@Adrian! posted:2. You want to avoid kickback voltage... high voltage transients developed in the layout from normal train operation...Motors are inductive and inductors produce voltage depending on the change in their current. VL = L di/dt. Big change in current makes big voltage. If your train bounces along the rail making and breaking contact or it has momentary disconnects going over switches and stuff, the Ldi/dt term is big since di/dt is discontinuous going from current to no current and back to current again. Those discontinuities generate voltage transients well over 100s of volts that have duration in the 1-100ns type of timeframes. These transients are not defects... these are part of normal train operations... but you need to stop them from propagating and damaging electronics which are mostly semiconductors like the ACT244 driver in the TIUs, or GRJ's buffer circuit and like PS boards in the locomotives themselves, (all have breakdown voltages in the neighborhood of 10-20V). This is what the TVSs are for. The big difference from current protection is these short 1ns voltage transients are a broadband event, (the Fourier transform of a 1 ns pulse is a sinc(w) going all the way out to a GHz.
Since these transient voltage pulses are GHz range they have wavelengths on the order of a few inches meaning they can bounce around a layout like a transmission line and the TVS you put way over here by the station won't suppress the transient over there by the train yard since the transient voltage pulse is physically only a few inches big wherever it currently is. (wavelength = c/freq). For that reason you have to put the TVS device physically near what you're protecting (well within a wavelength).
Follow-up questions pertaining to a layout with Legacy, LionChief and Conventional operation only (no DCS):
1) When discussing where to install TVS diodes inside the locomotive, usually the suggestion is to install them where the pick-up and collector wires attach to the circuit board. But, since the motor coil(s) become voltage spike sources in the event of a derailment, would where the motor(s) connect to the motor driver board be better locations to install TVS diodes inside the locomotive or would TVS installation in both places (where the motors connect and where the pick-ups/collectors connect to the circuit boards) be best?
2) If a TVS diode is also installed across each transformer secondary powering the layout, would there be a good case for installing any additional TVS diodes around the layout (assuming all Locos had individual TVS protection installed)?
