the battery, primary connectors, distribution box, and charging source. Fine stranded gives greater flexibility. The type of insulation also bears greatly on flexibility. RV and boat houses usually carry a selection. Ask to see different types. You may be surprised at the differences. You can use an automotive electrical system for charging if you also get ...
  6. A battery isolator (boat or RV house). Allows the automotive system to charge both batteries, and keeps the telescope battery from draining the auto system, and allows starting and stopping the motor during the process without problems. If the battery box is well secured, you can run the charging cables to a location in your vehicle and charge while on the road. The location does need to be adequately ventilated, since charging produces hydrogen gas, and you don't want your RV becoming a Hindenburg....

The battery box you got at the boat store will come with hold-down clips that can be attached to the floor or other storage location. They are for the strap that goes around the box to pass through, holding the battery securely in place. The bottom of the case will have recesses keyed to accept the clips, adding to the stability. Measure carefully when mounting them.

Use terminals properly sized for the wires and the terminals. Again, the people at the supply house can guide you.

Hope this is helpful... You can't stress safety enough around high capacity batteries. A shorted battery can put out enough power to melt very heavy wiring, and possibly cause the battery to explode. I've never been around one myself, but have seen the aftermath of two such incidents. Kind of like a tornado...I don't ever want to be close to one...

---------------------------------------

Subject: Inverter Issues & Questions --part 2      Top

From: Doug Groesbeck <douggroa_tattbi.com>

> The question is that can I run 2-400w converters daisychained together for an
> effective output of 800w 110vac without damaging the converters. I have all of
> my outlets in the observatory connected together so all I do is connect a
> small length of 12ga extension cord with 2 male ends from the converter into
> an outlet thereby energizing all of my outlets. What do you think? Can i plug
> another 400w inverter into an outlet that is already energized by an inverter
> to double to power?

I've followed all the responses thus far, and either I'm way off base or the rest of the folks missed part of your question. You're energizing the outlets by running a cord between the battery-powered inverter, then tapping the other outlets to supply your 110v items. So far, no problem there.

You want to add another inverter to the mix to provide a larger load capacity and connect it to the outlets in the same manner. Potential BIG problem there.

Unless you've very strictly paid attention to polarity coding on your wiring chain to the outlets, you could end up with crossed polarity - the second inverter would feed a hot signal to the neutral side of the chain. Result: a direct short. Something will give, most likely the second inverter you plug in - right as the bright sparks fly from the outlet. 110v outlets and switches have those brass and aluminum finishes on each side to provide the reference for polarity - brass for the hot side and aluminum for the neutral. You can in fact use either side of the switch/outlet for either hot or neutral, as long as the wiring is consistent on that receptacle (though you should follow code and keep the hot side to the brass for polarity-sensitive devices).

The other issue mentioned by several others is the frequency timing of the power supplied to the circuit. Two sources operating out of phase are going to make for some problems with consistent voltage.

So if you want a bigger load capacity on the 110v circuit, you're going to have to beef up the point-supply for the circuit. Consider either a single higher capacity inverter, or a small generator to feed the circuit.

--------------------------------------

Subject: Inverter Issues & Questions --part 3      Top

From: Mike Dodd <mdodda_tmindspring.com>

> Not daisy chained (indicating series connection). You want to run the
> outputs in parallel. This will keep the voltage the same, but increase the
> load capability to 800 watts. I THINK you can do this with AC. I
> know it works with DC. Come to think of it, there may be a phase
> difference trying to do this. Can anyone else comment on this?

Never connect the outputs of inverters together in any way. If you need more power, buy a larger inverter.

--------------------------------------

Subject: Inverter Issues & Questions --part 4

From: Roger Hamlett <ttelmaha_tntlworld.com>

From: Mike Dodd <mdodda_tmindspring.com>
> > Not daisy chained (indicating series connection). You want to run the
> > outputs in parallel. This will keep the voltage the same, but increase the
> > load capability to 800 watts. I THINK you can do this with AC. I
> > know it works with DC. Come to think of it, there may be a phase
> > difference trying to do this. Can anyone else comment on this?
>Never connect the outputs of inverters together in any way.
> If you need more power, buy a larger inverter.

To 'add' to this. If you are running 'normal' AC inverters, doing this, can give the same results, as 'crosswiring', between two phases on a three-phase supply. The potential for damage is massive. There are AC inverters sold, which are designed to do this - some 'off line' UPS systems, have a 'sync' connection, allowing the internal signal to be synchronized (for exactly this reason), but this is only common on very large units, where the manufacturers assume that even more power may be needed. I have a pair here, of 4KVA units, that do this, but the prices start at a couple of thousand pounds (and presumably similar in the US...). You can (of course) 'parallel up' the incoming battery supply, to increase the time that the load can be driven for, but to get more output power, the only simple solution is a larger supply. Remember there is nothing to stop you running different equipment from separate supplies though.

> On the subject of two inverters I would be concerned about possible
> phasing issues between the two different
> 110v outputs.

Yes.

> I could be possible to have an interaction between two powered devices
> if they share any common circuitry outside of the input
> power supply, could be sorta like the ground loop from hell.

Shouldn't matter. In general, the equipment being talked about comprises devices with their own 'mains' power supplies. These provide isolation between the incoming supply and the user (necessary...), and in doing so, prevent problems of this sort. What you say _could_ apply, if you are dealing with directly 'mains driven' equipment, and then joining parts of the latter circuitry together, but (fortunately), equipment that has connections 'available' to link to other kit, has to have these isolated from the incoming supply (except the ground connection, which can be common between multiple supplies), to meet current wiring regulations. The potential for normal 'ground loops' exists, but these would be the same if the equipment was on a common supply. If multiple inverters are used this way, it becomes essential that the ground connection is made common (and _only_ the ground connection), otherwise if there is a potential difference between the ground points, this potential will exist between the equipment...

---------------------------------------

Subject: Inverter Issues & Questions --part 5 of 5      Top

From: Jimmy McGuire <jxmcguire1a_tualr.edu>

By hooking AC power systems together, either in series or in parallel, you will let the smoke out of your equipment. This is an event I've always tried to avoid. I suggest you avoid it also. If the outputs of two or more inverters are COMPLETELY isolated from each other as verified by electronic test equipment, you MIGHT get away with connecting together the electronic equipment being individually powered by separate inverters, but even that is quite risky, since the equipment and inverter internal configurations are never guaranteed to remain in the tested config during all the operating modes.

rule

Subject: Power Supply Design URL      Top

From: Randolph Wilson <astroa_trwizard.com> Date: Mar 2002

Assuming that the Pictor CCD camera expects "nice" power, and given the cost of a CCD, some refinements may be in order. Radio Shack has a nice little book on power supply design, as do others. There are easy to implement solutions that provide clean, well regulated power, as well as features to protect your expensive equipment from possible harm. Take a look at:

<http://www.national.com/parametric/0,1850,645,00.html> to learn about some of what is out there.

rule

Subject: Power Supply Issues for LX200 Classic --part 1 of 3  Top

From: Doc G, Date: Dec 2002

----- Original Message -----
From: Bruce Gillespie<Brucea_tpcb.co.za>
> my AC Adapter and Inverter got stolen a while back and
> I am trying to setup a new power supply system. What I would
> like to ask anyone who might have some advice on the following:
>
> - How tightly regulated must the 18V be? What happens if the voltage is
> - over? under?Peak current draw? (I figure 2 Amps)
> - Anyone with a suitable circuit for a transformer/rectifier/smoothing cap?
> - What type of fuse and what rating?
>
> I would also like to include a 12V DC to 18V DC Converter in the box
> for when I have to use the system remotely - and cannot locate an
> off-the shelf DC-to-DC component here. Any pointers to web sites of
> suppliers in the USA and reference to brand names / part numbers?
>
> I tried using a laptop PSU that put out 19V DC up in the bush during
> the recent total solar eclipse (when it was not cloudy that is) but I
> found it could not handle the current and kept resetting the CPU in the
> scope when slewing on both axes so I need to build a transformer based
> one that can handle the current.
>
> Also the scope behaved very strangely. I have been using it for years
> with satisfactory tracking and GoTo capabilities. But this time I had
> real problems. I triple checked GPS coordinates (23 01 deg. S, 329 05
> W), time zone, GMT time, alignment stars (tried a few different), did
> two star alignments several times, with the problem being really way off
> Goto's. Even after stepping to the targeted and Sync'ing it seemed
> clueless, didn't even track properly. For instance aligned on Canopus
> and Betelgeuse, tried to GoTo Saturn and it lost the plot and slewed and
> 10 deg of course. Also some times it was giving a "Align Mismatch -
> Check Stars" message during selection of 2nd alignment star although I
> was 100% sure I had the right stars.
>
> Could this be power supply related? (this was off that laptop PSU) Or
> something else? Or am I missing something really obvious?

The voltages in the LX200 classic are regulated down from 18 volts. The digital boards work with 5 volts regulated and the rest of the electronics uses 12 volts regulated. Only the drive motors use the 18 volts directly.

In the beginning the LX200 classic used 12 volts. It was found that the motors sometimes needed more voltage and they went to 18 volts. This was very inconvenient and then they had a lot of transistor driver burnouts. Later still they started to add a pair of current limiting resistors to the motor circuits. This has apparently save most from a burn out problem. The motor currents are limited to about 1.5 amps. This saves the motors and the driver transistors.

What this all means in that the LX200 classic should work well with anything from 12 volts to 18 volts. I would not go higher and I would not let the voltage go below 12 volts. The regulation is not very important as long as it stays within this range. Rather than use a 12 to 18 converter, especially the Meade unit, I normally stack three 6 volt batteries in series. This gives a convenient 12 volts for some equipment and the 18 volts for the LX200.

At voltages above 18 volts, the regulators in the LX200 get very hot. If you are already in a hot climate, it may be too much. With a bell lubricated and well balanced LX200 you should be able to get reliable operation with 12 volts. Well charged batteries will usually be at 12 to 14 volts.

I cannot comment about the other problems though I suspect they are not power supply related unless the voltage gets below 11 volts. Then everything can go bonkers. (G)

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Subject: Power Supply Issues for LX200 Classic --part 2   Top

From: Jim Henson <jimha_tbga.com>

ScopeStuff <www.scopestuff.com> offers the #1218 DC-DC converter for the LX200 Classic. It has internal over-temperature protection that keeps it from suffering the heat related death that kills the Meade unit.

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Subject: Power Supply Issues for LX200 Classic --part 3 of 3

From: Hari Seldon

How about building a 18V battery pack out of three 6V cells. It works great for me and you will not worry about current draw or voltage regulation anymore. If you use solar panels to charge the pack, you can opt for 6V panels to charge each cell separately.

In any case, if you opt for an AC2DC make sure that your output is regulated for higher voltage. The scope can take lower voltage with no complaints. I believe peak current draw is around one amp. You should use a fast-blow fuse of 2 amps. There is a 2A fuse inside the control panel, but I believe it is not quite so 'fast blow'. Some time ago there was an incident in which the onboard fuse failed to blow as fast as it should and blew a capacitor on the panel. Instead, it blew a couple of 3A I had in-line from the battery pack.

If you insist on going the 12V battery route, then search the MAPUG-Astronomy Topical Archives to find the schematic diagram for a 12-to-18 dc solution and build it yourself if you're handy at electronics. Otherwise look for a cheap unit from Scopestuff at: <www.scopestuff.com>.

rule

Subject: Standard LX200 Classic Fuses --part 1 of 2   Top

From: W. Miller

In my 12" rev 4.3 Classic scope I use a 1-amp glass body fast-acting fuse (Littelfuse 3AG 312 series) in the fuse clip on the power panel. In my 16v power supply I use a 3-amp fuse. The 1amp fuse is on the downstream side of the LX200 power panel on/off switch. I first turn on my 16v supply and two large capacitors get charged through the 3-amp fuse. These caps (~10,000uf each), one in the 16v supply and one in the LX200, take a huge surge of current to charge up to 16v. In fact my 16v supply's 8-amp meter slaps over to full scale for an instant. Pushing all this current through a smaller fuse might cause it to blow. Then I turn on the LX200 power panel switch and a smaller surge of current flows through the 1amp fast blow fuse. I haven't blow a fuse since I set up this way.

Before this, I burned out my RA motor and Op Amps using only the 2amp slow blow fuse that came with the scope in the power panel and the fuse inside the Meade 18v AC adapter. These fuses were too big. Maybe a 1.5-amp fuse in the power cord would have blown, but I was not using the Meade cord. It is most convienent to change the fuse in this cord should it blow, but I don't mind getting out a screwdriver. The power panel came from Meade with a 2,200uf minature electrolytic capacitor on it.

I upgraded my power panel cap to the type typically found in power supplies for switching circuits. It has bolt on terminals, low internal resistance, and fits inside the LX200. I wanted a better capacitor since my 50ft power cord has considerable inductance. Using 12 gauge copper cuts it's resistance down but does not reduce the inductance.

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Subject: Standard LX200 Classic Fuses--part 2 of 2 Top

Michael Hart

The Bussmann MDL 1.5 amp fuse is in fact a time delay "slow blow" type. The particular version you describe is known as a spiral wound element. Older designs used two elements in series, one for short circuit protection, the other for overload protection. The older design is more expensive but eliminates current limiting effects of a spiral wound design. The original two element design is still available as an MDQ-1-1/2 amp.

rule

Subject: LX200 Power Cable Fuse?   Top

From: Mike Dodd <mdodda_tmindspring.com> Date: Dec 2001

>The ends of the power cable fuse are stamped "250v" and
>"Buss MDL 1 1/2", respectively.

That's a 1.5A time-delay glass fuse. The Digi-Key part number is 283-2179-ND (5/$0.91), and the Radio Shack part number is 270-1022 (4/$1.99).

>The Archive implied that "MDQ" fuses - though more expensive -
>had two types of protection. Yet there wasn't enough of a
>discussion to suggest that one would be preferable over the other.
>Or that 1.5A fuses are too much, too little, or just right, given a
>particular setup.

Digi-Key does list some 1.5A MDQ fuses, but: 1) they cost $80.33 for a package of 100 (no single-piece prices), and 2) currently there are none in stock.

I agree with Curt Tooley about not arbitrarily switching fuses. Many factors dictate the type and size of a fuse, and it's unwise to second-guess the designer.

This is a no-brainer: You can buy 4-5 replacement fuses for pocket change. Replace it and see if it holds. If not, replace it again. If it still blows, you have some other problem in the electronics.

rule

Subject: LX200 Classic Fuses Explained  Top

From: Michael Hart

> I have looked into the fuse issue and the news in not good. It is a close
> call between blowing the internal fuse and burning up the chip or even the
> motor. The internal fuse is a pain to replace. I suggest using a
> slightly smaller fuse in the line cord. It is a inconvenient when it blows
> but it is easy to replace. Standard fuses do not burn out very fast unless
> they are overloaded by 2 times or more. They have a time constant for
> heating and melting. This whole fusing issue is much more complex that
> you might at first think because of the several heating time constants involved.
> There is no good solution since the fusing burnout and device burnout were
> not coordinated carefully in the original design. Slow blow fuses and
> semiconductors do not like to coordinate well. The semiconductors often
> burn out faster than the fuses.

Doc G has a good understanding of overcurrent protection. I believe I can add a bit to this discussion and perhaps a solution:

Typical 1-1/4 X 1/4" fuses are available with at least four characteristics:

  1. Non-time delay, non current limiting
  2. Non-time delay, current limiting
  3. Time-delay, non current limiting
  4. Time-delay, current limiting

Certain fuse designs actually limit fault current to a value considerably less than the available fault current and operate extremely fast- a few milliseconds. They do this in a rather complex and important way.

It is not unusual for a non-time delay fuse to be slower at reacting to overcurrents than time-delay fuses, depending on the fuse and application. When designing in a fuse to protect a circuit we must consider many factors including available fault current, running current, peak current, circuit voltage, minimum melt time, total clearing time, AC or DC, frequency, temporary overload current, and the I2t rating of the weakest component in the circuit. These are off the top of my head- I'm sure there are more considerations.

The problem in applying a single fuse is it must be have a continuous current rating that is adequate for the entire circuit load while protecting individual components in the circuit. The fuse must react to short-circuits or overloads of different duration below any individual component's I2t without blowing needlessly during acceptable short-duration overloads. Then if we are using multiple fuses, we want the fuse to blow nearest the fault or overcurrent so that only the effected circuit is disabled. Finally, we must weigh the added static component cost into the design. For most consumer applications, a single fuse is used primarily to prevent fires while board level components are sacrificed.

Having said all the above, it might appear there is no simple solution to protecting the output driver chips. I believe Meade added two power resistors to the motherboard to help. If the Meade mods isn't deemed adequate (I still use the old motherboard), I believe there is a solution as follows, if there is really a problem to solve:

Protect the individual components using specialized high speed semiconductor fuses whose I2t let-through is LESS than the components I2t rating. I have knowledge and access to military grade fuses not typically available to the public. These fuses are very fast, used in redundant satellite, military and space applications, and may be quite small. Some require a magnifying lens to install.

To spec out a specialized fuse for an individual component, I need:

  1. Component I2t rating
  2. Operating voltage
  3. AC or DC
  4. Peak running current
  5. Peak cyclic current
  6. Peak available fault current

rule

Subject: Blowing a Fuse - Problem Solved! --part 1 of 3    Top

From: John Teel <mapuglista_tyahoo.com> Date: Aug 2001

Well I think I found the problem. As I said the fuse blows with nothing connected except the computer board (and the power panel board). I saw that it uses a 3-terminal 7805 voltage regulator to power all the digital electronics. First thing I did was see if the input of this device was shorted to ground and sure enough it is. According to Doc's schematics the 18vdc comes in to the input of this device and the only other thing connected here is a capacitor. With the ribbon cable unplugged the input is shorted to ground so that means it must be a faulty vreg or the input cap is shorted. Both of these would be very easy to replace. However, I'm going to wait to see what Meade says. If they tell me to ship the whole thing back and wait 4-6 weeks then I suspect I'll fix it myself. Ironically I design regulators for Texas Instruments and if Meade would have used a TI regulator instead of Motorola this wouldn't probably have happened. Actually my money is on the cap being the problem and I've read of others having problems with shorted caps.

Also I would think it would be safe to assume that nothing else was damaged by this short since it's at the input?

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Subject: Blowing a Fuse - Problem Solved! --part 2

From: Paul Rodman <paula_tilanga.com>

On my new LX200, the in-line fuse blew out of the box. It turned out that the power brick was reverse polarity...

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Subject: Blowing a Fuse - Problem Solved! --part 3 of 3   Top

From: Emery Hildebrand <emeryha_tearthlink.net>

Before turning my 1994 Classic LX200 8" (affectionately know as R2D2) on for the first time in 3 years, I decided to replace the battery first. Something must have gone wrong, because the fuse blew instantly when I turned it on. I blew 2 more fuses trying to see what could be the cause. Couldn't notice any problem at all. The scope booted-up fine as long as I had the power panel off. I tried reinstalling the face plate with both the declination and hand controller cords disconnected, and it still blew the fuse as soon as it was turned on. What a heart-sinking feeling! I noticed that the original fuse I took out was a 2-amp slo-blow. I thought that odd since the manual calls for a 1-amp. Then I scoured the message boards and found that most LX200s seem to have 1.5 or 2-amp fuses there.

The next day I picked up boxes of 1, 2, and 2.5-amp slo-blow fuses so I could continue tracking down the problem. Having had time to digest the situation was all it took. Since the mechanicals were smooth and free and I couldn't locate any obvious electrical fault, I looked for the most likely location for shorting out. The worst spot was the obviously tight clearance between the daughterboard that holds the fuse and the housing of the drive base. Before replacing the face plate this time I added a 6" strip of electrical tape to both the edge of the daughterboard and the upper, inner surface of the drive base opening. For this first test I used a 1-amp fuse.

Voila!!! it turned on without blowing! I cycled it several times and then ran it thru several polar alignment routines and then slewed all over creation. No problem. During this process I kept an eye on the built-in amp meter and it never lit more than 4 bars - and only 4 bars briefly. Wow! It felt great not only finding the fix, but not having to return lil' R2 for service. I'm gonna keep the 1 amp fuse installed since it's working. I have a gut feeling that the 1 amp will be adequate as long as I don't slew a camera into the base... but that has only happened once in all the years of using R2 so I doubt it will ever happen again. Anyway, I'll keep all those extra fuses on hand just in case.

Follow-up about a week later...

Last night was clear, dew-free and in the low 50s, so I took R2D2 out for a regular setup and checkout. It was the first real use its' had in 3 years. After changing the battery it retained my site info but lost date and time so I input those, did a tight collimation and crossed my fingers. It operated fine all night without blowing the 1-amp fuse. I guess that confirms the electrical tape fix worked. My neighbor has a 10" LX200 that he also hasn't used in 3 years. When he first fired up, his fuses kept blowing during the alignment procedure (but didn't blow immediately upon power-up as mine had). After similarly isolating his power panel with electrical tape his also appears fixed. Strange coincidence - or is this a known problem?

rule

Subject: LX200 12V Converter Circuit Top

From: John W. Downs

Summary on the Meade DC-DC converter, all Meade LX200 scopes will run on 12VDC - 18VDC. The processor board only sees 12VDC because of a on-board regulator. The motors run stronger at 18VDC, thus Meade made the change. The motors are rated to 24VDC so no problem there. Focusers such as the JMI zero image unit runs at one speed when the LX200 is powered by 18VDC, and two speeds at 12VDC. If you have anymore questions on the converter, I'll be happy to answer them OT.

rule

Subject: 120V-12V Power Supply Recommendations   Top

From: Colin Haig

David wrote:

>I have been thinking on using a Computer Power supply to power
>my 208xt and Kendrick Dew System. The Power supply says it's
>12V DC is rated for 10A and 5V DC is 25A. Any one see any
>type of problem doing this?

Your computer power supply idea will work probably work fine, but a couple of tips:

  1. Get an older style (non ATX) power supply, the kind used for AT-form factor boards. The ATX supplies need a couple of things to sequence them on properly, and have soft-off.
  2. It usually needs a load on the 5V side in order to work (the fan inside is usually enough for the 12V side). Usually the necessary load is a few hundred milliamps. So, you will need like a 10 Ohm resistor that's good for at least 3 Watts. You can also use the heat from this puppy as a dew heater ;-) or eyepiece warmer.
  3. You should probably put the two systems on as far separate wiring as possible. I.e. connect them back at separate 4-pin disk drive connectors, instead of bringing the wiring to a dual outlet at the scope. The Kendrick gizmo sometimes induces noise in the line with the big on/off currents.
  4. They really aren't well sealed for outside use.
  5. The 208XT should be okay, but some other devices actually expect +13.8V from lighter sockets. For example, the PC23C video cameras are fussy about input voltage because they have a built in 12V regulator that needs an extra volt or two to get past it. The 208XT power supply has some switching regulators, and a 5V jobby, so I don't think its gonna be a problem.

rule

Subject: LX200 Power Supply ( DC>AC vs DC>AC>DC )   Top

From: Gene Kahn

This has been covered from almost every direction but I had never seen any real number to compare using a 12VDC to 18VDCconverter compared to a 12VDC to 115VAC then 115VAC to 18VDC so I set up to do some comparisons of my own.

  • As a load I used 6 3ohm 25W resistors for a constant 1 amp
  • As a DC to DC converter I used a home built supply that runs on the standard LT1170 regulator
  • For the DC to AC, I used a 100 Watt inverter from Noteworthy/Toshiba
  • From the 12V source with the DC > DC the current was 1.66 Amps or 19.99 Watts in to 18 Watts out.
  • With the DC > AC > DC the source current was 2.08 Amps or 24.96 Watts in.

So if my setup/math is correct then the AC step wastes an extra 25% of your power.

rule

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