Reply to post: Old news. I did that with vacuum tubes in high school.

Old news. I did that with vacuum tubes in high school.

In my high school electronics class, early '70's, we were breadboarding five tube AM radios. You know, real voltages and currents, not this wimpy low-voltage stuff. One team had to build in a fault and the other team had to troubleshoot it. My team member and I realized that most of the class was simply "troubleshooting" by looking at the physical layout of the parts and jumper wires on the breadboard.

So we laid out the parts so the radio worked but with the parts scattered all over the place. For example, the antenna was no longer in the upper left-hand corner and the speaker was no longer in the lower right-hand corner and the Intermediate Frequency amplifier was no longer in the center.

One of the changes we came up with was to place the tube plate load resistor right next to the cathode bias resistor. If you're never worked on vacuum tube stuff, the plate resistor had roughly 250 volts applied with several milliamps flowing through it. The cathode resistor was the exact opposite. It developed its potential (voltage) by the flow of the current through the tube. So it had very low voltage and its components, particularly the cathode bypass capacitor, was rated at a very low voltage.

Anyway, the power supply was up on a shelf and we didn't have long leads so the 250 volt plate wire was stretched like a guitar wire to the far end of the breadboard because we had re-arranged the parts layout. (All of the connections were uninsulated alligator clips <- important point).

While the other team was trying to find the fault by pushing and prodding, they bumped the taut plate power wire. Its alligator clip moved slightly and now also touched the cathode lead of the tube. Not only did this give them a second problem to find, it put 250 volts across the cathode bypass capacitor rated at 6 volts DC. The ammeter on that Heathkit power supply pegged out at 250 milliamps.

Why didn't the fuse in the power supply blow? Because we were always blowing those fuses through mistakes so we put in oversized ones. Why? Because the instructor yelled at us for using up his fuses, of course.

Fascinated, we watched that power supply push over a quarter-amp through that poor capacitor but nothing happened. After a few minutes the two of us decided to wander to the other side of the room so we would not be near the breadboard whenever what was going to happen happened.

A few minutes later, with the two "troubleshooters" hunched over that board and oblivious to the high current, a very, very loud bang occurred followed by non-flaming confetti. The two students fell off their metal stools to the concrete floor but fortunately neither were injured.Smoke filled the lab.

They and the instructor were very mad at us but since we were far away when it happened, we denied any knowledge. We looked over the breadboard and pointed out the short circuit we already knew about. No proof meant no suspension.

That capacitor did not shoot anyone's eye out but it easily could have. Lesson learned about wearing safety glasses.