Couple new "toys". What to do?

[BrianG]

I got a 750w and a 1000w UPS system from work the other day where the batteries are dead. Apparently, it's cheaper to replace the whole unit than to replace just the batteries. Go figure. Anyway, they were going to throw them away anyway, so I brought them home and stripped them for parts I can use (heatsinks, coils, etc).

Each of these suckers has a pretty large power transformer in it with ~16GA 120v wires and ~10GA secondary wires. Normally, the battery hooks through a switching circuit to provide ~12v "AC" to the transformer and then steps up the voltage to 120v AC. Since these use 12v batteries as the power source, I figured I could hook it "backwards" and get ~12v from 120v.

The 700w one's output was 36v AC (with a center tap) unloaded. Just to see what it can do, I hooked a 0.75ohm load resistor to it. The output dropped to 29v AC. That's 38.66A, which equates to an output impedance of 0.18ohms. OK, that's a little higher than the 750w rating. But with a full-wave rectifier, I should still get a solid 16v DC @ 40-45A. Pretty good candidate for a decent linear 14.4v PS!

The second transformer (1,000w) was a little larger with a little bigger windings. The output of this one was 15.9v AC unloaded with no center tap. With a 0.75 ohm load, the output dropped to 15.1v AC, which equates to 20.1A. Calculated output impedance is 0.04 ohms. With a 0.46 ohm load, the output was 14.6v (calculated 31.8A). Even though this one is capable of higher power and has lower output Z, the output V starts out too low (unloaded) to be useable for a PS at heavy load unless I make it a strict 12v PS. Then, I could use this to its full potential up to 65-70A.

Cool, eh? Or is it just me that thinks it's cool?

[_paralyzed_]

That's pretty cool. I wish I had your knowledge, i'm still trying to remember ohm's law. It's coming along..... Those would be some robust power supplies, build 'em and sell 'em on here, we're all power hungry!

[BrianG]

I do plan on making supplies out of them, but you ever try to regulate 50-75A? Talk about lots of transistors! Normally you'd need to parallel several to be able to handle the current required. However, transistors don't parallel well unless you take precautions and even then it's not the best. FETs would be the logical choice for ease of paralleling, but not sure if FETs can be used effectively in this way.