I think I already know the answer to this question, but figured it wouldn’t hurt to ask. We have an Athearn that fried it’s 3rd decoder since Christmas last night. This time, it lasted about a month before it bit the dust. I re-worked all of the electrical connections when I reinstalled it (so I know they were good). I think the motor in the unit is simply pulling too much current for the decoder and that’s what’s causing them to go bad. I know they’re gone the loco starts to short the layout. I pull the doecoder out of the locomotive and the short stops. We’re using Digitrax DH126 series decoders in the model.
The problem could be intermittant shorts between the motor and the frame. In the DC set up the bottom brush of the motor got its power from the frame. For a DCC set up the bottom brush needs to have a wire attached and TOTALLY insolated from the frame. It could be the insolation is iffy and if the motor shifts slightly it is shorting out the decoder.
I had a problem with the Athearn BB motors when I was converting a few to DCC. My problem wasn’t excessive current, the older Athearn motors are notorious for voltage spikes from the arcing of the brushes to the commutator.
I use two 18 volt 5 watt Zener diodes (1N5355) back to back across the motor power wires. I have never had a decoder go bad in any of my Athearn BBs with the diodes installed.
Under normal running conditions the BB motors can draw close to 1 amp. At nominal running speeds with 20 cars a BB motor will draw a little over 700ma. Locked rotor the BB motor can draw over 1¼ amps.
Best thing to do with these is to swp the motor clips - but be careful removing them since they hold the whole motor together as well as contain the brushes and brush springs. The one with the extra prong sticking out on the bottom is used to make contact with the loco chassis, if you put that one on top, or at the least, clip off the extra prong, you make it much less likely to get contact with the frame, which will instantly fry a decoder.
In addition, you need to insulate the bottom of the motor well. Don’t use electrical tape, get some Kapton tape. It’s much more resistent to abrasion so it won’t wear through. Kapton tape is highly desirable for decoder installs. No, it’s not cheap, but it’s not THAT expensive. Not compared to 3 fried decoders.
I also use the same method as Mel, but with ABS plastic sheet and I don’t cut off the tab. I put the brass plate on a piece of flat steel bar and with a nail set, I hit the tab back into place where it was punched out, then file the area flush. I also insulate the trucks from the frame by cutting off the steel plate where the bolster rests and replace the steel plate section with a fibre washer so it sits at the same height…but I run DC…I do it because I don’t want a hot frame and it produces better truck pick-up overall, helpful for constant lighting circuits with LED’s.
I should have added, that one of the decoders got nuked because of this. A small peice of decoder wire that I had trimmed down became wedged in and bound the gears. I thought I had the problem sovled (ie, I thought this WAS the problem) after I removed it.
For the diagram above, I assume the circut is across the orange and grey decoder wires to the motor? Is that correct? How do you wire them on your installs? soldered together in a peice of shrink wrap??
And to everyone else, I think the motor is fairly well isolated. But since it’s easy enough to pull the motor out, I’ll do that and double check it. And yes, I do need to get some kapton tape.
I’ve got 1 spare decoder left, I’m not sure I’m brave enough to drop it in yet.
As someone who told a person that they’d been throwing away good lightbulbs for years, I recommend NOT throwing away “burned out” or “shorted” decoders unless you see the nasty looking spot on the decoder.
Myself, I’d store it away with a piece of orange tape on it–my color for POSSIBLY defective. Or maybe in a marked envelope or baggy.
I was surprised a couple of years ago to buy a “DCC Ready” Athearn engine and find out the engine hadn’t been insulated from the frame. They added the lightboard with a DCC receptacle etc. but hadn’t insulated it. Once I took it apart and took care of that, it worked fine.
My normal thing is to use heat shrink with the spike protection diodes. I normally parallel them positive to negative with a red wire on one end and a black on the other end, polarity doesn’t make any difference. Small neat package and protects against voltage spikes.
