Protection for PLC output card relays

AB2005 · Dec 13, 2006

Dear All,

I want to connect MOVs (Mattel Oxide Varistor)with parallel of coil of each relay (220VAC) which are energizing through PLC out put Relay card for providing protection to the output card relays contacts against high voltage spikes occur when relays de-energize. But I haven’t experienced ever before.

Please help me for selecting the proper MOV.

TimWilborne · Dec 13, 2006

AB2005 said:
MOVs (Mattel Oxide Varistor)

Can't help you out but just wanted to say thanks, I always wondered what that stood for

elevmike · Dec 13, 2006

AB2005,

How big is the coil in question? You need to select your MOV based on the amount of energy expected to be produced by the collapsing coil, measured in Joules, (watt seconds). There are maybe a dozen specifications for MOVs. Your main concerns for arc suppression is the working voltage and the capacity for energy absorption/crossover.

brucechase · Dec 13, 2006

Not an answer to your question, but the correct spelling is Metal Oxide Varistor. See below

http://en.wikipedia.org/wiki/Varistor

Mattel is a toy manufacturer known mostly for Hot Wheels and Barbie dolls (and of course the Ken action figure - he wasn't a doll).

http://en.wikipedia.org/wiki/Mattel

Check with the starter manufacturer on recommendations.

allscott · Dec 13, 2006

I have never seen an MOV used like this but I guess it would work. In most of the machines I have seen a RC snubber is used to accomplish this.

I will try to dig up some info on sizing.

Edit:
Found this http://164.54.212.3/control_systems_manuals/D405_wiring.pdf

It's a manual for a DL405 but on page 25 is some good information on sizing a snubber

TConnolly · Dec 13, 2006

The RC snubber is a better way to go for an inductive coil. The RC snubber will supress transients created when the coil de-energizes. A MOV does not help supress a transient. It lets the transient thru when the transient energy is greater than the energy rating of the MOV - a MOV is rated in Joules. MOVs are frequently used across solid state output devices such as SCRs and Triacs to protect the semiconductor switch by letting high energy transients go around the semiconductor switch.

keithkyll · Dec 14, 2006

Alaric said:
The RC snubber is a better way to go for an inductive coil. The RC snubber will supress transients created when the coil de-energizes. A MOV does not help supress a transient. It lets the transient thru when the transient energy is greater than the energy rating of the MOV - a MOV is rated in Joules. MOVs are frequently used across solid state output devices such as SCRs and Triacs to protect the semiconductor switch by letting high energy transients go around the semiconductor switch.

Agreed. MOV's won't supress the arc across the relay points. A snubber will.

AB2005 · Dec 14, 2006

Dear Allscott,

Thanks for providing a useful link.

Protection for PLC output card relays

AB2005

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TimWilborne

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elevmike

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