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I recently purchased a few electromagnets. I want to drive these with transistors, but I want to build in protection, as these don't have any available specs re: max operating temp, duty cycle, etc. (It was all I could really find for a diy project)

Assuming a 1 amp NPN, standard driving circuit driving a coil with a flyback protection diode, how would I guard against possible fire if the coil fails as a short circuit? (Ie, I would assume that if the coil suddenly becomes a short circuit, the transistor would conduct too much current from C to E and would start smoking, and possibly lead to fire)

(LED to indicate fault condition would be nice also)

How could I, in addition, detect if the coil fails as an open circuit? (maybe just an LED to indicate this failure mode)

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  • \$\begingroup\$ You can add a temperature probe on the coil and a fuse. \$\endgroup\$ – Damien Mar 5 at 6:42
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    \$\begingroup\$ If the coil fails as a short, you will have some nanoseconds (or a few microseconds, based on the volume of the transistor) to detect a non-saturated VCE and then shut off the base drive. A high-voltage transistor will have about 10 microns collector-buried-layer, thus you will have 10 microns of silicon to heat up; the thermal timeconstant of 10 micron of silicon is 1.14 microSeconds. So get an analog comparator with less than 100 nanosecond propagation delay, and be able to sweep out the saturated base charge in another few hundreds of nanoseconds. \$\endgroup\$ – analogsystemsrf Mar 5 at 7:53
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A fuse would protect against fire. But unless you massively over-rate the transistor, it will probably blow before the fuse.

An LED, with a suitable series resistor, connected across the fuse would indicate that it had blown. But that won't help if the transistor blew first.

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