protecting 24vac motor controller - zener, MOV or both?

[oldmanbiker]

I had a recent power failure that blew out routers, hard wired smoke detectors, etc.  The most painful was a pic-based motor controller that runs off of 24vac.

Now that I've replaced everything, I would like to protect the motor controller.  From what I've read, I could use opposing Zeners on the controller board where the ac input comes in.  I also read that Zeners are slow, and that MOVs are faster.  But MOVs are imprecise on voltages, they have a range where they begin kick in.

Does this suggest the best protection is opposing Zeners and MOV, with a series fuse?

In any case, what is the normal way to determine the correct voltage for the protection components?  Surely it must be significantly higher than 24v to prevent false trips.

Thanks

[ante]

I believe it’s a good idea to put a MOV on the primary side of the transformer (which I presume you’ve got for the 24VAC). For 230VAC I usually put a 275V MOV as a protection, it has worked fine so far.  ;)

[oldmanbiker]

I can't put it on the AC side of the transformer, because it is in the attic somewhere.  Exactly where, I'm not sure.  Anyway, I read somewhere that protection components are best placed on the board, right where the power enters.  The idea is that inductive surges from lightning, etc. are also handled.  In my case, there is maybe 50 feet of cable between the transformer and the circuit board.

[ante]

I suggest a fuse (in series) and a 32V MOV (in parallel) on the power input of the board.

[oldmanbiker]

Thanks for the info.  I've been searching for a 33v mov, but there doesn't seem to be any.  Here's one that is close, a 30vac mov.  Open circuit voltage on the transformer is 26 vac.

V47ZA3
Max applied voltage:    30 V Acrms,  38 V DC
Max energy, 2ms:        4.5 J
Clamping voltage:      93 V
Varistor v0.1mA:          47 (42-45) V

[ante]

Should be fine for the job!