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Old 14th November 2002, 06:30 PM   #1
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Default Power Supply Capacitors

Let's say I want to put 40000uf of filter caps on each voltage rail. I need them to be 100v caps. Assume I have these two options available:
1. one 100v, 40000uf cap for each rail
2. two 50v, 40000uf cap in series for each rail

Option 2 is cheaper, which is the direction I'm leaning.

Are there any disadvantages to using two 50v caps in series to make a 100v cap?

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Old 14th November 2002, 06:58 PM   #2
Variac is offline Variac  United States
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I don't think 2 50v in series makes a 100v cap
I don't think they work this way.
Proceed cautiously
I don't know
any more
Mark
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Old 14th November 2002, 07:01 PM   #3
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Equation for series caps capacitance:

C = 1 / ( 1/C1 + 1/C2 +... )
so 2 40000uF 50V caps in series = 20000uF 100V

Parallel caps just add.

You would need to put resistors across the series caps to equalize voltage to avoid making a bomb. (Capactors have unequal leakage currents.) I would just buy caps with proper ratings.

This has been done before, but mainly for things like 900V where electrolytics aren't available and high capacitance is needed. I have use this to put some 800V .56uF film caps in series / parallel to make a 1600V .56uF cap. I never build normal projects.
What are you building?

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Old 14th November 2002, 07:07 PM   #4
UrSv is offline UrSv  Sweden
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First you have to make sure that the caps are charged equally or you may exceed the voltage rating of the individual capacitor. This is obstacle number one.

Second, when connecting caps in series they behave exactly the opposite of resistors that is when you connect then in series the resulting capacitance will be smaller than the largest cap. The formula for series caps is as for parallelling resistors:

1/Ctot = 1/C1 + 1/C2 ..... 1/Cn

In this case you end up with half the size as they are equal and you need to invest in twice as much.

Go for the correct option. Choose the right voltage and buy a size that when parellelled gives you the total capacitance needed a nd the best price/performance.

Parallelling caps gives you a total of the sum of the individual caps so 4 times 10.000 uF is "the same" as 40.000 uF single cap "all other things being equal".

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Old 14th November 2002, 07:37 PM   #5
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Good call Darrell and UrSv, I forgot that series caps use the same equation as parallel resistors. Shame on me...

Sounds like I need to find some 100v caps. Know of any good sources?

I am planning to build one of Anthony Holton's 400w symmetrical mosfet amps for some HT subwoofers.

It will be my first project, so I'm sure I'll have lots of questions. There is so much good info on this forum, but sometimes it's hard to find. Thanks

awhiteguy
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Old 14th November 2002, 07:43 PM   #6
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Supplier for caps:

http://www.mouser.com
http://www.digikey.com

Request a catalog from both or download the pdf. You can get about anything from one of the two.

I think this amp may be a bit much for a first project. I think it's best to learn with a smaller cheaper amp before you smoke alot of expensive parts. (I have) I started with the pass labs zen.

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Old 14th November 2002, 08:44 PM   #7
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Default Re: Power Supply Capacitors

Quote:
Originally posted by awhiteguy

2. two 50v, 40000uf cap in series for each rail

Option 2 is cheaper, which is the direction I'm leaning.

Are there any disadvantages to using two 50v caps in series to make a 100v cap?
To make it work properly, you must have charge equalizing resistors across the caps. This distributes the voltage.

http://www.elfa.se/elfa/produkter/se/20/2021348.htm

The resistor above fits "computer grade" caps from RIFA. They can be bought but it works perfectly with any kind.
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Old 14th November 2002, 08:53 PM   #8
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Another idea:

Use a center tapped transformer and connect the center tap to the connection between the two 50V caps. This will also maintain equal voltage. Thats how my ps works except I have the tap grounded to provide +- 35V.

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Old 14th November 2002, 09:02 PM   #9
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I don't think I can do that if I am using the center tap as ground for +-70v. Is this correct?
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Old 14th November 2002, 09:11 PM   #10
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Quote:
Originally posted by awhiteguy
I don't think I can do that if I am using the center tap as ground for +-70v. Is this correct?
I didn't realize it was already wired that way. You would have to use 2 CT transformers, so 100V caps should be the simplest option.

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