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MOV and Fuse at input of offline SMPS?

treez

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Our contractor is doing a 200W Offline SMPS (240VAC). He is putting the MOV directly after the mains input connector…then the fuse. This is because the MOV sinking surge current is not wanted to blow the fuse. If the MOV gets damaged it will blow up anyway…so there’s no point putting it downstream of the fuse, would you agree?

We just hope we are not violating some safety reg?
 

biswaIITH

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Our contractor is doing a 200W Offline SMPS (240VAC). He is putting the MOV directly after the mains input connector…then the fuse. This is because the MOV sinking surge current is not wanted to blow the fuse. If the MOV gets damaged it will blow up anyway…so there’s no point putting it downstream of the fuse, would you agree?

We just hope we are not violating some safety reg?
Yes You are right. Generally we put MOVs just after the fuse at the Input section
 
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    treez

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Easy peasy

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If the MOV goes - you generally want the fuse to blow - unless the MOV has it's own fuse

a well blown MOV will usually destroy some other part of the ckt by coating it with hot conductive stuf ...
 
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treez

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If the MOV goes - you generally want the fuse to blow - unless the MOV has it's own fuse
Thanks, i see, but does "MOV downstream of fuse" runs the risk of fuse nuisance blowing due to normal MOV surge current?
 

    FvM

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Yes - it is a nuisance but the R of the fuse at high currents is enough to blow the fuse and protect the equipment - without necessarily blowing the MOV, - esp if the mov is large enough - easier to replace a fuse than the whole equipment - we have built a bit of lightning protection kit - and the key is very large mov's, inductors, and suitably rated fuses ...

The fuse limits the destructive impact of the mov when it goes also ...
 
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