Quick question

Have you set the engine on the track with the wires to the decoder disconnected?  Just want to eliminate the possibility of a short in one of the trucks.

Scott Chatfield

Quick answer

Yes. It has been on the track with no wires connected to the decoder and there was no short.

In short....

It sounds like a short.... In the decoder that is.  Hard to say what could cause that all of a sudden, unless there is a zeiner diode on the board.  I've seen those die because of a sudden voltage spike.  I don't have one of those decoders handy to look at.

Scott Chatfield

Thanks

Thanks for the answer. I'll take a closer look to see if there is any sign of damage.

I've had that exact failure

I've had that exact failure mode happen to me on a Proto2000 2-8-4 with two different brand sound decoders.  works fine..shut it down..next time you power it up it's shorted.  It would be great if we could failure analyze the shorted decoders but that's not going to happen anytime soon..if ever.  I'm beginning to wonder if we need to do "soft" turn-ons and turn-offs instead of just suddenly shutting the power down (or on).  

BTW..I would really be surprised if decoder manufacturer's put zener's into their circuits...too expensive.  They are probably depending on the parasitic capacitances of their H-bridge power FETs...but who knows..no circuit diagrams come with these decoders...someone would have to reverse engineer one...over my pay grade  I'm not even sure these deooders utilize H-bridge circuitry.  

Same problem

I had the same issue with a BLI 2-8-2 (N scale).  The brand new loco ran fine for 3 days and then it suddenly shorted out after running for a while.  Sent the loco back to BLI for repair.

H Bridge Motor Drivers

Most decoder designs are actually fairly simple - for an Electrical Engineer. Designs for simple mobile and accessory (see Dr Bunza's blog) decoders can be found online. For commercial decoders it is the microcontroller  firmware that is the big mystery. I have reviewed and debugged decoders from many manufactures, and every one used an H Bridge to drive the motor(s). Some Large Scale decoders are designed with four discrete transistors, usually FETs, but most are designed with Half or Full bridge ICs. For efficiency many speaker are also driven by full bridge ICs. Most of the decoders I have dissected were designed to detect short circuits, but that does not mean that the designs, including the firmware, are foolproof.