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Old 2nd March 2011, 03:39 PM   #1
rookie amp builder is offline rookie amp builder  Canada
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Default ripple capacitors

hello all again

i have a quick question about capacitors. I'm STILL building the leach amp and have a bunch of different caps i want to use for the power supply

2 @15000uFD 100V
4 @33000uFD 50V
10 @680uFD 250V

now my question is these are to be used for + & - 90volt rails. id like to use all of these caps (just cause i have em). now when i wire them all up my plan is to use "1" "2" & "5" per rail, so im going to wire 2 of the 50v in series to make single 100v cap, from what i understand about caps, in parallel you get the same voltage and double the capacitance, in series you get double the voltage and HALF the capacitance, is this correct??? meaning to say once this is all said and done will my total capacitance per rail be 34900uFD and is there any benefit/fallback to this arrangement.

Thank you again
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Old 2nd March 2011, 05:15 PM   #2
Boofers is offline Boofers  Canada
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Location: Ontario
Yes man.

Caps in parallel - Total capacitance is Cparallel = C1 + C2 + C3. The voltage rating is what you read on each cap.

Caps in series - The total capacitance is found by Cseries = 1 / [(1/C1) + (1/C2) + 1/C3)] Note that this formula is the same format as calculating resistance in parallel.

With series voltage it gets a bit more tricky. Yes you add the voltage ratings together to get a total voltage. The danger is if you use different values of capacitance in series you can potentially exceed the voltage ratings (because with different capacitance values you drop a different voltage across each cap) If you are using 2 x 33mF 50V caps, you should be ok, but as a general rule you use caps with voltage ratings 20% to 25% higher than your max voltage. With 90V rails, 100V caps are cutting it a bit close.

I generally don't put caps in series becasue the series resistance and inductance are added together, but hey sometimes you use the parts you have right?

Check out this cool power supply design program. You can maybe use it to explore different cap configurations.

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Old 2nd March 2011, 05:28 PM   #3
agdr is offline agdr  United States
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ripple capacitors
Here is exactly what Boofers said in an app note from Daewoo from a google search. Check out item #15, series caps, pg 3.

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Old 2nd March 2011, 07:23 PM   #4
Boofers is offline Boofers  Canada
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That is a nice article agdr.

It is interesting that the formulas for voltage across series caps show that the series capacitor with the smallest capacitance will have the highest voltage drop.
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Old 3rd March 2011, 12:07 AM   #5
artu is offline artu  Chile
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Parallel capacitors will also decrease ESR so the resultant cap. with be more fast, it should handle better fast demands of current transients.

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Old 3rd March 2011, 09:39 AM   #6
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
Buy the correct capacitors for your project.
If your supply rails are a nominal +-90Vac then you need >=110V capacitors for the smoothing. 150V and 160V are available but they get much more expensive the further above 63V they are.

Select a transformer that allows 100Vdc caps to be used safely. probably <=62Vac
regards Andrew T.
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Old 5th March 2011, 04:57 PM   #7
john_ellis is offline john_ellis  United Kingdom
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Andrew T's advice is worth heeding.
But you could also add parallel resistors to your caps to force the voltages to be the same. They should be dimensioned so that the RC time constants are the same or you could still get a dynamic voltage division which causes one of them to get to be to high.
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