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Old 21st October 2011, 08:17 PM   #1
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Default Filter Cap Voltage Rating

Is there a rule of thumb or best practice for choosing the voltage rating on a supply capacitor? I have two transformers, one puts out 36VAC and the other 53VAC. Would a 63Vpp rated cap do or would I have to go to 80Vpp for the bigger supply?
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Old 21st October 2011, 08:27 PM   #2
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The cap should be ok with either output. I usually go with something like 20% to 50% "overhead" but I'm not sure how that fits with any "official" RoT or best practice. BTW, power supply filter caps are usually rated for WVDC (working volts direct current), not Vpp.
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Old 21st October 2011, 08:40 PM   #3
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Originally Posted by sofaspud View Post
The cap should be ok with either output. I usually go with something like 20% to 50% "overhead" but I'm not sure how that fits with any "official" RoT or best practice. BTW, power supply filter caps are usually rated for WVDC (working volts direct current), not Vpp.
Great! Thanks. Is WVDC the rated output voltage or the peak voltage (i.e. top of the 120hz sine curve)? I know they are fairly close, just trying to get a picture in my head of what manufacturers have in mind when they rate a cap.
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Old 21st October 2011, 09:00 PM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by somejohndoe View Post
Is there a rule of thumb or best practice for choosing the voltage rating on a supply capacitor? I have two transformers, one puts out 36VAC and the other 53VAC. Would a 63Vpp rated cap do or would I have to go to 80Vpp for the bigger supply?
AC volts are rms values, don't forget the √2 ratio between that value (a pure mathematical construction), and the real peak value.
In addition, a transformer voltage is specified fully loaded, and for the nominal mains voltage. Add 10% for unloaded condition, another 10% for mains variations, and you arrive at 1.414*1.1*1.1=1.71.
With 36VAC, that's 61.6V. 63V is just sufficient, with no additional margin.
With 53V, you're completely out, and 80V is not sufficient either, you have to go for 100V.
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Old 21st October 2011, 09:09 PM   #5
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I'll confirm the ratings with some meter measurements before I proceed. The 53v transformer was pulled from a unit that had 80V rated caps.
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Old 21st October 2011, 09:13 PM   #6
jcx is offline jcx  United States
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Bad advice 63 Vdc rated Cap is not enough for 53 Vac

53 VAC * 1.414 = 75 Vpeak

xfmr also have "regulation" issues - they give a higher V unloaded so that they meet their VA spec under load

then there is line variation - design for 10% high line V

I would use 20% over the peak rectified V as a starting point for the Derating calculation, usually another +20-25%

stacking these factors you could make a rule of thumb of using Cap Vdc rating of ~2x the xfmr Vac #

so I would use 100 Vdc rated Caps with the 53 Vac xfmr, 75-80 with the 36 Vac xfmr

If you are using it center tapped for a dual supply then each polarity is only 1/2 the peak and you could use the 63 V rated C

I have seen many underated C in commercial supplies - it can work - but it is bad design

Last edited by jcx; 21st October 2011 at 09:20 PM.
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Old 21st October 2011, 09:29 PM   #7
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Thanks, Elvee. I assumed the outputs mentioned were open circuit and not the rated voltages. 53 volts is sorta odd to me for a transformer.
somejohndoe, ideally the cap will charge to the peak voltage and stay there, so use that value + the safety margins spelled out in Elvee's post. As a diy'er, I have to rely on the manufacturers' ratings, ie that "stay there" voltage. Including worst case and a little CYA margin is a necessary habit when selecting virtually any component.
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Old 21st October 2011, 09:55 PM   #8
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The xformer is a CT-ground with each side being 53v. Looks like 80V might be running close to the wind. I might double up a couple of 10000uF 100V for each rail.
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Old 22nd October 2011, 08:22 AM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by somejohndoe View Post
The xformer is a CT-ground with each side being 53v. Looks like 80V might be running close to the wind. I might double up a couple of 10000uF 100V for each rail.
Today's capacitor are much less tolerant on overvoltage than before: 35 years ago, you could go away with murder, apply a quasi- permanent overvoltage of +25% to a part, and it would happily survive.

If you try the same kind of trick with a modern cap, be prepared for a loud surprise.....
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Old 22nd October 2011, 11:26 AM   #10
AndrewT is offline AndrewT  Scotland
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JCX is right. Listen and learn.

One cannot do DIY electronics unless one learns how to do the arithmetic.

Otherwise, all one can ever aspire to, is copying exactly someone else's project, which is hardly DIYaudio in it's widest terms.
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