@Adrian! posted:Uh, if you go back to the original thread when we figured all this out together we had two fixes.
One was to upsize the TVS from the tweezer sized ones MTH originally had in there to a larger part (You want a larger one that can take a hit.... but not one so large that the added junction capacitance shunts out the signal swing... that's how we arrived at the SA5.0A part).
Two was to add a "beefy clamp" out made out of rectifier diodes in parallel. The TVS clamp fast, but since they're a reverse junction device with a tiny junction area they don't do well with sustained currents (like more than a few ns long). Regular rectifier diodes (like the 1n4148 we used in that post) are fine with sustained current (they're made for rectifying in power supplies) but being a forward junction device have a super duper long recombining time of like 8-10ns and don't turn on super fast.
So you put the rectifiers in parallel and have them at a slightly lower effective threshold voltage than the TVS. That way if a long event happens you still get the fast clamping of TVS, but 10ish ns in, the current is handed over from the TVS device to the "beefy clamp" so the TVS doesn't fail from sustained current.
Once you do this, the TVS parts fail very rarely. I think we blew 2 total in 4 years on 30 TIU channels total. Our club is basically the railroad equivalent of a demolition derby so the setup is definitely stress tested.
I could read up, but I thought I'd just ask. I highly respect your work here.
What are you using for power supply to the rails?
I only run a fraction of the time. I have both Lionel PH180s and a MTH Z4000 connected to my O scale, and Bridgewerks DC connected to my G scale, all with 10 amp auto fuses connected inline. I run large consists all the time.
After maybe 15 + years, I've never had a TIU failure. So I'm wondering what's the difference? Is it mainly the amount of trains running? Something else?
I'm always tempted to make these mods and then I don't.
I appreciate the great work and I expect to see this show up in future MTH hardware releases.
Please don't take this the wrong way Adrian. Just thinking out loud.
BTW, I have blown a lot of ten amp fuses!
