Mains CMOY (dual secondary trans) ?
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A3rd.Zero

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I have a transformer with a dual secondary which runs at 8.5V. This is the powersupply I designed for it. Any suggestions? Do I need the filter resistor? Thanks for your help!



Milo
 
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groggory

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I think you could very easily leave that resistor out.

However, I can't read what values of caps you are running. So I can't compute what the filter ripple will be. That is about as simple a power supply as can be had, but should do the trick.
 
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tangent

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I'd leave the filter resistor in. It suppresses a lot of ripple, and because you have a low-impedance output, you're not sacrificing much by having it in there. It's a good tradeoff.
 
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groggory

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Tangent, maybe you can just enlighten me....

How does a resistor supress ripple?

I understand how capacitors filter out ripple. And the usefulness of inductors. But I don't understand the use of resistors in power supplies. Could you just give me a mini tutorial on power supply design? Thanks
 
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tangent

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Quote:

How does a resistor supress ripple?


By itself, it doesn't, but in conjunction with that second cap, it forms an RC low-pass filter. f=1/(2*pi*R*C) This is good because the ideal DC power supply is a 0Hz low-pass filter.

Let's say R is 10 and C is 2700uF. That means ripple would be down -3dB at ~6Hz, and one decade above this (60Hz), ripple is down a further 6dB. Ripple keeps dropping 6dB/decade beyond this. Increase C and R to reduce ripple still further. There are practical problems with increasing them too far, though.
 
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A3rd.Zero

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The caps are 470uf and then 220uf. the Resistors ended up being 22K. I ended up using a single secondary transformer because the two winds on the dual were much to different. With some hand matched pos resistors as a voltage divider I actually am within .01 volts from the ground on each side and on the scope it looks really nice. Im quite impressed for just making this one out of scraps I had lying around. Thank you for your help guys!

Milo
 
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