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Under-rated fuse question
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Jester:
I'm working on a project that requires a load resistor to be applied to the 120V Mains for 15 seconds once an hour. The circuit is in a fairly small plastic enclosure.

The temperature rise for 15 seconds once an hour is not a problem, however assuming the single fault condition that the circuit gets latched on continuously, the circuit will over heat catastrophically. The resistor is 500 \$\Omega\$, so 240ma and about 30W. If I insert a 200mA  160ma fuse in series, the fuse burn time @ 240ma is approximately 100s 200s. Assuming we can tolerate temperature rise of 30W for 200s, can I rely on the fuse to be long time reliable with 24 heating cycles a day to 15% 7.5% of the trip time?


Siwastaja:
The fuse will most likely be fine in your conditions (i.e., won't nuisance blow), however, 200mA nominal fuses that guarantee blowing up on 240mA in any time are going to be rare, or unobtainium. In other words: it won't blow up in the fault condition (unless you are very lucky with the actual fuse), and your device catches fire.

You'd need to use something like a 125mA fuse to be sure enough, and then it would be a problem with your 15 second cycle.

Thermal fuse coupled with the resistor is the way to go. Adjust the coupling so that it doesn't blow in the normal case.
Tomorokoshi:
What type of device is turning on power to the load resistor? Can you program the duty cycle of how the device is controlled?
ejeffrey:
I wouldn't want to rely on a fuse stressed that much long term. Can you use a thermal fuse or even a bimetallic thermal switch?
Neilm:
I have seen resistors that have a "pinger" on them. If the resistor gets too hot the pinger opens and disconnects the resistor. The downside for this is it is a single shot device and you would have to replace the whole resistor. Also they are not that available. A thermal fuse that is pressed up against the resistor would be a reasonable option
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