Potentiometer ? I think!

I have an Athearn power supply that hit the floor yesterday.
It seems the Potentiometer (I think that is what you call it) was damaged. It no
longer works to control the power. It is just always on.
I put a meter on it and it showed as bad. I took it apart and could see no damage. It seemed like a simple device. I made sure the small wire brushes were touching the board and put it back together, but that did not help either.
I also tried putting a potentiometer on that I had laying around and of course it smoked, like I thought it would since this is D.C..
What makes the one in the power supply so special that it does not smoke and where can you get replacements?
I realized that these are cheap power supplies, but I am curious.
Thanks Chris
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Chris skriver:

So am I.
Would it be possible for you to take a picture of the broken "potentiometer" ?
Klaus
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Klaus D. Mikkelsen wrote:

I can do that. I will post a picture later tonight.
Chris
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Klaus D. Mikkelsen wrote:

Here is a pic
http://s261.photobucket.com/albums/ii77/sunshine5666 /
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wrote:

The round black thing on the left side of the circuit board is a trim-pot that has been adapted for use as the throttle control for the powerpack. That, in fact, is the potentiometer - or one of them.
At the bottom of that round black area you can see three black squares with little legs that go through the circuit board. The two outer legs are a fixed resistance and will measure some value depending on what is printed on the potentiometer body. Read there and see if there are some numbers like 103, or 10k, or 100k or something like those. When you do, I'll tell you what the outer leg measurements should be.
The center leg is the wiper of the potentiometer and that measurement from the center to either outer leg will vary depending on where the knob is turned. From the limited view of what you have photographed, it looks as if you have a simple unregulated rectified circuit with one leg of the bridge rectifier variable. This is not a good powerpack. You'd be spending good money to repair something you can buy in a thrift shop for a dollar. Toss it and buy a better one. Seriously. -- Ray
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Ray Haddad wrote:

It says 20k.

I agree with spending money to fix. That will not happen. But I may have something laying around. I do not think I will spend to much more time on this, but I have learned a few things, so I am happy.
Now I wonder if I can get some of this "Stimulus" money for my railroad. Amtrak is getting some! :)
Chris
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Chris wrote:

You didn't point out which component you're speaking of. I'm going to assume it's one of the two potentiometers at the top center of the photo, one of which appears to be attached by wires back to the power pack. It appears to be a carbon film pot. It *may* be bad, or not. The way to check is with an ohmmeter across the two outer terminals. The center terminal is for the wiper. If it reads infinity or some other very high value across the two outer terminals then it is bad. Normally for this application you might have something between 1kohms and 10kohms. However, I also see, on the circuit board, a black, 3 terminal device almost exactly in the center of the photo that is likely either a transistor or SCR. Assuming that power to the track is actually being controlled by that device, if it is shorted you would have the "always on at full speed" problem that you describe. If you have a multimeter with a diode check function, and you know how to use it, you can determine if that is shorted and the cause of your problem.
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Chris skriver:

And next a sharp picture of the circuit board alone. Both sides.
If the original potmeter was blown, and the new mounted blew too, then perhaps the error is somewhere else.
Shorted transistor perhaps?
Do you hava an ohm meter ?
Klaus
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Klaus D. Mikkelsen wrote:

Yes I have an ohm meter and have been checking things out. The "potmeter" (new term for me, learn something everyday) is bad. I have also been checking resistors and they so far check out. Have more checking to do.
Chris
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On 2/19/2009 10:10 AM Chris spake thus:

It smoked because too much power went through it, not because of AC or DC. See below.

Not sure, but what you probably have there is a power supply with a rheostat, not a potentiometer.
Rheostats and pots are basically the same things--variable resistors. However, potentiometers generally only handle small amounts of power, typically about half a watt. A rheostat can handle considerably more, like all the juice going out of the power pack.
Does the control have wire windings that are contacted by a sliding brush? If so, it's a rheostat. You might be able to find a replacement, either at a "real" electronics store (forget Radio Shack!), or, more likely, online: check electronic surplus retailers, etc.
You can probably determine the resistance of the rheostat by measuring across the full winding: this is the resistance value you want for a replacement.
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Chris wrote:

At your friendly local hobby shop. Ask for a "power pack." And toss the damaged unit.
Really.
Those train-set power-packs are not worth the hassle of repair - unless you want to branch out into the electronic side of the hobby, in which case I suggest you build an elementary transistorised throttle. You can use the damaged unit as a source of AC, if the connections to the transformer are still good.
Have fun!
Cheers,
wolf k.

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I am a retired physics professor and always used Sargent Scientific Catalogue for electronic meters pots etc. You get the good stuff there. there is a web site but I don't have the link. Google Sargent Scientific Supplies. JOHn
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NICHE541 wrote:

That is a cool site. I see I will be spending some money there. Started making a list already.
Thanks for the site.
Chris
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Wolf K wrote:

As in my last line I know they are cheap but I am curious. I like to play around with things. It is part of learning.
Chris
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When you replaced the potentiometer (aka pot) did you replace it with one with the same resistance value? That makes a big difference in how the device will work and if it'll "smoke" or not.
Most electronic components (including pots and rheostats) have some sort of identification imprinted on them. Pots might have something like "A2K" or "502B" or some other number/letter combination. The number is its total resistance and one of the letters indicates logarithmic or linear taper. You should replace the burned out pot with one that has the same properties.
I also never heard of "potmeters". Ohm-meter is used to measure them as they are just variable resistors.
Peteski
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snipped-for-privacy@my-deja.com wrote:

I learned that the hard way "I smoked it"! Had some more out in the shop and put it in today and it is now working again.
Chris
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