[SOLVED] VARISTOR Placement doubt

[satiz]

Hi all,

I am planning to designSMPS circuit. In that for input protection I am using 275 varistor after the fuse. but one of my friend told we need to place the varistor before the fuse. Any one can please suggest where we need to place the varistor exactly and what will happen if we are placing the varistor before the fuse?

Please find the attached Images for reference

Image having two varistor RV1 and RV2 which one is correct



Thanks in advance

 

[Relayer]

Hello satiz,
The fuse has to be placed before the varistor.
The varistor is there to quench any mains voltage spikes and transients within the tolerance of the fuse. i.e. The fuse will blow, thus minimizing any damage that may be sustained by the varistor.
Varistors used in any design should be replaced after x amount of years. Why? Their molecular structure changes over time due to spikes etc. In my early X-Box days, I've had to replace the varistors in many an X-Box PSU, due to them virtually exploding, and in some rare instances, causing the PSU to catch fire.
I would recommend you encase your varistor with heat-shrink tubing. This will minimize the chance of a fire and cosmetic damage to other nearby components.
Regards,
Relayer

 

[schmitt trigger]

Varistors go into a low impedance mode to clamp the voltage surge via brute force.
For that reason, the fuse must be place before the varistor.

As a matter of fact, to put emphasis on what Relayer mentions, I've also seen varistors blow up.
They spew debris all over the board causing further damage, and as such you can see many designs where the varistor is sheathed with tubing.