Question about MOVs

[etnietering]

Hi, I salvaged 2 small MOVs out of an old power supply, and was hoping to put them in my logisys. The only places I can see for them to go are in parallel with the bleeder resistors across the primary caps. These spots are labeled VD1 and VD2....the power supply I got them out of had them labeled MOV1 and MOV2. They're in the same place schematically in the old power supply, but I want to make sure I'm putting them in the right spot. I drew up a schematic of the input side of my power supply, and VD1 and VD2 are the places where I think the MOVs go:


Can anybody clarify this for me? I don't want to put them in and have it 'splode...

Thanks!

[everell]

Re: Question about MOVs

Take a look at thread named "Power supply build quality pictorial.closed.use part 2", page 19, post #372 about Compaq HP-Q250GF3. Ends with page 20, post #387. It has a discussion on the MOV, in particular a pair of MOV's that blew up.

[etnietering]

Re: Question about MOVs

OK so either they do go there, and that PSU got hit with a huge surge which blew them up, or they don't go there and that will happen to mine when everything's fine...which is it?

[Travis]

Re: Question about MOVs

The MOVs there are to protect primary components from excessive input voltage (like when you turn the voltage selector switch to "110V" but plug it to a 220V line). They're rated at ~200V and will conduct above this point. If they hold on for a while before breaking up, the fuse will blow, protecting the primary circuit.

In the end you have two MOVs and a fuse blown, which are easy to replace, instead of primary caps, primary switchers, bridge rectifiers or so(spreading e'lytic everywhere and causing fire probably).