Exploding MOSFETS

I didn’t see any sparks though..

My MOSFETS arrived today. I connected them into a circuit that controls the power to a telephone bell using a Raspberry Pi PICO. They worked quite well. For a while. Then the ringing stopped. I was pretty sure what the problem was. It’s all to do with collapsing magnetic fields. The bell is operated by a coil which turns magnetic when you put power through it. This attracts the “donger” (for want of a better name) which strikes the bell making it ring. Then, the coil is turned off and another one turned on to move the “donger” in the opposite direction towards the other bell. If you do this around 20 times a second you get the ringing sound you get in old films and TV shows.

The snag is that when you turn off a circuit containing a coil the magnetic field the coil has generated collapses, and when it does this it induces a voltage in the coil. This behaviour is used to good effect in transformers and car ignition systems but in this case we end up with a bunch of voltage with nowhere particular to go except back up the wire into the MOSFET that just turned it off. Sometimes you get lucky and he pulse causes no damage. Other times you don’t. I’ve been kind of lucky. Only one of my MOSFETS is broken. The good news is that I know exactly what to do. I need to put a diode across the coils to short out any induced voltages and stop them causing damage.