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Yesterday afternoon I decided to fire up the layout and do some switching. I set both loops to 18 volts and addressed the engine on the outer mainline with the Cab1, with the intention of crossing over to the inside loop. The loco moved about two feet and stopped, headlight flashing! I looked down to see the red light lit on the Z4000 and quickly shut everything down. None of the circuit breakers popped nor did any of the inline fuses. However, there was the odor of fried electronics present.

I disconnected the Z4000 from the layout and it tested out okay. I then put the meter to the tracks and found that both loops were shorted even though they are independent form each other. What the hey, everything was working fine the previous day! To be certain I removed all of the cars and engines from the layout, but the short was still present.

Further testing seemed to point to a problem with the TVS box. I had purchased this unit from Scott's Odds N Ends way back in 2013. Here's what I found.

TVS 004

I'm guessing there was just one too many voltage spikes over the past seven years! For the time being I'm going to use the two inboard circuits which are previously unused and are testing okay, at least until I can investigate some better methods of TVS protection.



Images (1)
  • TVS 004
Original Post

Very interesting, I am not familiar with this item and apparently the company that bought scott's odds n ends does not have a web site.  Do I see one TVS totally blown apart and 3-4 fried? 

As you continue your investigation, I am sure many of us with discrete TVS units installed would like to learn more on what you find. The common wisdom I thought I had read was a TVS likes to fail open so you don't know they are not working.  these did the opposite big time it appears. if not too much of a pain, how were these connected to your layout?

Hokie 71 - There are four pairs (one hot connection & one common connection per pair) on this unit so you can hook up four separate loops. It's installed "inline" on the feeders from transformer to layout buses. There is also a single spade for a ground wire. I was using the two outside pairs for my two mainlines.

I never had the unit apart before the incident so I don't know how the diodes looked before totally frying. Apparently when they cooked it created a short in each of the two spade pairs. The other two unused pairs still test okay and not electrically connected.  

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