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UPDATE on Lights causing LM-3S to overheat?

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CPRail:
Hi Greg,

Ask and you shall receive!

The resistors come up just behind a bunch of diodes on the board. I've marked the resistor connection and where the headlights connect (this is the "front" of the board - gyralites are connected to the "rear" part of the board with a similar setup). I've also circled the resistors.

Resistors I attached to the LEDs are 1KΩ - I used the same resistors when wiring up my Rapido B36-7 and it works fine.

I'll have to measure what is coming across those resistors.

For the common, where is the best place to measure?

gregeusa:
usually the common wire is blue... let me look at the manual:

ok, blue wire in the diagram:
https://ringengineering.com/RailPro/Documents/LM-3Instructions.pdf

so 1k should be fine, I guess I could not see that some of the lines on the resistors were brown, not black... it would be very good to test the resistors with an ohmmeter to be sure, they sure looked black...

have you tried the decoder with the leds connected but not the motor?

Greg

CPRail:
To clarify, I did not add the resistors on the board, that is how it came. I added the 1KΩ resistors in line with the LEDs as per the below photo. Maybe I have too many resistors in play?

I note that the board I have in my loco is NOT what the Athearn P/N shows as being...

Maybe I have dud LEDs. Now that I soldered the 1KΩ resistors to them, is there a way to test the LEDs without them being hooked to the loco? Can I connect them to an AA battery, like one can do with an incandescent bulb?

gregeusa:
so more resistance should lower current, so that should not cause overheating

having shorted LEDs could, or if they were wired in backwards.

disconnect the LED and resistor and test on your supply voltage, you have not indicated your max track voltage, but again guessing it is 12 volts or so.

Greg

G8B4Life:

--- Quote from: CPRail on April 16, 2025, 11:38:00 AM ---Here are the results (see photos). The grey items on the bottom of the board sure look like resistors to me, but I'm not an expert - I only play one on TV.  One location is marked R1.

All running was done on test rollers as I don't have a continuous loop of track. Throttle was set at 75% for all tests. Load was set to about 25-30% on the HC. There was a 5 minute cooling period between the first run (in reverse) and the subsequent run (in forward). I removed the shell immediately after each running test to check temps.

Test 1 was 15 minutes in reverse, headlights/gyralites were off.

Test 2 was 20 minutes in forward, headlights/gyralites were on.

After Test 1 the grey resistors where cool to the touch, the LM was slightly warm.

After Test 2, the front grey resistor was rocket hot (headlight), the rear was uncomfortably warm (gyralites), the LM was noticeably warm - you could feel the heat from the outside of the shell.

While you can see the LM temperature increasing, I suspect if I had an actual load and was able to let it loop, it would increase faster.

Next steps?

--- End quote ---

You didn't run the test without using the motor, ie standstill. however I don't think you'll need to.

Yes those resistors are resistors, The R1 marking confirms that but we did need to confirm it; I've not seen that body colour on that size resistor before. The value is 150 ohms though (brown-green-brown-gold).

Anyway, I'm confident I've been able to ID the board now (consequently lifting the LM off the board may have given this away too). It appears to be an older Digitrax product that Athearn put in their models and I found a home drawn circuit of the board here: http://www.pacificcascaderailway.com/converting-an-athearn-pc-board/. It's certainly looks like it's designed for bulbs/globes/lamps, and in fact I'm surprised you get the LED's to work at all (circuit simulation says it shouldn't!)

Anyway from what I can work out is your passing about 90 mA on each of the front and rear outputs which is about 1.2 watts of heat generating power each output at the same time, so no wonder the poor LM was toasting. From what I can see you can either toss the board and start again with a new motherboard designed for LEDs or you'll have to get rid of more components on the motherboard; namely the 4 smd diodes at each end, and maybe more.

- Tim

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