Question about Varistor

[hkqq2010]

Dear guys,

I has a Bestec ATX300P5WB as follow, which stop working,



Thanks to "everell" who provides its similar 5Vsb circuit to me, I found that TNY276 of 5Vsb circuit may be defective. I will buy one in the near future.

Meanwhile, I found 2 varistors (TVR10241) as follows also burn out,



I want to buy them with TNY276 on next time I visit the shop. But I am wondering why it use a 240V varistor. Can I buy two 180~200V varistor to replace them? Any part no provide?

Any suggestion is very welcome !!!

[Behemot]

Re: Question about Varistor

What is your mains voltage? It depends on that.

For 230 V I now use B72207S0231K101 (230 V working voltage, 360 V conductive voltage). They start to conduct sooner so it means they will probably fail sooner as well, on the other hand they provide protection from even lower surges.

It is also generaly good idea to cover varistors, thermistors and this kind of components with heatshrink tube. It can contain most of the energy of the exploding component.

[hkqq2010]

Re: Question about Varistor

What is your mains voltage? It depends on that.

For 230 V I now use B72207S0231K101 (230 V working voltage, 360 V conductive voltage).

Thanks for your reply. My mains is 220V.

But the design of Bestec is quite different. It use two varistors, each across one of the primairy caps. Therefore, it is equivalent to two varistors in series across the mains. So I think I should use two varistors with 1xx V to replace them. Any suggestion?

[Behemot]

Re: Question about Varistor

You haven't stepped up for the standard 230 V?

Ah, I see. In full filtration circuit, there should definitelly be also one varistor before the rectification cascade to supress the spike before it can even hit primary caps. Anyway, there is as much as 311 V after rectification and if you have 230 as well, than 325 V. So I think 160V, or maybe safer 170V varistors would be good.

[hkqq2010]

Re: Question about Varistor

You haven't stepped uf for the standard 230 V?

Ah, I see. In full filtration sicrcuit, there should definitelly be also one varistor before the rectification cascade to supress the spike before it can even hit primary caps. Anyway, there is as much as 311 V after rectification and if you have 230 as well, than 325 V. So I think 160V, or maybe safer 170V varistors would be good.

Thanks for your valuable info. I will get 2 x 170V varistor.

[tom66]

Re: Question about Varistor

I would go a little higher than 170V... as that's just 240Vac. Maybe closer to 200V?

[Behemot]

Re: Question about Varistor

Yeah but it will than start to conduct way over 300 V. I doubt 200V input caps will like that, their rated spike voltage is not so high.

[hkqq2010]

Re: Question about Varistor

Ah, I see. In full filtration circuit, there should definitelly be also one varistor before the rectification cascade to supress the spike before it can even hit primary caps.

I had double checked it, but I found no varistor before the rectification cascade.
I think with this design it will suit for both 230V and 110V mains. The PSU has a switch to select the mains between 110V or 230V. I was always wondering how a 230V protective Varistor to protect the PSU which use in 110V mains.
Of course, this design cannot protect the bridge rectifier.

Anyway, there is as much as 311 V after rectification and if you have 230 as well, than 325 V. So I think 160V, or maybe safer 170V varistors would be good.

Yes, the rectified primiary voltage of the PSU is about 330V.

[hkqq2010]

Re: Question about Varistor

I would go a little higher than 170V... as that's just 240Vac. Maybe closer to 200V?

Thanks for your suggestion.
But I would choose 170V, which is more closer to 330V/2=160V. I think it will be more protective.

Free service manuals database: http://www.tgohome.com/ServiceManuals

BTW, the above site you provided is cool. It will be very useful to me in future.
Many Thanks.

[Behemot]

Re: Question about Varistor

Just remember, 170 V DC (thats 130 V AC/RMS). If you'll buy 175 V AC, thats for 225 V DC and they start conduct at arround 270 V. (datasheet for mine varistors)

As for the switch, well, for 115 V it uses boost circuit so there is 325 V anyway on the input side. For that reason when there are these two varistor accross the input caps, they will protect them even on 115 V mains. And usually as there are very short spikes (ms wise), they don't care about the bridge rectifiers as they (usually) use 600 V ones even in el-cheapo PSU's (in higher-quality PSU's you can find even 800V or 1000V) which are able to sustain few kilovolts for fractions of second.

[hkqq2010]

Re: Question about Varistor

Just remember, 170 V DC (thats 130 V AC/RMS). If you'll buy 175 V AC, thats for 225 V DC and they start conduct at arround 270 V. (datasheet for mine varistors)

Thanks for your reminding, I will go to choose 130V AC varistor.

BTW, althought I know varistor is use to protect device from surge, I don't know its exact mechanism. Please forgive my foolish, I want to know how the varistor protect the electronic device. Does it work like fuse?
When there is surge exceed a specific voltage, it becomes an electrical crowbar to let the current go throught it to ground, instead of going inside the device. Is it correct? Does it burn out like fuse? One of my varistors becomes several K ohm, the other even close to short.
Does the varistor give protection by damaging itself?

As for the switch, well, for 115 V it uses boost circuit so there is 325 V anyway on the input side. For that reason when there are these two varistor accross the input caps, they will protect them even on 115 V mains. And usually as there are very short spikes (ms wise), they don't care about the bridge rectifiers as they (usually) use 600 V ones even in el-cheapo PSU's (in higher-quality PSU's you can find even 800V or 1000V) which are able to sustain few kilovolts for fractions of second.

I see ... thanks for your explanation.
But I wondering why Bestec used 2 x 240 Vac varistor in series, which is equivalent to 480 Vac varistor, I think. Is it a bad design?

[Behemot]

Re: Question about Varistor

They survive for longer time, most manufacturers do that. You are correct, over some specified voltage the varistor becomes more and more open and conducts the spike to ground. And yes, it damages itself, it has both limited maximum power it can handle, and also some number of "conducts" it can do.

Very much like thermistor…the less you use them, the longer they survive. That is one of the reasons why NOT to turn off and on PSU completelly every day. (second reason is you have to buy CR2032 battery every year )

[hkqq2010]

Re: Question about Varistor

They survive for longer time, most manufacturers do that. You are correct, over some specified voltage the varistor becomes more and more open and conducts the spike to ground. And yes, it damages itself, it has both limited maximum power it can handle, and also some number of "conducts" it can do.

Very much like thermistor…the less you use them, the longer they survive. That is one of the reasons why NOT to turn off and on PSU completelly every day. (second reason is you have to buy CR2032 battery every year )

I see ... Many Thanks.

[Behemot]

Re: Question about Varistor

You're welcome

Also if the one varistor of yours is nearly open, I suggest changing it. With low resistance under normal circuimstances, current will flow through it to ground and it can go soon

[hkqq2010]

Re: Question about Varistor

You're welcome

Also if the one varistor of yours is nearly open, I suggest changing it. With low resistance under normal circuimstances, current will flow through it to ground and it can go soon

Yes, I intend to replace all of them.

Btw, I had another PSU which stop to work. After I fixed, it now turn to work, but with some intermittently. Any suggetion which part I should check of it?

[Behemot]

Re: Question about Varistor

What do you mean, intermittently? What does it do (or does not do) exactly?

[hkqq2010]

Re: Question about Varistor

What do you mean, intermittently? What does it do (or does not do) exactly?

Sorry, typing eroor.
It has intermittently noise.

[Behemot]

Re: Question about Varistor

That can be many things and not all af them are easy to repair.

Usually thirst thing to check is capacitors, if they are bad, quite often compensation or other stuff gets broken (starts to oscillate, uses too low/too high frequencies and so).

What load (if any) you have on the PSU? Can be crossloading too.

[hkqq2010]

Re: Question about Varistor

That can be many things and not all af them are easy to repair.

Ooooo ... too bad to hear " ... not ... easy ..."
Anyway, I will try to fix this problem for my hobby.

Usually thirst thing to check is capacitors, if they are bad, quite often compensation or other stuff gets broken (starts to oscillate, uses too low/too high frequencies and so).

Thanks for the suggestion. I will test them one by one.

What load (if any) you have on the PSU? Can be crossloading too.

I use a PC with AMD Athlon 64x2 and no graphic card to test it.

Thanks for your reply. I will try to fix this cheap PSU when I have time.