|One fine day near the tail end of summer 2011 in Seattle, I decided to build a little +12 / -12 volt DC linear bench power supply to use for some analog synthesizer experiments. I harvested one of the many dead PC power supplies from the basement for the metal box, but before I could throw the guts away (switchers are electrically noisy, and I wanted a quiet little linear one) I noticed that it had a whole bunch of parts on it that would work just fine in a power supply project. Makes sense, since it had been, in its previous life, a power supply.|
|I prototyped the +12/-12 supply on a breadboard. It's nothing more than a full wave bridge harvested off another dead PC power supply, a couple of capacitors, and 7812 and 7912 regulators being fed from a 22 VAC CT 2A transformer that I found in a line lump power supply. It probably won't source more than 150ma or so, but that should be plenty for my purposes. By the way, heavy wall warts and line lumps contain 1 to 2 amp iron core transformers. If the connector on an AC line lump has 3 or more pins, there's a better chance that it contains a center tapped transformer.|
|The single-sided dremel method has been serving me well for fast, simple, no-chemical circuit boards. I just draw the board right out on the copper with a pencil until I've got it all worked out, then dremel away. I went ahead and added a diode for a half wave, another capacitor, and a 7805 regulator to get +5 V DC while I was drawing out the board.|
No doubt you just looked at that picture with the 12.08 volts on the meter and
wondered what that junk on the right is and why the nutball who wrote this mess
left it in there. Here's the dope: After I removed the power supply guts from the box,
I traced from where the line voltage entered the board and saw that it went
through some filtering, then entered a full wave bridge and some smoothing
capacitors rated at 400 volts. It was all laid out along a strip about 1 inch
wide along one edge of the board, and I could see that if I were to saw the board
down a line to isolate that part of it, I'd wind up with a high voltage DC power
supply of some kind. Like any normal person who hoards dead ATX power supplies,
I can't resist an easy experiment involving high voltage DC of an unknown value,
so I excised the interesting portion of the ATX power supply and powered it up.
When I poked the capacitors with my voltmeter leads, I discovered that I had a 340 volt DC power supply. Disclaimer time: Don't attempt this. 340 volts can kill you. It's direct to the main line too; there's no isolation transformer, so after killing you, it'll burn your house down using your flaming corpse as the igniter. So anyway, I drew the schematic out and stared at it for a bit, then flipped the 115/220 volt switch to 220, telling the remains of the board a big fat lie: That it had retired to the south of France and was having 220 volts AC being applied to it. The DC output voltage was halved to 170 volts. 170 volts is perfect for lighting up Nixie tubes!
Thus did the Electronic Gods spork to me: if I ever build an analog synthesizer, it should probably include at least some Nixie tubes. What are Nixie Tubes you ask? They are soft, warm, glowing conveyers of numeric information, more beautiful than my camera can render. Here is a Nixie tube display on a frequency counter measuring an A note from one of my first oscillators. Sadly, that oscillator is too awful to mention further, but I did want to mention the basically found-for-free source for 170 VDC power supplies that I stumbled across to anyone that might find them handy. Happy hardware hacking to all!