A favorite at Case Western Reserve University where I went to school in the late 1980s was the exploding electrolytic capacitor. In our dorms, there was a switched outlet in the bathrooms above the mirror over the sinks. Most people used it to plug in radios. When you left the bathroom and turned off the light, the radio would automatically turn off, saving electricity.
Well, we would go into the bathroom with a flashlight so we wouldn’t have to turn on the main overhead lights and energize the socket, stick a 47 uF (the optimum value, IIRC) cap into the socket, and wait for the next victim! Flip the switch, light goes on, cap goes boom!
It usually scared the #*$% out of him or her!
One time after a few beers, I didn’t have any caps handy, so I just wrapped some 20 gauge wire tightly around a pen and shoved it in the outlet. It worked nearly as well; instead of an explosion, there was a zap, and a burning, smoking, stinking coil of wire fell into the sink!
Brings to mind the "Little Jiffy Fuse Tester" we built and left in the shop.
Just a simple little box with a pilot lamp, heavy duty pushbutton and heafty power cord.
The instructions on the cover stated: "Plug into an outlet, lamp should light. Press button and release. If light is now off, the fuse was good."
The operative phrase, of course "fuse WAS good". Surprising how many EE students (and even professors!) only caught on AFTER they had "tested" the fuse!
Good times indeed!
As a technician at Linear Power (remember those amps, back in the monster car-audio days?) there were occasions when the board-stuffers would load large, filter electrolytics backwards which would blow-up with quite a bang when power was applied. They would also occasionally leave out the zener diodes from the shunt-regulators allowing the full rail voltage of +/- 63VDC to be applied half-a-dozen op-amps. Those would give a good pop leaving little craters where the dice would explode. We’d also put small 10uF caps in the female end of an extension cord, place it under a desk, then plug in the male end around the corner giving the prep people quite a scare. Fun times.
My father once told me how, while working in commercial refrigeration in the '40s, he could take a motor starting capacitor and charge it from a 110 ac line (of course peak voltage charge random). The cap (with screw terminals) was then left in the employee's phone booth as a doodling object. He said it was always funny how the secretaries and office guys simply ignored it, but every tech and mechanic, while talking, just had to finger the two terminals simultaneously.
In a two way radio I was a tech with a sig gen on my bench that just happened to cover broadcast FM. One morning I found out what station a co-worker listened to and intermittently turned on the sig gen for random seconds with no modulation. He never did figure it out.
A certain engineer at Harman-Kardon many years ago, when the company was still in NY, would reverse the electrolytics in power amps under test while people were at lunch.
It's remarkable that no one was hurt. It's also no surprise that this person went on to a string of jobs thereafter. When he finally left one some years ago, more-or-less against his will, a good many people quit early that day and went out to celebrate.
In going to college in the 70’s we all had dial type phones in each of our dorm rooms. One trick was to remove the “Green” wire from the terminal block on an un-suspecting neighbors phone. Then call them up and leave my phone off the hook. Disconnecting the green wire caused the phone to continue ringing when answered. Short of re-connecting the green wire or disconnecting the phone there was little you could do to stop the ringing. (Phones in those days did not have connectors and were hard wired.) I usually hung up after about 20 minutes or so. Of course the neighbors caught on after a couple of times and would fix their phone only to pull that prank on the next victim.
Speaking of flashbulbs, many years ago (early '80s) I bought some "Magicubes" which were 4 flash bulbs in a plastic case. The nice thing about Magicubes was that they didn't required a battery, but used a striker to strike the base which contained a fulminate which initiated the flash. I rigged up a paperclip to trip the striker and placed it in a coworkers desk drawer. When he opened the drawer a string pulled the paperclip and tripped the striker. It was quite a wakeup!! I still have the cube and some spare bulbs in the back of my desk drawer. Maybe one of these days...
A charged condensor from a car's distributor was a favorite for my eigth grade prinicpal. At recess he would walk around on the playground and yell "Here, catch" and toss it to you. We took it as fun and made him teach us how he didn't get shocked himself. This was in 1960. Today, I'm sure that he would be in a lot of trouble. But in my opinion he was a great "hands on" teacher.
We had just learned about inductors and reactance in my basic electronics class and after the lecture, one of students came up and asked what I thought would happen if he stuck a small 10uH inductor in the lab bench wall socket (the class also doubled as a lab).
I told him to figure it out, you have 120vac at 60 hertz an that if it was me I would think twice before doing it; I believe the current would be quite high.
By then I was off on the other side of the class talking with my instructor and the other students- and then a BLAM! I can here pieces of the poor inductor ricocheted off the ceiling and walls.
Fortuntely it was the end of class and most students had left.
My respect for instructors in general went up another notch.
You just never know what kind of students you will be getting...
I wonder what job he has now???
The flashbulb thing is pretty funny, but the ones involving capacitors across mains powerlines and high-voltage charged electrolytics are just unbelievably stupid, cruel and dangerous, even for college kids. There are all sorts of ways pranks like that can go badly - or fatally - wrong. It's no wonder we engineers have a reputation as socially inept nitwits.
Blog Make a Frequency Plan Tom Burke 17 comments When designing a printed circuit board, you should develop a frequency plan, something that can be easily overlooked. A frequency plan should be one of your first steps ...