bi-directional zener diode and fuse questions

[Opus17ech]

Hello all,

 I recall a previous discussion on this forum about using bi-directional zener diodes to keep voltage spikes from frying the electronics in your locomotives.  I think the recommended rating for these diodes was 36 volts.

 If I were going to do this when wiring my layout, where would I locate these in the circuit?  Would there need to be one diode at every lock-on?  Or would just one of the diodes across the transformer posts be sufficient?

Also, what would be some recommedations for using fuses?

Any advice would be greatly appreciated!

 Thanks!

Opus17ech

[lionelsoni]

These are called "transient voltage suppressors", "TVSs", and "Transzorbs" (The Vishay trade name).  You would need just one across each transformer output circuit; but you can locate it at the transformer or at the lockon or wherever else is convenient along the circuit.

You get the voltage rating by multiplying the maximum AC voltage by the square-root of two (1.414) and making whatever reasonable allowances you need for transformer and TVS-rating tolerances.  For example, if you have a type Z transformer putting out 24 volts maximum, you need to allow for the peak voltage of 34 volts.  The Vishay 1.5KE43CA has a standoff voltage of 34.8 volts and so should work for this transformer.

Here is a link to the data sheet:

http://www.vishay.com/docs/88301/15ke.pdf

Be sure that you get the bidirectional version, with the C in its part number.

Some people like fuses.  I don't, because they need to be replaced when they blow.  I prefer automatically-resetting thermal circuit breakers like those designed into traditional transformers.  They model the thermal behavior of the transformer and layout wiring, tripping only when those get too hot.  Multiple-output Lionel transformers should have circuit breakers added, one for each output circuit, either internally or externally.  Otherwise, there is no overcurrent protection for short circuits between output terminals.

[chuck]

If I were going to do this when wiring my layout, where would I locate these in the circuit?  Would there need to be one diode at every lock-on?  Or would just one of the diodes across the transformer posts be sufficient?

Since you're real goal is to protect the electronics in the locomotive wouldn't the logical place be across the power feed's to the circuit boards in the locomotive?  I don't think anyone is doing this, but it would seem to be the most logical place.

They are usually installed across the feeds to the track from the power supply.