Author Topic: Bench Power Suppy question  (Read 1449 times)

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gizmo

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Bench Power Suppy question
« on: July 01, 2005, 12:03:40 PM »
Not really a renewable power question, more of an AC theory question for anyone in the know.


I'm building a new variable power supply for my test bench. My existing suppy is 1.5vdc to 25vdc, 4 amps max. Great for testing windmill regs and chargers. The new one has a dirty great transformer and should supply 15 plus amps. Common design, LM317 driving a bunch of 2955 power transistors. The transformer I have is from an old UPS, and has an output of 28 volts AC. After a bridge rect and filter caps this ends up as 39v DC. A little high, slowing cooking the 2955's. I need to drop this voltage down to about 28 volts DC, or about 19 v AC from the transformer.


I have another transformer I can use, but its output is 15vac, giving 21vdc after rectification. Not quite high enough. On its primary there is another set of windings, output of about 30vac. If I connect this winding in series with the existing primary, but reversed phase, I get my 18vac on the secondary. Wonderfull. But is this a safe way to do this. Primary current is 0.3 amps using the standard primary, no output load. If I add the reverses winding in series with the primary, total primary current jumps up to 0.6 amps, no output load. So the transformer is drawing more power at idle. I let it run for a few minutes then tested to see if it was getting warm, but I couldn't detect any heat at all.


Would it be safe to use the transformer like this, or will it eventually start cooking internally. Both the primary windings use the same thickness wire.


Glenn

« Last Edit: July 01, 2005, 12:03:40 PM by (unknown) »