Just a Small Fire
At work we’re cleaning out and recycling a number of older computers. In keeping with good data practices, we want to wipe the disks before we do so. We decided we needed a degausser to do this. But they cost a lot of money. So, I decided to make one. No problem, I’ve made them before for various purposes.
It’s just a coil of wire that line current goes through. Since the line current already switches direction 60 times per second, it does a pretty good job of degaussing.
I wrapped the necessary 30 AWG wire around a cardboard form I made that was the same shape as a disk drive and a little larger in size. I checked the maximum current for 30 AWG wire, and de-rated that some for safety sake. Then I figured the necessary resistance for the line voltage to allow the desired current. I wired a 1/4 W resistor in series with the coil and the power cord.
You might see this coming now. Yup, I forgot to allow for the power that the resistor had to dissipate. The 1/4 W resistor was way too small for the power level when I plugged it in to test it. I had the right resistance, but the wrong power level. About 200 times too small.
Naturally, the resistor burned up immediately and set the cardboard coil form on fire. Just a small fire, only an inch tall. And I blew it out quickly after I pulled the wall plug. No problem!
Of course, a coworker had just picked that moment to put her head in the door and saw the flame. Great timing. Every one at work had a great laugh, my boss included.
So, I’ll rebuild it later using a light bulb as the resistor — that works and can handle the necessary dissipation. And it won’t risk setting off the fire alarm.