varistor use / damage 1

[DYV1973]

LS,

On a pcb near the 230 VAC connections, there are 3 varistors towards the switching power supply (for 24 VDC and 5 VDC).

Q1: Why are there 3 varistors (between PE and neutral, PE and live, live and neutral), instead of 1 between live and neutral?

The pcb is fed by an isolated power supply of 230 VAC, in which peaks are very unlikely. There is no switching in the system either.

Q2: What can still cause that varistors suffer from burn out (they are of MOV S14K275 type).

Thanks,
DYV

 

[Skogsgurra]

That's quite a coincidence. I am writing about that kind of problems right now.

Why three varistors? That is usual in some applications where the designer isn't sure if the grid will be TN or IT. Nothing suspicious there.

No switching you say? What about the 5 and 24 V switchers? They produce lots of switching transients, usually at a very high frequency and those transients are reflected back to grid and whatever there is.
A varistor is highly capacitive and most do not tolerate HF current above a certain limit. I think that is what happens. Put a decent EMC filter between varistors and switchers and see if that doesn't help. It usually does.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

The varistors are probably not there because of anything the product might be returning to the power lines. Varistors can only take a limited number of hits. They actually wear out. Proper protection demands varistors between ALL lines entering a device because a spike can be any polarity on any single line. Three MOVs protects from a spike on either power line with reference to the case or ground (differential) or between the power lines themselves (common mode).

For the MOVs to actually work they require an impedance between them and the spike source. Usually the normal length power cord provides the minimum.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mikekilroy]

as stated, you need all 3 to attempt to fully protect input.

are you saying you have a device with all 3, and all 3 blew at SAME TIME?

if so, please tell where you are located.

if in usa, probably only close by lightening strike would cause all 3 to blow at one time.