Diference between MOV varistor and bidirectional transient voltage suppression diode

[fjpompeo]

Hi,

I really appreciate to know the differences between a bidirectional transient voltage suppression diode or transzorb (E.G. 1.5KE20AC) and a MOV varistor (E.G. S20K20) and if possible typical applications of each one.

Thanks in advance,

Fernando - Call sign PU2PLL - SP - Brazil

 

[treez]

MOV will "die" after quenching a certain number of transients. (its leakage current gets higher and higher after each "quench", and eventually it has so much leakage current that it just dies and goes short circuit......blowing the fuse that its downstream of in "crowbar" fashion.

A MOV is more rugged than a TVS…….though with low level transients, a TVS will “live” longer than a MOV……its just that a MOV is beefy enough to be able to withstand and quench enormous transients…whereas a TVS is not.

Also a MOV more often fails short, which blows the fuse, and is the required outcome……..a TVS is more likely than a MOV to fail open , which is bad, because the fuse will not blow….or say if the fuse is replaced then an open TVS means that the supply actually has no transient protection.

Whereas a mov is likely to fail short and stay short, so replacing the fuse will not allow the supply to work again…..which is the required outcome……in other words, if a mov fails, then you cannot just replace the fuse….you must replace BOTH the fuse and the MOV, which is the required outcome.

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if you were going to put a transient protector downstream of the mains input filter, then you woudl put a tvs there and not a mov...because the tvs could repeatedly quench any input filter ringing without dying...whereas a mov would die earlier if given this duty

 

[FvM]

To get a basic idea, compare the v/I characteristic in VDR datasheet with the breakdown and clamping voltae specifications of transsil diode. VDRs are faster but have softer characteristic than transsil.

 

[treez]

Yes, and it is a legal requirement to use a mov at the mains input of a line connected smps...with a fuse upstream of it....TVS is not permitted at this place.