Varistor for Bestec ATX0300 D5WC

[docchan2000]

I got a blown up varistor ( vr44) located bet the two big capacitors.I don't know what is the value of it or what voltage to replace it?.Pls.help. can I just put any varistor there like 100.v?

[budm]

Re: Varistor for Bestec ATX0300 D5WC

Can we see the pictures? Are the caps connected in series or parallel? Voltage rating of the caps, 250V? 450V? The more information you provide the better we can help you.

[docchan2000]

Re: Varistor for Bestec ATX0300 D5WC

Thanks for the reply. I don't know what voltage to put but it looks like this picture which I screen shot from hkqq2010.

[docchan2000]

Re: Varistor for Bestec ATX0300 D5WC

Here is the specs of my atx

[budm]

Re: Varistor for Bestec ATX0300 D5WC

You are going to have to remove the board and take hi-res pictures of the whole top side and the bottom side of the board, and then good closed up pictures around the VRxx.
Or you will have to locate the same power supply to see what the print on the VR are.
Right now I cannot tell how those VRs are connected to the circuits.

[fzabkar]

Re: Varistor for Bestec ATX0300 D5WC

If there is no other fault with the PSU, then power it up (without varistors) and measure the voltages at the varistor pins.

[momaka]

Re: Varistor for Bestec ATX0300 D5WC

I think the varistors should be across each cap. I looked at pictures of two ATX-300 units I have (12E and 12Z), and they both have the varistors across the primary caps. Probably 180V or 200V-rated varistors. These Bestec PSUs don't have APFC, so the primary caps are rated for 200V. You should see half of the AC rectified voltage across each - i.e. about 162V.

If the PSU is not in a mission critical PC or you don't care whether it gets killed from a voltage surge, then remove those varistors and run the PSU without it.

By the way, this thread should be in the Power Supply Design and Troubleshooting forum.

[docchan2000]

Re: Varistor for Bestec ATX0300 D5WC

Thanks for all your reply.I am new in this site and not quite familiar in posting.
Is it OK if I just put a 300v varistor there?I'm using this PSU for my HP desktop.I'll try to take some nice pictures tomorrow.

[fzabkar]

Re: Varistor for Bestec ATX0300 D5WC

Here is an ATX PSU schematic:
http://www.pavouk.org/hw/en_atxps.html

Here is the manufacturer's original circuit diagram:
http://web.archive.org/web/200901051...power/2038.zip

Here are lots more DTK PSU schematics:
http://web.archive.org/web/200901051.../circuitp.html

[docchan2000]

Re: Varistor for Bestec ATX0300 D5WC

Here are the pictures. The red Varistor is the new one, it is a 100v varistor but I'm not sure if i should replace it with a 200v or 300v one. Thanks

[momaka]

Re: Varistor for Bestec ATX0300 D5WC

Is it OK if I just put a 300v varistor there?I

No point in doing that.
The two varistors in your PSU are installed across each 200V capacitor. The idea behind a varistor is to clamp any power surges that go beyond the 200V rating of the primary caps. If you put a 300V varistor, then it won't protect the capacitor. However, you can't use a 100V varistor either, because the voltage on each cap will be around 165V and may go up to 170 V when your line is 5% high. So that's why you need a varistor rated between 180-200V.

I would say just don't bother putting it in there. It won't destroy your PC. It's just that *IF* the line voltage to your house goes high due to lightning/thunderstorm or other technical issues (both of which are very rare events), then your PSU will likely fail and blow a fuse. Since this is a Bestec PSU, chances are nothing else will blow when the PSU blows up, because Bestec PSUs tend to be well-engineered. So again, I just wouldn't worry about that varistor.

[docchan2000]

Re: Varistor for Bestec ATX0300 D5WC

Thanks for the immediate reply momaka. If the varistor serves nothing in powering up my unit when it exploded...why can't it power up my Desktop?does this mean that there is more than the varistor that got messsed up?

So I need to remove that red varistor that i just put and check on the other parts?

[fzabkar]

Re: Varistor for Bestec ATX0300 D5WC

Make sure the 120V/240V selector switch is in the correct position. If you are in a 240V locale and you have selected the 120V position, then this will cause the varistors to explode.

[Per Hansson]

Re: Varistor for Bestec ATX0300 D5WC

The 300v varistor is still better than nothing.
And besides most caps have a voltage surge rating.
I.e. they may be ok with a short over voltage of 50% for a couple milliseconds.
Check the datasheets

[fzabkar]

Re: Varistor for Bestec ATX0300 D5WC

What would happen if one of the capacitors were slow to charge up? If the two capacitors are not perfectly balanced, wouldn't one of them have a higher voltage than the other, and could this be a possible failure scenario for the varistor?

[momaka]

Re: Varistor for Bestec ATX0300 D5WC

If the varistor serves nothing in powering up my unit when it exploded...why can't it power up my Desktop?does this mean that there is more than the varistor that got messsed up?

Yes, it's possible there are other bad components. I explain below.

The 300v varistor is still better than nothing.
And besides most caps have a voltage surge rating.

Most caps have a surge rating of 25%, not 50%. That would make these 200V caps being able to handle up to 250V for a very short period of time. So the 300V varistor still wouldn't do anything useful here.

So I need to remove that red varistor that i just put and check on the other parts?

If that is 100 V -rated varistor, then most definitely remove it. Otherwise it will blow up as soon as you plug in the PSU in the wall.

Since the original varistor exploded, as you noted, then that means other parts could have been damaged as well.

The first things to check would be the fuse, bridge rectifier (black rectangular box with part number "GBU..."), and thermistor (that green, disk-shaped thing next to the bridge rectifier).

You will need a multimeter to troubleshoot these (and the rest of the PSU). May we ask how familiar you are with using one? Reason I want to know is so that we can give you troubleshooting instructions accordingly.

What would happen if one of the capacitors were slow to charge up? If the two capacitors are not perfectly balanced, wouldn't one of them have a higher voltage than the other, and could this be a possible failure scenario for the varistor?

I think that could happen only if one of the two balancing resistors across each cap went open. But they are usually in the range of 150 KOhms and up, so they are very unlikely to go open circuit due to an overload.

[docchan2000]

Re: Varistor for Bestec ATX0300 D5WC

I just do some you-tubing and readings in the net.I know not much about this things but I like doing things on my own because of the self fullfilment aside from saving a lot of money. I got to have that multimeter and test those parts you mentioned momaka. Many thanks and more POWER💪

[docchan2000]

Re: Varistor for Bestec ATX0300 D5WC

Many thanks to all those who contributed to this thread...budm,fzabkar, Per Hansson and momaka.Greatly appreciated jt.��

[momaka]

Re: Varistor for Bestec ATX0300 D5WC

I got to have that multimeter and test those parts you mentioned momaka.

Good. Let us know what you find with the testing.

- The fuse should ideally read 0-1 Ohms across it (depending on how good your multimeter is).

- ^ Same goes for the the thermistor, except anything between 0-10 Ohms is generally acceptable. Make sure there are also no physical cracks or missing pieces in its body, as that might indicate a pending failure.

- Bridge rectifier is basically 4 diodes connected in a special way.
Between the two AC (~) legs, you should get infinite resistance (i.e. open-circuit, or OL on your meter), regardless of how you connect your multimeter probes.
With red (+) multimeter probe on positive leg (+) of bridge rectifier and black (-) multimeter probe on negative leg (-) of bridge rectifier, you should get infinite resistance again. Reversing the probes should give you approximately two diode voltage drops (800 - 1600 mV) on diode test mode on your multimeter.

[kc8adu]

Re: Varistor for Bestec ATX0300 D5WC

thread moved.
btw last time i saw these fail the customer lost a lcd tv too.
i went over to check things out after i repaired the power supply and tv and found my hunch to be spot on.high neutral.it was loose at the pole.