# 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
Can you post a link to the type of thermal fuse you intend to use?

#### ACharnley

##### Member

(I've ordered in 600mA, 1A and 1.5A varieties)

#### alec_t

##### Well-Known 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.