During the first discussions on the forum about using TVSs, way back when, the 1.5KE33Ca size was recommended. I posted a question about using two PH180s in parallel using a TPC400, and would this need a different size TVS? The response I received (can’t remember by whom) stated the 1.5KE33Ca was adequate since the maximum voltage is still only 18V, and the lower clamping voltage was better.
During this discussion someone questioned using the 1.5KE33Ca with transformers that could go higher than 18V, like the old ZW and Z4000. The consensus was that using the 1.5KE36Ca for these higher voltage transformers was better and since they still provided protection at 18V, the 1.5KE36Ca became the standard for the forum (this is my recollection anyway). I had already bought 100 of the 1.5KE33Ca size and my plan is to use them since my track is powered only by PH180s through TPCs or PowerMasters.
Because of all the TVS discussions going on now about failure rates and modes, I thought I’d re-evaluate the decision to use 1.5KE33Ca before I get too far along in wiring my new layout. So the question is would the 1.5KE36Ca size provide any better protection than the 1.5KE33Ca? In other words, would the 1.5KE36Ca be less likely to fail on my 18VAC system? I'd rather not buy new TVSs, but I will if it makes sense.
I think the answer should be formula driven, so hopefully there will be no disagreements. Thanks.
ADDED: Found this in some old notes; guess this is where I got the 33V idea:
"If you are using a brick as your source, I would use a 30V TVS rather than the 36V version. The 36V version is more appropriate for PW transformers that have a voltage boost for the whistle/horn function. There is no boost in voltage for a brick.
18V x sqrt 2 x 115% (for high line voltage) = 29.3V A TVS does not cut in abruptly at its rated voltage, rather it just starts to ramp up there. If 30V makes you nervous, try a 33V TVS."
