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How can you check that MOV is damaged?

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alphadog

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I have 3 issues that i wanted your advice on them please.

1.
If I use a thermally fused MOV (the fuse gets open when temperature is too high), how can i know when the internal fuse in the MOV got open?
http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Varistor_TMOV.pdf

2.
What does that picture mena?
And how can i conclude from it when the fuse gets open depending on temperature?


3.
Does anyone know about a MOV that comes with PTC tight to it?


Thank you very much.
 

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Sceadwian

Banned
The graph is pretty self explanitory. Between 85 and 125C you have to linearly derate the value you chose for your fuse.
So if you use that fuse in a 105 degree environment you have to derate it's current value by 50%. So if you were planning on using a 2amp fuse, you need to use a 4amp fuse, or you're going to get false trips.
 

Sceadwian

Banned
Not a clue sorry =)
 

Willbe

New Member
I have 3 issues that i wanted your advice on them please.

1.
If I use a thermally fused MOV (the fuse gets open when temperature is too high), how can i know when the internal fuse in the MOV got open?
Feed the MOV from AC through a high value resistor so that you get max 1mA or 10mA current through it. Look for the typical V-I curve for an MOV on a scope; if the fuse is open you will just see an AC waveform on the scope.

". . .how can i conclude from it when the fuse gets open depending on temperature?"
The graph you show doesn't tell you.
Fuses [and coils and semiconductors] have an I squared T rating; if a 1A fuse opens in 3 sec at 2A, then its rating is 12 amps-squared-seconds. This formula applies for part of the "trip" or "clearing time" curve.
These are on the Web but are hard to find.
Of course, a 1A fuse holds 1.1A or so indefinitely so the curve doesn't apply at this end of the curve, and at enormous current the clearing time also deviates from the formula.

It's I squared because heating in a resistor is proportional to I squared, and the ultimate temp rise which melts the fusible link is proportional to time.

Fuses also have an "interrupting" current rating. A residential 120v/240v supply may have a short circuit current of at least 20,000A, so a 15A fuse for this application has to be able to safely interrupt 20,000A.
 
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