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Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
When looking at various designs for power supplies I notice that the filter cap has large amounts of capacitance frequently (but not always) in tens of thousands of mircofarads. My question deals with design criteria. Is there a lower or upper cut-off of such capacitance?
In rough designs where a non-specific transformer is mentioned for a 12v supply I've seen the filer cap listed as 22,000 uf; another example has one of greater capacitance. Other times I've seen them substantially smaller. I looked at the possibility of a relationship to other components but have yet to find any. It almost seems they pick a large number out of the air....(?)
If a larger level of capacitance has greater filtering capability, is there a level that should not be LOWER? By examining the design how can a person decide this capacity? Aside from smoothing the signal, is there something the filter cap does that I'm not taking into consideration?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Zb wrote ...
It is suggested to use 2200µF for every amp of current draw (fullwave rectified, 50Hz).
Voltage matters too, using your formula the output will have constant ripple voltage which may be negligible at high voltages but might cause problems at lower ones.
The answer is 1) consider maximum allowable ripple voltage for your application, 2) size your cap so that the maximum load ripple does not exceed allowable value.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
It depends on the current of the load and application of course. If you know the percentage ripple you want and the current drawn, you can use Vrpp = I / (2f * C) For example, your 12V power supply with a 10% ripple supplying 2.5A would need a 22,000 uF cap.
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
I've been wondering, if you are making say a 12V 30A power supply which might have some ripple, can you put a bunch of 10,000 uF caps, some 1 uF poly caps, and some 0.1 uF caps, that way the large ripples are taken care of by the big caps and small ripples by the ceramics?
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Is There a Size Cutt-Off for Filter caps?
