@bmoran4 posted:..It is argued that placing the TVS closer to the sensitive electronics is beneficial, but you can do it right on the transformer - that is what I do.
Brain, as one of your followers and one who greatly respects your opinion, I want to make it clear I'm not looking to start any kind of argument, rather a discussion. I have heard from others, who like you, have placed TVS diodes across their transformer outputs and elsewhere around the layout, and this has worked for them. Until I read the explanation linked below from @Adrian! , I too was unconvinced that placing TVS diodes inside the Locomotives may be best for protecting them, and that doing so would be a PITA. After reading this, my opinion changed in favor of placing the diodes inside my locos.
Maybe you've already seen Adrian's explanation, but in case you haven't, it's interesting to ponder.
@Adrian! posted:It's not so simple actually. You want the TVS right up against whatever it's protecting:
There's a little bit of RF design principles here (sorry it's complicated)... let me try to explain it easily:
What happens when a train derails is the motor and bouncing contacts produce transient events (fast changing voltages and currents over a short time duration... like 1-10ns range). These transient events are short in time which means they are very wide-band in frequency (fourier transform describes this).
They can be up to the GHz range, which means they behave like RF waves on the wiring and track (because they have a comparable wavelength in the 10s of inches regime). Once you are into this wave propagation behavior it means that the whole wire length is not the same voltage at the same time. So just because you null out the voltage at one point along the wire with the TVS between the train and the TIU doesn't mean the transient voltage at the TIU or train is zero because of the waves bouncing up and down the wire.
If you want the basics it's here: Transmission lines
So the only way to be 100% sure the transient voltage at the protected device is safely held down by the TVS is to physically place it very close to the thing your protecting (like within 1/10th of a wavelength). That's why it has to go right on the board inside the TIU.
Hope that wasn't too poorly explained!
Here's a link to the original topic and reply
