I have a automatic batching control that has a varistor as protection
Is connected between line and ground appears to be in short circuit but I don't know for sure if I am testing it correctly. Does somebody know how I can test it?, without it the control works fine but I like to replace it. Please it's urgent. If somebody can help me I appreciate it.
Thanks...

 

Resistance of a varistor decreases at increasing voltage. At a certain point, it behaves as a short circuit. In this way, it draws more and more current letting the incoming circuit breaker opens.
If the test voltage of the measuring device is higher than the rating of the varistor, you will measure a very low resistance.
If the voltage rating is not written on the varistor, use a 1.5V battery and a known resistor in series with the varistor. Then measure the current. Thus you can calculate varistor's resistance.

 

A varistor will generally fail shorted. This is like most solid state devices. With most meters the failed varister will test shorted and the solution is replacement. Most varistors of various brands and of various sizes can take about 10,000 hits of low level spikes and one really big hit then they short. Your circuit is ready for a new $0.50 to $2.00 part and you will be back in business. By the way a varister is somewhat like 2 zener diodes nose to nose and the reversed biased one will conduct when the inverse voltage exceeds the rating, generally over 150% of the system norminal RMS voltage.

 

Since varistors are high voltage devices that breakdown when their trip point is reached I would assume any varistor with an Ohm meter reading of less than say 10 K was bad. Other signs that that they are bad are visible discoloration, spots, and even holes as they tend to fragment when they see lightning.


Fred Townsend

 

It very likely did it's job and gave it's life for your circuitry. Testing is simple, at the applied voltage (peak) or below it should be a high impedance. (OPEN). if it's shorted, it absorbed more energy than it's rating. At or below it's energy rating it can recover, but either repeated surges or very large ones tend to cook them. This is not bad, by taking out the breaker or fuse, it decouples your electronics, possibly saving them. It is a bad idea to run without one, because the surge that cooked it will very likely fry your equipment as well and you know that at least one such surge occured. I would replace it immediately and if repeated, fix or condition your power source.


Regards

cww

 

I think the varistor is conected in paralel with the primary of the transformer. The short is the winding

 

Varistor normally used to protect circuits from high voltage surge. If voltage surge, higher than varistor voltage, appears accros the circuit to be protected, varistor acts as low impedance to suppress the surge. In normal situation, varistor acts as capacitor. Varistor will damage if its voltage rating is exceeded. Short circuited varistor could well be as a result of excessive voltage surge. This also sugest that high surge do occurs therefore protecting the circuit is crucial. Do not wait until the surges kill the circuits.

Regs.

insk

 

Disconnect it from the rest of the circuitry. Testing varistors is difficult, because they show as an open circuit when they're good, and as an open circuit when they've been burnt out. If yours shows as a short circuit, replace it.

Paul Baker

 

If in doubt, replace it. Most common varistors cost less then a dollar.

Wayne Shimanis

 

Responding to W. Hinton's (Sun, Dec 29, 10:49am) reply:

While your perception of its function is that of back-to-back zeners, the varistor is actually a voltage controlled resistor, as suggested by Haken Ozevin. In fact it is a two-terminal semiconductor device having a voltage-dependent nonlinear resistance.

For the complete description visit, yep you guessed it, "IEEE Std 100-1992" covering the definitions of Electrical and Electronic terms!

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]]