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Old 24th July 2004, 06:09 PM   #1
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Default Dual rectification off single sencondary transformer question

Hi,

Can I cheat using 1 transformer or will I have to buy another one for a negative bias supply?

Click the image to open in full size.

http://www.geocities.com/super_bq/Misc/PS_Example_1.JPG

BQ

PS: please click on http://www.geocities.com/super_bq/Misc/
and click on PS_Example_1.JPG as Geocities doesn't allow direct linking.
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Old 24th July 2004, 08:11 PM   #2
SY is offline SY  United States
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"Y'know, funny looking. Yah, sure."

In any case, you can get by with a single transformer (assuming it's got the right voltages), but not the way you're drawn it. You want to ground the transformer CT, which is the common between the positive and negative rails' return leads (ie, the negative lead of the positive supply and the positive lead of the negative supply). The feed the positive rail filter with the + output from the bridge and the negative rail filter with the - output of the bridge. Your output voltage will be on the order of +/-1.4 times the voltage on either side of the CT. IOW, for a 400VCT secondary (200-0-200), your outputs will be on the order of +280 and -280.
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Old 24th July 2004, 09:27 PM   #3
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Default But require different output voltages

SY,

I understand Dual Complementary FW rectification. Thanks for your example. However, I wanted to know if it's possible to have 2 different supply voltages off the single secondary?

With a 75-0-75 transformers. I would like 212VDC (done by single bridge full wave). As you can see in the drawing, I also want a 106VDC supply (which taken off the 75 vac & CT lead to GND).

I've never seen it done this way and wonder if it presents too much loading on the single 2ndary winding of the transformer?

BQ
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Old 24th July 2004, 10:35 PM   #4
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No, it won't work. You've drawn a short circuit when the two bottom rectifiers of the bridge are conducting.
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Old 24th July 2004, 10:57 PM   #5
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Old 24th July 2004, 10:58 PM   #6
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What Charles said.

At the cost of more ripple and lower efficiency, you could ignore the CT, ground one end of the transformer, then take a simple half wave rectifier off the other end. You can use the CT and another rectifier to get other voltages.
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Old 25th July 2004, 01:18 AM   #7
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Default Thanks Guys!

SY,

moamps was kind enough to submit a drawing and shows what you are explaining. With that drawing I can get the 210VDC and 105VDC off a single 2ndary. I would not of believe you can do full wave bridge and half wave at the same time.

THANKS!!!
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Old 25th July 2004, 02:44 AM   #8
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BQ,

Consider Pete Millett's scheme for 2 rails from 1 CT winding. Wire a FW bridge to the ends of the CT 150 VAC winding, that gives you your high rail. Connect an additional diode's anode to the CT. You take the low rail from the additional diode's cathode. Both rails are full wave rectified. The grounded pair of diodes in the bridge serves both rails.

You should be fine as long as you don't exceed the capability of the power trafo secondary. With cap. I/P filters, you can access only 1/2 of the secondary's VA rating.
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