Can placing MOVs on a 240 V input ever be a bad thing?

Question

The power supply of my home lighting keeps failing. So far I have had ten identical power supplies fail. Upon inspection the thing that is always damaged is the electrolytic capacitor after the bridge rectifier (the power supply has no step-down transformer). I have always fixed the faulty capacitors by replacing them with new ones.

Aside from the power supply just being badly designed, I can only think of two reasons why they are failing: from getting too hot, or from surge voltages.

I can't really fix the temperature problem with how things are; the only thing I can do is buy electrolytic capacitors that are rated to survive in that environment; 10000 hours @ 100°C and from surge voltages that exceed the capacitor's voltage rating. I'm pretty sure they are using very cheap capacitors - that's why they are failing so fast.

To solve the surge problem I am thinking of placing a MOV on the AC input of the circuit so that the capacitor and the bridge rectifier won't be exposed to surges. The original circuit does not have any protection against surges. I was wondering if my adding MOVs to the circuit can cause problems, because I am essentially modifying the circuit. I don't think there will be, but I am just making sure.

Answer 1

From Littelfuse, here is an estimate of MOV lifetime vs. energy absorbed.

In my experience they always fail shorted until they burn enough to physically come apart.

Answer 2

So far I have 10 of the same power supply failed. Upon inspection the thing that is always get damaged is the Electrolytic Capacitor after the bridge rectifier (the power supply has no step down transformer). I have always fixed the faulty capacitor by replacing them with a new one.

We'll need to see a legible picture of the front and back of the PCB with the capacitor on it, as well as the ratings of the capacitor you have used as a replacement.

There are billions of devices out there that have bulk DC capacitors attached to line rectifiers. It's not very hard to get them to last long enough for the capacitor to fail from "old age".

If you buy the capacitor from reliable sources, e.g. DigiKey, Mouser, etc. but not Aliexpress or eBay, then if the capacitor is still failing then you've not used a high enough rating. Rectified 240V mains at high line is about 370VDC peak. You need 450V capacitor, and its ripple rating must not be exceeded. You will have to measure the ripple current through a replacement capacitor, and consult the datasheet to make sure you stay well within the specs. Typically some derating would be applied, so e.g. you'd want to stay below 1/2-1/3rd of the rated ripple current for longevity.

The hours rating on a capacitor only applies when the capacitor is used within its ratings.

You've assumed that the capacitors fail due to overvoltage, and that may well be the case, but it'd be prudent to eliminate more mundane causes of trouble:

1. Exceeding ripple current rating.
2. Exceeding DC working voltage rating in high line condition.
3. Forgetting that the full bridge rectifier output voltage is 1.4 times the RMS sine line voltage, e.g. a 350V voltage rating is insufficient, and a 400V rating would leave almost no margin.

In cases of under-rated capacitors, the replacement may be physically bigger or use a different package, so the replacement may need to be mounted outside of the PCB, connected by short lengths of hookup wire, and a bigger case would need to be used to house the PCB and the new capacitor.

Answer 3

Good to ask. MOVs may not solve your problem with power supply failure. Do be careful, because some MOVs fail into a low resistance state and can get very hot; they can even explode. A metal box is good. MOV after fuse is good. MOV on separate fuse is good. MOV close to delicate SMD parts is bad.