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Old 26th August 2003, 08:29 AM   #1
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Question AC Cap for DC use

Hi,

Someone tell me that the AC cap (e.g. for motor starting etc) can be used in audio amp as a normal DC cap.

Is it possible?

If yes, how to calculate the votage spec?
(e.g. AC 500V cap = DC XXXV?)

Thank you for your help.
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Old 26th August 2003, 01:27 PM   #2
dhaen is offline dhaen  Europe
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Hi TunaFish,

I have used AC caps in DC circuits using: AC V=DC V*1.4
They are still working.....

Cheers,
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Old 26th August 2003, 08:14 PM   #3
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Default Go ahead and try it.

I hear alot of people use them. Shouldn't be any problem with breakdown, as dh says. Sonic qualities will be interesting, depending on type, oil, pp, terminations etc.

Good luck.
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Old 26th August 2003, 08:17 PM   #4
Fuling is offline Fuling  Sweden
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I use a lot of GEC paper in oil AC caps in my projects.
They are rated 370V AC and Ive run them up to 480V DC without problems.
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Old 26th August 2003, 09:54 PM   #5
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The only caps I use now are GE or ASC PP in oils for the PSU.

Pete Millett made a post on the Tube DIY board at Audio Asylum a while back after he'd emailled ASC for their opinions on the safe DC working voltages from their AC cap ratings. The 1.414 John mentions above was about the minimum in most cases.
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Old 26th August 2003, 10:02 PM   #6
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Quote:
Originally posted by dhaen
Hi TunaFish,

I have used AC caps in DC circuits using: AC V=DC V*1.4
They are still working.....

Cheers,
You've got that the other way around:

DC V = AC V * 1.414

Rectified DC voltage is the square root of 2 * RMS AC voltage.
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Old 26th August 2003, 10:06 PM   #7
dhaen is offline dhaen  Europe
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Apologies for my syntax.

I meant that a 1000V AC cap is good for 1400V DC

Cheers,
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Old 7th September 2003, 03:42 AM   #8
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Hallo,
I have some of those AC use only paper in oil caps. They are are marked outside with the warning against PCB use. What does that mean? If we move the cap ouside the chasssis, then it's safe then?
My point is: if you can put or fix the cap on the PCB careful, then why it is dangerous?? Leakage? Very unlikely, right??
cchen
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Old 7th September 2003, 03:51 AM   #9
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Wait a minute!
Those old schematics around 50's use a lot of this type of capacitor as the first line defence against high voltage in power-filtering!
But, how do they sound??
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Old 7th September 2003, 06:41 AM   #10
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I know what PCB is now!
What a dumb post I've done!
cchen
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