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Protecting a shunt with a resettable thermal fuse

ACharnley

Member
Quick question regarding thermal fuses which I've not had the pleasure of using, until today. I just need to double check I have the right approach.

Circuit is variable voltage, fixed current of circa 500mA which I'll refer to as current

If I have a transistor shunt set at 36V it could, in an edge case, heat up and destroy the transistor. If I were to use a thermal fuse in-line with a rating of <current it would trip before the 36V is reached while >current would never trip.

This assumes the heat from the transistor makes no difference, but if the fuse is placed right beside the transistor then I can use a fuse value of lets say (current x 2). So the fuse doesn't self-trip, it relies on the transistor heat to bias it.

Thanks, Andrew
 

alec_t

Well-Known Member
Most Helpful Member
Can you post a link to the type of thermal fuse you intend to use?
 

alec_t

Well-Known Member
Most Helpful Member
In the datasheet each fuse has several different current ratings. I can only guess at their significance (hold current, trip current, ....... ?). Which one are you relying on?
I'd be interested to hear how things pan out. Keep us posted.
 

ACharnley

Member
Current is fixed at 500mA, I'm hoping the heat from the transistor can raise the 1A fuse to trigger at say 60c. I purchased a few of each to see which works best. There's also a reliance on the heat transferring from the transistor to the fuse before the transistor over-heats.
 

ChrisP58

Well-Known Member
The second table on page 2 shows how the hold current changes at different part temperatures, dropping to a bit less than 50% at 85°C from the nominal 25°C value.

The working temperature will increase due to the current through the part plus heat from the adjacent transistor, but it will also loose heat due to radiation and convection.
 

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