# Interposing Relay Issue



## JRaef (Mar 23, 2009)

Why are you using 60VDC for the relay coil?


But to respond to your question, diodes, like any solid state switching device, are subject to damage by rapid changes in current or voltage. Your coil is not likely ever to pull enough current to damage a diode, so it's most likely voltage spikes doing it. So to that end, yes, an MOV would help. The tricky part is in not knowing exactly HOW bad the spikes are. Most likely they are over 1000V, but how much higher is a guess. If you use MOVs with too low of a value, they pop immediately and are useless; too high of a value and the diode shorts before they act. Either way though, you must understand that MOVs are pretty much one-shot devices; they need to be periodically checked for integrity. An RC Snubber across the coil may be a better approach.


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## I&ETech (Mar 30, 2016)

Thanks for the reply JRaef.

We are not using 60VDC for the coil. We send 120VAC to the relay and there is an integrated circuit in the relay socket. It's part of the OEM design.


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## cmdr_suds (Jul 29, 2016)

Sounds like they are using the AB 700-HL series 120vac terminal block relay


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## I&ETech (Mar 30, 2016)

The relay modules are Phoenix Contact brand.


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## just the cowboy (Sep 4, 2013)

I&ETech said:


> We send a 120VAC signal to the socket and the rectifier circuit sends 60VDC to the relay's coil.
> 
> .



This is why he asked about 60VDC to the coil.


One every year out of 1000's is great especially after years of service, I would not add anything.


Cowboy


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## joebanana (Dec 21, 2010)

So, you have what I'm assuming is a half wave rectifier in the socket, what about a flyback diode across the coil?


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## I&ETech (Mar 30, 2016)

It is full wave rectification and possibly a varistor. The varistor depends on the type. I'm still looking into it.


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## cmdr_suds (Jul 29, 2016)

A number of those "terminal block" type relays are made like that. Wish I knew why. 
Maybe they just don't make those skinny relays in 120Vac flavor?
I'm sure the diodes, varisistor, ect is just the interface circuitry to go from 120vac to 60vdc


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