How do I know the rating for a varistor?

Power Circuit Protection
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1

My 32" Blaupunkt tv stopped working. I opened it up to have a look and found a blown varistor. None of the varistors on the board have any markings. How do I know what to replace it with?

IMG_20240929_135139~2
IMG_20240929_135139~21920×1440 202 KB

2

You need the schematic circuit diagram to be certain.

The schematic symbol on the PCB under the part and the “C” prefix of the reference designator imply it is a capacitor not a varistor.

If you’re really certian it’s a varistor and want to make an educated guess at the specs:

  1. Remove the varistor from the circuit
  2. Power up the circuit (it should be working now because varistors are never needed for operation)
  3. Measure the voltage, AC or DC, that would normally be across the varistor.
  4. Choose a range of varistors with a maximum working voltage higher than the measured voltage.
  5. Measure the diameter of the existing varistor and choose one from the range you chose htat is the same or larger in diameter (size is good proxy for the other varistor specs)
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3

Thanks Paul. I’m not certain at all what it is. Makes sense that CB120 would indicate a capacitor though. Do the same steps you described work for rating a capacitor? I’m tempted just to leave it out, if it’ll work without.

4

No.

If a capacitor is bad and has no markings, the only way to know the specs is from the manufacturers schematic and or parts list.

Simply removing it won’t fix anything.

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5

Hello,
Welcome to the DigiKey TechForum. I recommend trying what PaulHutch mentioned. I see you said if it works you may just leave the part out. Not a good idea, the part is there to protect against surges and you would lose that protection.

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6

This appears to be an SMPS (Switch Mode Power Supply). Sometimes when things go wrong SMPS can fail catastrophically. Also, varistors have a limited life when they are subjected to power surges, especially at their peak rating. This varistor may have seen more than it’s fair share, so I would not leave that out of the circuit. PaulHutch has a very simple method to estimate the specs for a replacement. Just remember, if this uses a 240 VAC input voltage there will be considerably more VDC so mind your fingers, those are lethal voltages!

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