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Old 27th July 2001, 07:49 PM   #31
Eric is offline Eric  United States
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Petter: thanks for the clarification!
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Old 14th August 2001, 01:46 AM   #32
haldor is offline haldor  United States
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Quote:
Originally posted by William
Dear Grey,

Thanks for your comments. I did have 4 rectifiers in my pre-amp before (that was before the old transformer failed). Since the replacement transformer has a centre tap, so I used two rectifiers instead as it would still have full-wave rectification. I did connect up correctly to produce 350V d.c. Yet, the result ended up to be unexpectedly poor. A friend of mine said that I should use 4 to make a bridge and he believed it would be better to have 4 working instead of two. As I have no electronic background, I just could not figure out the reason.

William
Hi William,

There are two ways you can connect a center tapped transformer with two diodes. One way will end up with one full-wave rectified DC output of half the voltage you would get with a bridge rectifier, and the other way you will end up with half-rectified +/- DC outputs.

Bear with me, here come some really bad ASCII schematics.

Full-Wave Rectified single output supply

Diode
{-------|>|-------|-----|------0 + Supply
}.................|.....|
{.................|.....- Filter Cap
}--0 Center Tap...|.....-
{....Ground.......|.....|
}.................|.....|
{-------|>|-------|.....0 Ground
Diode


Half-Wave Rectified Dual +/- Supply

Diode
{-------|>|---|---0 + Supply
}.............|
{.............- Filter Cap
}.............-
{.............|
}-------------|---0 Center Tap, Ground
}.............|
{.............- Filter Cap
}.............-
{.............|
{-------|>|---|---0 - Supply
Diode

The only way you could have got the same results changing from a bridge rectifier to 2 diodes would be if you doubled the secondary voltage of the transformer and only needed one supply output. You will get a slightly higher output voltage than before since you are now only have one diode drop in the circuit instead of two.

If you have + and - supply outputs then what you did was change from full-wave rectified to half-wave rectified which means that your filter caps are now undersized for the amount of ripple they going to see.

Phil

Second try. Drat! That didn't work very well. The spaces in my ASCII schematics just collapsed down. I have revised it to use ... instead of spaces. Just ignore the ....'s in the drawing.

Phil



[Edited by haldor on 08-13-2001 at 08:51 PM]
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