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Choosing capacitors voltage rating
Choosing capacitors voltage rating
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Old 15th July 2019, 11:26 AM   #11
Vovk Z is offline Vovk Z  Ukraine
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Choosing capacitors voltage rating
Quote:
Originally Posted by shovon View Post
Because I had previous experience with 16v capacitors in series. with a 20vac (20-0ac) single power transformer with fullwave rectifiers bridge with 2 16v 6800uf capacitors in series , I was getting around 28.6 DC voltage but after 20-25seconds later both of those 16v capacitors got exploded.
80VDC cap for 50VAC nominal voltage is ok. 100V - could be ok too .
I usually prefer to choose the higher one, because it's more reliable (but it's my own problem).
Let's do some math.
50VAC nom *1.1 *1.41 = 77 VDC max (it's maximum VDC, but it won't be so high in average).
You should test your mains voltage if it is usually at a higher limit (118-120V or higher) then you really can consider 100 VDC capacitors. Else, if it isn't so high - 80V will work ok.

And don't use caps in series except for very high voltages (when really there aren't such nominals - it's the only reason. Of cause don't forget to put parallel resistors with caps to align voltage).

Last edited by Vovk Z; 15th July 2019 at 11:29 AM.
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Old 16th July 2019, 02:39 AM   #12
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by benb View Post
Do not put capacitors in series to make a capacitor of twice the voltage rating. Due to several reasons, mostly gross differences in electrolytic capacitance values, they may not charge up to the same voltage.
I have seen it done but they put balancing resistors across the caps.
Otherwise one cap charges up more than the other.
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Old 16th July 2019, 06:26 AM   #13
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by nigelwright7557 View Post
I have seen it done but they put balancing resistors across the caps.
Otherwise one cap charges up more than the other.

Bleedeer resistors, or bleed-down resistors, are used in many power supplies.
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Old 16th July 2019, 10:03 AM   #14
DF96 is offline DF96  England
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There is a school of thought which says that series capacitors will adjust their share of the total voltage so that they each draw the same leakage current, hence there is no need for balancing resistors. I am not entirely convinced by this.
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Old 16th July 2019, 11:18 AM   #15
kodabmx is offline kodabmx  Canada
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Choosing capacitors voltage rating
Much like the "debate" on whether or not you "need" to use grid stoppers, it's a 1 cent part. Don't be cheap and lazy, include it. Even if it's not needed, it won't hurt either.
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Old 16th July 2019, 02:39 PM   #16
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by DF96 View Post
There is a school of thought which says that series capacitors will adjust their share of the total voltage so that they each draw the same leakage current, hence there is no need for balancing resistors. I am not entirely convinced by this.

Indeed, that "school of thought" is the self-proclaimed internet guru's without any education to back up their claims.
These days the phrase used is "fake news".
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Old 16th July 2019, 10:51 PM   #17
TonyTecson is offline TonyTecson  Philippines
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Choosing capacitors voltage rating
Quote:
Originally Posted by DF96 View Post
There is a school of thought which says that series capacitors will adjust their share of the total voltage so that they each draw the same leakage current, hence there is no need for balancing resistors. I am not entirely convinced by this.

that school of thought is valid if the two caps in series are exactly alike in capacity and esr, otherwise, they will not balance out..for safety's sake i will never do that without the resistors...

i saw some of these work out when repairing atx psu with half bridge topology, the caps were supposed to share 300 vdc to 150 each, but did not, so what i did was to measure them in an esr meter and true enough, they were not the same as they were labeled, so replacing them with identical caps, restored balance...

theory is fine until some factors come in to upset them...
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Old 17th July 2019, 02:19 AM   #18
Rick PA Stadel is offline Rick PA Stadel  United States
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Yep, you gotta have quality data on leakage variations, both part to part and lot to lot. Then choose your balancing resistors to swamp that by 3:1 or more. Big caps often have big leakages; one shipment could be better or worse by 10 or 15:1.

Still better (already mentioned / credit due others), only use them in series if there's no other option.

Regards

Last edited by Rick PA Stadel; 17th July 2019 at 02:25 AM.
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Old 17th July 2019, 02:25 AM   #19
TonyTecson is offline TonyTecson  Philippines
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Choosing capacitors voltage rating
ecaps are notorious for tolerance...
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Old 17th July 2019, 03:10 AM   #20
Rick PA Stadel is offline Rick PA Stadel  United States
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O.K. - you got me -- capacitance differences have to be considered, too. Thanks, TT.
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