Varistor gets hot quick,why?

I bought a beefy 30A 250V relay,in order to protect components,by the way the wall socket in my area is 220v.i used two 10D271K varistors,one conneted to relay's core.another connected to relay's contact.i burned the one in connection with core of relay.i didn't realised the problem at first until i replaced another 10K271k.this time i touched the parts after i plugged it in.i could locate the problem because i finger told me the varasitor connected to core gets very hot.then i realised i should replace 12D271k with a bigger value.but i don't know which one is OK.maybe 10D471K.the databook shows 10D471K withstand 470V spike.has anybody experienced the same problem?

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rickharris2 years ago

Why not use a 5 volt relay? then you don't need all the rest of the circuit

gada888 (author)  rickharris2 years ago

The 30A relay i'm using is also rated 5v. I choose this as it is triggered by a SSR-Opto. i thought it could be safer.i tried to make another circuit with a 5v 16A relay,The principle is like what you suggested,This time it works.but i'v been told this version should by plugged in a ground fault socket.

Ok working with mains voltages is always risky and you should safeguard your self and others any way you can.

gada888 (author) 2 years ago

The diagram is here.


Why. Are you taking full current THROUGH the varistor ???? That's not what they're for.

Also, given the usual ratings of SSRs, they are protected to full mains voltage+transients.

gada888 (author)  steveastrouk2 years ago

Yes,you are right.I think i misplaced some component.

rickharris2 years ago

A circuit diagram or even a picture of what you have will help some what.

On a general issue If it's too hot then it is passing too much current.

I am not at all clear why you have put a voltage dependent resistor across the relay what are you trying to do?

I suspect his varistor is turning on as the AC cycle peaks.

I still don't see what the Op is trying to do?

Me neither.

gada888 (author)  steveastrouk2 years ago

I replaced the 10D271K far the relay works as i heating parts anymore,I need time to give it a good test.bcz i 'm fiddle with the AC 220v.

I fiddle with a voilin, never 220VAC

Varistors are often used to quench voltage spikes.

I do not know the specs for the varistors the OP used.

Sounds like they are the wrong type [voltage wise].

Why did he use a relay to protect components?!

How big is your Varistor the size of your head because that is what you would need for 30 amps.

gada888 (author)  Josehf Murchison2 years ago

I don't understand.but i noticed the the same value Varistor come in diffirent size.i don't know why.

Current is why their are different sizes, it sounds like your Varistor can't handle the current. If you have a Varistor that takes 10 amps and you put 30 through it you will fry it.

gada888 (author)  Josehf Murchison2 years ago

Thank you very much,my relay board still have problems.when i connect it to lamp.the lamp flicks.i could hear the sizzling of i presume the problem lies in what you suggested.i'll take another varistor.

different size --- different power handling capability

He only says a relay rated at 30A. We don't know the actual current. MOVs are to handle spikes - we don't know the spikes. I am still puzzled why he thinks a relay will protect his equipment. . Is he using MOVs to 'cancel the spikes caused by relay chatter, or quench line spikes?!