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Saturday, December 19, 2009

Linear Power Supply (Hot Glue and High Voltage)

I'm sure that you have heard the old saying "you can never have enough power supplies".... I have found this to be true on many occasions. It seems that when trying to build something new and exciting I am lacking the required voltages and currents to make it go. So whenever I get the chance to build a power supply I take it (or if I have nothing better to do).
This one started with a power supply that had been modified by someone, then thrown in the garbage. It looked like it had a bunch of good parts, including a Variac, so I took it home. The first thing that I noticed  after getting it home was it had stickers all over the front of it in what appeared to be Korean. These were above and below a bunch of holes that looked like they were for switches and lights etc... Most of the holes were empty or had obviously incorrect switches and lights stuck in them. After opening it up it was clear that the insides had been cut, drilled, replaced, burned up, replaced again, drilled, cut, glued and burned up (again). I think it started out as a power supply and was then 'improved' in such a way that the 'improvements' came bursting out.
I decided to take all the major components out and clean and check each one. For the most part it all seemed to work. I tried to figure out what the previous owner was thinking when they added / changed everything but it was such a mess inside that I figured it wasn't worth the time. There were all kinds of connections made by twisting wires together and hot-gluing them very near 'weld spots' on the steel enclosure - those fun looking weld spots that happen when you get a lot of current flowing from a wire into the metal chassis and the paint burns off and blobs of molten copper go splattering all over.
Anyway after cleaning it all up I cut some new holes and mounted some new switches and lights. The original filter section seemed to be OK so I left that alone. It needed a new cooling fan, capacitors and some other things. The Variac was not an original piece in this so I changed how it was mounted and slipped some 0.09"  thick PTFE sheet between the coil windings and the metal chassis. Oh yeah  - I also added a ground so the chassis would actually be grounded and not 'live' as it was when I got it.

There is a lot of junk crammed inside at this point but it seems safe and doesn't look like it's going to catch fire or electrocute me. I don't think I would run it unsupervised for any length of time but these kinds of unknowns and uneasy feelings just make life spicy and interesting, exactly what garage experimentation is all about.

Above is a photo of the schematic of what I ended up hooking up. I decided to tap into the AC output from the Variac so I could get some variable AC voltage if I need it someday. The DC output is just the rectified and filtered AC. I can get 0 to 200 volts DC out of it and about 0 to 140 volts AC with no load with 120 volts AC input from the wall. Nothing special but may be useful for something one of these days. The DC seems to be filtered really well as under load with my oscilloscope there is no ripple. I haven't bothered figuring out what the power factor is to get a better idea of what the real output will be but I don't care right now... it works! The easiest way to figure out it's capabilities is crank it up until it glows red inside then back off a quarter turn on the Variac.

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