A Harsh Lesson

    Turns out power resistors have a nasty side to them, I had heard long ago that they could be dangerous if overloaded but until today I didn't know exactly how bad this could be. I had connected the 12V supply of an ATX power supply to test the 12V voltage sense wire, I wanted to see if I could potentially use this wire to sense the voltage at the load rather than just the termination inside the PSU, to acquire a reasonable voltage drop to test this idea I connected the 12V supply to a 1Ω 50W power resistor attached to a heatsink thinking this would be good enough for the short period needed to test it. I knew very well I would be send 12A into the little fellow, had I done the maths at the time I would of realised I was dumping 144W of power into a resistor designed for 50W and most certainly would of abandoned the idea or at least connected several more in series. I had 4 resistors/heatsinks at my disposal which connected in series would of given me 4Ω and 3A each, a very safe 36W to have carried out my experiment.

    But I didn't do this and proceeded with using just the one, I was aware after a few minutes that it was getting hot but decided that it would be safe for a little longer, I would of avoided the resulting explosion a minute or so later if I had stopped there when I though about it.

    Without warning there was a loud pop and the end of the power resistor shot across my desk. The resistor was not flat on my desk but instead about head height when it shot past my field of view dragging a red hot resistance wire behind it, Had I been leaning forwards at the time I would of taken that in the bloody ear burns and all. I did not avoid all injury though, the ceramic core shattered and spewed sand like fragments across my desk and into my right eye which is still sore even now several hours after the incident.

    What did I learn about this little incident and can pass onto others? Don't abuse power resistors thinking that they can take the punishment, they wont simply burn up like their smaller counterparts. even with heat sinking you can easily overload them so always do the maths before using one as a dummy load to make certain it can be done safely.