[MrWild]

Sometimes too much assumed knowledge can bite you on the a$$. (chair) Such was the case as I built my power supply. My power supply consists of two 12.5v. 20a. transformers wired parallel/series - primary/secondary. I like to test each step of the way so I don’t have problems by the end of a project and no idea where the problem cropped up. The first thing I did was to figure out which of the taps on the primary side made the required 12.5v from (U.S.) house current. Once I had found the correct pair, the third was cut short.

After mounting the transformers to the sub tray of my power supply box, I wired them up. The red wire of one was soldered to the black of the other on the secondary sides (series). Grabbing my special $3.95 Harbor Freight VOM, I took a reading and found 25.6v ~AC. Great, real close to the ballpark. Then I added the Full bridge rectifier. Now I had read that I should expect 1.414 times the AC input for DC output. So grabbing the HF VOM once again, I applied it to the posts and got… 23.8v –DC. Like HUH???

So having more free rectifiers than common sense (I was given two 200v 10a.) I thought hmm, lets experiment, so I tried hooking the transformer output to the DC +/- side of the rectifier. I don’t suggest you try this at home kids. Remember that 20a the transformers put out? Well, rectifiers don’t exactly like having the inputs crossed. I don’t know why… Needless to say, I let the smoke out of the rectifier and got to see neat sparkles. Nothing dramatic, but kinda cool in an Oh $hit sort of way. I wrecked the second rectifier when I bent the terminal and it broke upon straightening. So much for the free ones, time to turn to known good ones of 600v 35a capacity.

Wiring in a known (new) good one, I again tested for volts and found THE SAME THING! What happened to the 1.414 times? I sat and thought about it for a bit and thought maybe my VOM is taking an average over time reading. The higher volts come in pulses and maybe it needs the capacitors to store the pulse. I hadn’t read about this anywhere, but it seemed to make sense. Grabbing the donated used 50v caps of 10k, 10k, 22k uf capacities, I wired blacks to blacks and silvers to silvers, then black to negative and silver positive. Powering on once again (the lights actually do a brief dim like turning on a fridge), I tried the VOM to the output and low and behold, I got my 33.6v.!!

Turning it off I ate and let it sit for about 45 minutes. I’ve read capacitors can hold a charge for quite some time, but 45 minutes? Surely they bled off after 45 minutes. Still I’m cautious around electricity and remember powering small lawnmower caps up from the high tension side of the lawnmower then tossing them to friends in Power Shop class. Not one to catch a flying cap, I decided to take a wrench lying near and shorting across the cap terminals. HOLY $HIT!!!!!! Let me tell ya that 42k uf at 33v makes a very loud (my deaf ears heard it) SNAP! Bright flash. More flash than the rectifier sparkles even. I touched the wrench a second time to be sure and got another smaller spark before the caps were fully discharged. Forty five minutes and still a YOWZER. I now have two 1k ohm, 1w resistors in parallel across the terminals for a 500 ohm 2w bleed down resistor.

Remember to be careful out there folks, electricity kills.

[2muchstuff]

On some of my hobby work I use 3-70,000uf 75 volt caps charged to 50 volts to weld brass and stainless steel wire together. I definitely makes a good pop.