A big screen TV that someone threw away, and I am trying to fix it.

Immediately after the 110 v plug (power in) there is a Varistor.

I haven't pulled the board yet, but I tested for continuity across the two leads of the Varistor and I get no beep.

Doesn't that mean its blown / failed / open?

Thanks
Gary

 

A continuity test shouldn't cause a beep, because a varistor has much greater resistance than, say, a fuse. Try it on the Ohms range of the meter. The varistor is presumably an inrush current limiter whose resistance drops as it heats up.

 

Varistors are transient voltage suppressors. They're only in their "low" resistance state when they're conducting. A beeper or ohm meter can't put a line voltage MOV into a conducting state...

 

Can I connect a voltage through it, via my Variac to test it?

Thanks

 

Can I connect a voltage through it, via my Variac to test it?

Not ideal. Better to measure the voltage across the varistor when you try to turn the TV on. If it's the full line voltage, then the varistor is bad.

 

As pointed out in another recent thread, Varistor are very rugged and difficult to "break" because the physical construction is little more than a block of ceramic with two electrodes. When varistors fail, it is usually spectacular. If you can read the part number on your varistor, it is almost certainly good.

A beep from a DVM, particularly a single beep probably means that it had mistaken the capacitance of a component for resistance.

 

@Gdrumm

Inasmuch as non-CRT TVs have no use for degaussing circuits, the likely application for said varistor is line-surge protection -- Such a varistor (specifically an 'MOV') will appear 'open' under normal conditions, whereas a low resistance reading is indicative of failure (and should be accompanied by an open OCP fuse/breaker)

Better to measure the voltage across the varistor when you try to turn the TV on. If it's the full line voltage, then the varistor is bad.

I'm confused -- It seems to me an High-Z (i.e.'undeployed') MOV would drop the full line voltage across its terminals --- Is there an implementation I'm unaware of?

Genuinely intrigued
HP

 

You guys need to agree on what you are talking about. Is the device wired in series with the AC line, or in parallel with the AC line? Is it wired as an in-rush limiter or as a spike suppressor?

 

Is it wired as an in-rush limiter

Now I get it! --- Thanks @MikeML!!!

 

Now I get it! --- Thanks @MikeML!!!

My questions still are unanswered. I dont think the OP knows or knows the difference...

 

My questions still are unanswered. I dont think the OP knows or knows the difference...

Without a schematic I suppose there's nothing for it but 'tracing' the circuit...

Best regards
HP