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Old 26th September 2001, 02:06 AM   #1
Chad is offline Chad  United States
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I'm building an http://www.aussieamplifiers.com N-channel amp it uses +70 and -70 volt dc rails. Can I use PSU caps rated at 75 volt dc with a surge rating of 95 volts? Is this cutting it to close?
Thanks in advance, Chad
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Old 26th September 2001, 02:35 AM   #2
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Chad,
The better working voltage rating for the capacitors would be 20% in-excess of rail votlage.( ie 84V or higher, my choice would be 100V for its difficult to find capacitor with working voltage rating between 80V and 100V and surge voltage even greater).

- XL
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Old 26th September 2001, 08:45 PM   #3
JBL is offline JBL  Canada
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Generaly you do like Xavier said but you can do it with 75v cap. There life are going to be shorter but i have some old amp here(maybe 10 year) that have their capacitor at their limit(50v rail 50v cap) and they are still working fine.

Putting cap in parallel help to get longer life(I think).
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Old 26th September 2001, 09:00 PM   #4
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I think you won´t have any problems with the caps. They always have a little headroom anyway.
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Old 27th September 2001, 01:19 AM   #5
Chad is offline Chad  United States
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So if I put two 50v caps in series across the voltage rail it would give me 100v power handling? What would happen to the cap value, would it stay the same or be cut in half? A buddy of mine has some extra 10,000uf 50 volt caps if I series wired them would they be 10,000 uf 100v or 5,000uf 100v. Thankyou all for your help I've gotten myself in over my head with this project, so now I'm trying to save my investment and get it working.

Chad
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Old 27th September 2001, 01:32 AM   #6
Geoff is offline Geoff  United Kingdom
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Two 10,000uF 50V caps in series would be the equivalent of one 5,000uF 100V cap.
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Old 27th September 2001, 01:36 AM   #7
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Chad,
If you run the caps in series, the capacitance will be cut in half. However, you *must* use a resistive voltage divider to assure that the caps each see exactly half the voltage. Place something like a 100k resistor (the value isn't critical) in parallel with each capacitor. A 1/2W will be sufficient.
The reason is that the voltage will divide across the caps in proportion to their actual capacitance. Given that electrolytics are notorious for being +-20% on their capacitance value, the results will be unpredictable, and could easily exceed the voltage rating for one of the caps. The resistors will force the voltage to divide evenly.
Tube folks do this all the time. In the first leg of the Pi filter in my amps, I've got a couple of 350V caps in series (with resistors, of course) in order to create an effective 700V cap bank for a 575V rail. I could have used two 300V caps, but 350V parts came my way cheaply, and I couldn't say no.

Grey

[Edited by GRollins on 09-26-2001 at 08:38 PM]
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Old 27th September 2001, 01:38 AM   #8
drewd is offline drewd  United States
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The capacitance of capacitors in series behaves like resistors in parallel, so the capacitance would be (c1*c2)/(c1+c2). When the capacitors are in parallel, the capacitances add.

In series, the voltage rating will double, while in parallel, they will remain the same.

Thus, your 5000uF capacitors in series will be the equivalent of a 2500uF capacitor at 100v.

In parallel, the capacitors will be the equivalent of a 10,000uF capacitor at 50v.
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Old 27th September 2001, 02:26 AM   #9
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Though u may connect two capacitors in series with divider network be careful not to exceed the ripple currents of the capacitor. Also note that ac ripple voltage will increase because of a drop in capacitor value which may lead to hum problems.

-XL
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Old 27th September 2001, 06:57 AM   #10
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Using resistors as a voltage devider is needed to be sure one of the capacitors doesn´t get excess voltage across it. It´s a good idea to keep your caps in safe operation.
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