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Old 10th December 2003, 05:30 AM   #1
azira is offline azira  United States
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Default Transformer rating, to DC

I have an old amp that is dusty and has a bad channel. Time to take it apart. I figure I can salvage the parts. It happens to use a single IC for the amplifier so I -hopefully- can reuse that at some time.

But what I am really interested in is the Transformer. It has 5 wires coming out of the secondary, one is black and one pair is red and the other pair is blue. The red pair have 66.5V RMS accross them and the blue pair has 11V RMS accross them. I'm confident that atleast the red pair was centertapped since it was used for the amp.

What I want to do with this is convert it into a bench supply for future experiments. Is my math correct for calculation my expectations?

Secondary * 1/2 * sqrt(2) - 2*Von
66.5V RMS * 1/2 * 1.414 - 1.4V = 45.6Volts (I should expect to get +/- this or slightly less for ripple at DC)

I am guessing that the blue pair isn't center tapped because it has no continuity to the red pair. I should be able to use this as a single supply of the full voltage if I ground one end of the rectified voltage right?

Thanks.
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Danny
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Old 13th December 2003, 12:33 PM   #2
paulb is offline paulb  Canada
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This sounds correct, except I don't understand what you mean in the last paragraph.
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Old 13th December 2003, 12:47 PM   #3
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Regarding the last paragraph: yes. Given you've checked there is no connection between the blue wires and the others, the blue is an independent secondary winding. So you can use a full-wave rectifier (4 diodes) and put the + and - wherever you like. If you connect - to ground then you should get some +15.5Vdc after smoothing from the +.
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Old 16th December 2003, 09:54 AM   #4
djk is offline djk
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"I am guessing that the blue pair isn't center tapped because it has no continuity to the red pair. I should be able to use this as a single supply of the full voltage if I ground one end of the rectified voltage right?"

If you are looking for low current for opamps, you could ground one blue wire and use two half-wave rectifiers, one for + and the other for -.
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Old 16th December 2003, 04:41 PM   #5
azira is offline azira  United States
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I suppose I could use the low voltage rails for some kind of preamp circuitry and the high voltage rails for the power amplifier stage. That's another idea.
I really want a variable bench supply for testing but the local surplus store is SO expensive that I've sort of set out to make one myself. This xformer would be perfect for that as well.
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