Colossal Capacitors + College Dorm Room
A Cautionary Electronics TaleBy David Massey
Electronics has been a hobby (okay, obsession!) of mine since I was old enough to hold a soldering iron. When I was a kid, I was always building or rebuilding something, and my hobby eventually became my occupation. Back in the early 1970's when I started college as an electrical engineering student, I was living in the dorms at what was then called Florida Technological University in Orlando, Florida (now called the University of Central Florida).
As a freshman, I didn't get to choose my roommate. It seemed the school made every attempt to match me with someone that I had absolutely nothing in common with. I suppose maybe they wanted me to diversify my circle of friends beyond engineering students, but the roommate they chose for me was an English major! What were they thinking?! My roommate never seemed to care about what I was building on my side of the room... that is until the day I powered up my pirate radio station for the first time and went for a long walk around campus with my FM radio receiver to see just how far the signal traveled. My FM Transmitter – Pirate Radio 90.5 The monophonic FM transmitter consisted of three transistors, the third transistor was the actual RF oscillator transistor using a 25pF capacitor in parallel with a coil consisting of three turns of solid copper wire around a pencil (pencil is removed once the coil was formed). To tune the transmitter, I either stretched the coil windings apart a little or squeezed them together.
Since the FM transmitter design was simple and not crystal controlled, any variation in the power supply voltage caused the LC tuned oscillator stage to change frequency, and the power supply ripple voltage would modulate the FM signal and end up as an annoying hum in the radio receiver. I knew I had to find a very large filter capacitor for the power supply to reduce these effects.
My Power Supply – A Disaster Waiting to HappenThe power supply was put together from surplus parts from an old IBM main-frame computer that was donated to the engineering department for students to salvage parts. I got my hands on a huge capacitor (probably 3 inches in diameter and 7 inches tall) that had about 15,000 microfarad capacity rating at 10 working volts and 15 volts surge. In my haste to build the 12 volt power supply for the transmitter, I didn't pay attention to the word "surge" so I figured 12 volts on a 15 volt rated capacitor would be just fine for my pirate radio station.
The Smoke Test – Figuratively and LiterallyAfter everything was wired up, the final step was the "smoke test." After turning on the power and adjusting the tuning coil to the only clear spot on the FM dial (90.5 MHz), I took a walk around campus with my transistor radio to see just how far my signal was going. Everything was going great with the reception of the signal until I got near the library building and suddenly the clear, strong signal died. I didn't think to put a fuse in my power supply circuit, so I knew it wasn't a blown fuse, but I didn't expect to find what I did when I ran back to the dorm room. When I opened the door, there was my roommate laying on his back sideways across his bed in a daze. He managed to mutter, "something blew up on your side of the room!" Our beds were directly behind the chairs for our desks, so he literally flew backwards and landed on his bed in fright when he heard the explosion. I don't think my non-techie roommate ever spoke to me again, but hopefully I inspired him to write a great essay.
The Aftermath – Lessons LearnedIt was immediately apparent to me what had happened – my power supply capacitor had exploded! It couldn't take the overvoltage very long before it began to heat up causing the pressure inside to exceed the maximum pressure the rubber seal on top could withstand. It literally blew its top and spewed its guts all over the room. The ceiling was covered as well as our books, desks, floor and wall next to the power supply. It wasn't easy cleaning up the mess I made, but that didn't stop me from rebuilding the power supply with a proper capacitor and going back on the air again.
Later that year I added a 19KHz subcarrier signal to the audio input, which faked FM radio receivers into thinking I was broadcasting in stereo (remember the little red light that would come on when a commercial FM station was broadcasting in stereo?). I even had a phone patch to take calls from students for song requests.
It was fun while it lasted, but I had other things I wanted to spend my time on (like studying), so I sold the radio station gear to a hippie for twenty dollars.
The moral to this story is to make sure you don't exceed the rated "working voltage" of a capacitor, and in fact you probably want to operate well below the rating just to be sure. Unless, of course, you are trying to show an English major roommate how exciting electronics is in an attempt to try to get him to change his major to engineering... or maybe he'll ask for a room transfer instead!
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David Massey is a senior engineering technician living in Kennesaw, Georgia. He is married and has six children. His technical interests include electronics and computers, amateur radio (call sign WD4OWA), electric vehicles, alternative energy such as solar, and energy storage technologies.