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Old 10th August 2005, 06:23 AM   #1
maxw is offline maxw  United Kingdom
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Question simple transformer/rectifier problem...

I know this is simple (even for me!) but I am not sure what I am doing wrong.......I am using a 20VA 2x15VAC toroid here, and using a brainGT rectifier (well half of it) hoping to get +/-22VDC similar to this diagram:

Click the image to open in full size.

But look at my measurements, I cant work out what the problem is and why I dont get +/-22VDC at the pink part.
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Old 10th August 2005, 06:25 AM   #2
maxw is offline maxw  United Kingdom
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hmmm, maybe I have to wire the purple and red wires from the transformer together....

EDIT: no, because if I had that wrong I wouldn't get 42VDC at the output of the rectifier, right?
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Old 10th August 2005, 07:32 AM   #3
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Join the white and red together but just put them in the 'spare' connector block terminal. This is your 0V. Connect purple to your white striped clear wire and yellow to your non-striped clear wire.

You will need smoothing caps on the output of the bridge and a small load on the caps in order to measure meaningful DC.
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Old 10th August 2005, 07:44 AM   #4
maxw is offline maxw  United Kingdom
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Quote:
Originally posted by richie00boy
Join the white and red together but just put them in the 'spare' connector block terminal. This is your 0V. Connect purple to your white striped clear wire and yellow to your non-striped clear wire.
That is exactly how it is connected in the pic!
perhaps its on a hard angle to see though....

Quote:
Originally posted by richie00boy
You will need smoothing caps on the output of the bridge and a small load on the caps in order to measure meaningful DC.
So its probably fine and I just cant measure it without some load?
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Old 10th August 2005, 11:13 AM   #5
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Where are the caps?
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Old 10th August 2005, 11:44 AM   #6
maxw is offline maxw  United Kingdom
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Sorted. As stated by richie00boy I needed some caps. I connected some equipment up to it and the voltages are perfect.
I had no idea you needed capacitance and load on a simple rectifier for it to work....oh well, you learn something new every day

Thanks!
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Old 10th August 2005, 06:27 PM   #7
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Hi Maxw,
Have you got the output voltage from the regs that you require?
I would expect the max (unloaded ) DC from the rectifiers to be about +-23V. When loaded this will drop to 20.5V. This will be the feed into the regs. The drop through the regs and the ripple have to be subtracted from the 20.5 to find the maximum usable DC voltage for your circuit. I would expect this to be about +-16Vdc.
To get +-22Vdc I think you need 21VacX2 windings or a little above.
My easy way to choose a transformer for a regulated supply is Vac = Vdc for medium voltage outputs. Correct the required voltage by subtracting 2volts when above 50Vdc or adding 2 volts when below 12Vdc.
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Old 10th August 2005, 10:55 PM   #8
maxw is offline maxw  United Kingdom
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Quote:
Originally posted by AndrewT
Hi Maxw,
Have you got the output voltage from the regs that you require?
I would expect the max (unloaded ) DC from the rectifiers to be about +-23V. When loaded this will drop to 20.5V. This will be the feed into the regs. The drop through the regs and the ripple have to be subtracted from the 20.5 to find the maximum usable DC voltage for your circuit. I would expect this to be about +-16Vdc.
To get +-22Vdc I think you need 21VacX2 windings or a little above.
My easy way to choose a transformer for a regulated supply is Vac = Vdc for medium voltage outputs. Correct the required voltage by subtracting 2volts when above 50Vdc or adding 2 volts when below 12Vdc.

hmmmm, well I haven't fully tested it out yet with reg psu etc. Thanks for the tip about voltage drops, I will bear that in mind and see how this goes, I may get a bigger transformer anyway and with a higher voltage
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