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Old 19th September 2001, 01:37 AM   #1
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I'm a bit unclear on what capacitor should be used in terms of voltage rating.

On a previous post I read it was safe to run 32VDC on a capacitor rated @ 35VDC.

My understanding and from what others say - a 20% safety margin was the norm? My current project ; Pass's Balance Line Stage quotes "Substitute capacitors should have a 100+ volt rating". The power supply voltage it uses is 80V and after regulation it drops to around 60V. So why spend the extra $ when a 63V cap will do in the 60V circuit?

I know electrolytic capacitors can have a tolerance deviation of +/- 20% That is a huge variance and what does the tolerance imply if running capacitors at near 100% rated voltage level? I know capacitors explode like a bomb when exceeding their rated voltage. But I also have seen them explode within normal voltage range.

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Old 19th September 2001, 02:32 AM   #2
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The voltage rating of a capacitor is based on several things. Most importantly, it should not be lower than the expected voltage across it. If the capacitor is located after a regulator, then the voltage rating doesn't need to be much above the regulated voltage (a few percent is normally OK). If the designer is paranoid, they will sometimes set voltage ratings on the output capacitors so that a regulator failure won't kill the capacitor. If there's a crowbar circuit, or if the circuit can function with the higher voltage (with some added noise or hum), this would work just fine.

On the other hand, the filter capacitor that's located after a rectifier needs a healthy voltage margin. This is because the mains voltage is poorly regulated. A variation of 20% (up or down) is quite acceptable to the power company, and to the electricians who wired your house. There are few commercial devices that aren't designed with this in mind.
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Old 19th September 2001, 03:00 AM   #3
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First off, the +-20% margin on electrolytic capacitors isn't on their voltage rating, it's on their capacitance. It has no effect on their ability (or lack of it) to stand rated voltage. The voltage rating means what it says.
When Nelson says use 100V caps for an 80V rail, you've got to keep in mind what the standard voltages are. In general, the voltages in that range are 75V and 100V. (You can find other voltage ratings between those two, but they're rare.) Since he's already past 75V, obviously it's necessary to jump to the next higher voltage.
Where this 20% idea came from is a mystery to me. I've never seen it in a book, and all the equipment I'm familiar with (both stereo and mainframe computer) runs between 5-10% less than the rated voltage, and that's only to allow for line spikes and such. Actually, like most things, you can push it a bit if you like living dangerously. I remember one amp back in the 70's that ran a rail about 5-8% *over* the rated voltage of the power supply caps.
On the other hand, the company is longer around. Draw your own conclusions.
Anyway, for an 80V rail, I'd run a 100V cap, unless I came across a 90V or so, but that's not a common voltage. For a 60V rail, you could run a 63V as long as the rail is regulated. For unregulated voltage, that would be cutting it close, so I'd recommend a 75V cap in that case, since the common voltages generally jump from 63V to 75V.
A related point is that, given quantity price breaks, it may be cheaper to buy 10 (or 100, or 1000) of the higher voltage rating, and use them throughout the circuit, rather than buy 2 of this and 2 of that and 4 of the other one...
Yes, electrolytic caps can explode, but I wouldn't use the phrase 'like a bomb.' The only ones I've had literally explode are the smaller ones that don't have a vent (let's say 'like a firecracker'). Large computer grade caps have a rubber plug that pops out, releasing the pressure. I've had a few of those go, too, but they're pretty tame; just messy and smelly.
It's your money. If you feel that it's worth jumping two or three steps or more in voltage, go for it. Just make sure that you put enough voltage on it so that it forms, or you won't get the capacitance you're paying for.

Grey
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Old 19th September 2001, 09:46 AM   #4
AuroraB is offline AuroraB  Norway
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PSU filter Cap's, and others connected directly to unregulated PSUs, should have appx. 20% ( or more) higher rating over nominal for several reasons.
Your power company has a nominal tolerance for the power they deliver,- here in Europe +/- 10% is rather common,- but it can often be more than +10% if you live close your mains supply transformer.
Also,- usually the nominal voltage for toroid transformer are spec'ed with ref to full load, where as E/I cores and similars are spec'ed at idle. Thus, - pushing the voltage load on cap's can give a few rather unwanted surprises...........
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Old 19th September 2001, 08:41 PM   #5
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I forgot to mention about the life of the capacitor. Surely running the capacitor at 95% of it's rated voltage would not last nearly as long as running it at 80% of it's rated voltage. I've heard you may only get 30% of the life of the capacitor if you run it AT rated voltage. Comments?

For power supply filtering, I figure the 20% margin is right when using toroidal transformers in an unregulated power supply.

Biggest capacitor explosion i've seen was in my friend's stereo in his truck. He blew one of those big capacitors and the whole truck inside was filled with paper like shrapnel.
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Old 19th September 2001, 09:24 PM   #6
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+/- 20% usually means it's lacking 20%. That's how they save on materials. 470mF would be about 376mF.
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Old 20th September 2001, 08:31 PM   #7
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It doesn't really hurt to use a higher rating isn't it? From what i've heard a higher rating for an electrolytic cap would probably have a lower ESR. Comments?
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Old 20th September 2001, 11:04 PM   #8
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I had an idle moment, so just for fun I called my power company to get their input.
Nominal voltage here is regarded as 120VAC. The fellow said that if you were near a transformer that you might get a little over--no more than 122VAC--and that the lowest they allowed was 119VAC.
He was amused/horrified that anyone would think that +-20%, or even +-10% was acceptable or normal in any way.
Low voltage isn't really that much of a problem, so let's look at high voltage. 122V represents a little over 1% increase over the nominal voltage. Supposing that you were to choose, say, a 32V rail. A 1% increase would lead you to 32.32V...clearly still within margin for a 35V cap.
If in other contries the voltage varies more, then that should be taken into account, but here in the US 20% overvoltage would be 144VAC. When's the last time you heard of an outlet that measured 144V? If you're measuring wall voltage that high, you need to have a talk with your power company. There's no need to put up with that kind of nonsense.
People start talking about things like this, and it gets passed by word of mouth without anyone ever checking things. Eventually it becomes something "everybody knows" without any factual basis.
Running the cap at rated voltage should not reduce the life to 30%. By definition, running a cap at rated voltage should give you 100% of the rated life. At a guess, it sounds as though there was a substantial amount of ripple current there. Lots of ripple means someone did a poor job designing the power supply, not that the cap is at fault.

Grey
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Old 21st September 2001, 01:04 AM   #9
Geoff is offline Geoff  United Kingdom
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Grey

If your mains supply differs so little in the US then you are very lucky. Here in the UK, the statutory obligation is to maintain the mains supply between 230V +10% and -6%. In reality, I have measured line voltages, in different locations and over a period on time, of between 205V and 255V. Add to this the variation of up to 10% due to transformer regulation (no-load to full-load) and the design of power supplies, particularly regulated ones, (and the rating of capacitors) becomes far more problematical.

Geoff

[Edited by Geoff on 09-20-2001 at 08:15 PM]
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Old 21st September 2001, 02:00 AM   #10
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Geoff,
As I said above, if you live elsewhere, take local conditions into account. That's part of why companies have Export models of their equipment which vary internally from the domestic versions.
The power I have at my current house is the worst I've ever had, but even then it only varies by about 2V, or roughly 1%. I'm assuming that it's load related, as I'm nearly at the end of the distribution line. In my previous house (same power company, and only about two miles away as the crow flies), the power was rock steady...always. I don't remember it ever varying as much as .5V. I don't remember where the transformer was in relation to the house.
Given that the weather here is mild now (mid-September, highs in the mid to upper 80s, lows in the upper 50s to mid 60s...all degrees Fahrenheit), I ought to measure my voltage to see what it's doing with minimal load on the line from air conditioning, etc. I'll try to remember to do so when I get home.
Naturally, this is a separate concern from noise on the line, which we've poked at elsewhere. The AC line voltage I have now is the worst I've had in that aspect, too, but excepting really big spikes, then it wouldn't enter into the voltage question. Even then, you'd be better off tackling the problem with MOVs, caps (small ones across the incoming AC line, not the bulk caps), or fuses, rather than going to something like 10kV caps on the off chance that it might save your bacon in the event of a lightning strike.

Grey
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