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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 29th Aug 2007, 7:17 pm   #1
Andy Day
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Default capacitors in series - pitfalls?

Hi folks. I have on occasions stuck two caps in series/parallel when I haven't had one of sufficient working voltage, perhaps more by luck than judgment with no apparent problems. Ie 2 lots of 2 X 0.1 250v in parallel, and then each pair in series - if you see what I mean!

I'm doing up a Philips 462A, and have made some 0.05 500v caps by sticking two 0.1 250v in series inside the waxed paper casing.

Can anyone think of a situation where this is unsuitable, messing up phasing of signals or something I've not thought of?
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Old 29th Aug 2007, 8:12 pm   #2
Ray Cooper
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Default Re: capacitors in series - pitfalls?

People keep asking variants of this question, don't they? Perhaps the answer needs archiving. Try this for size:-

No, two 0.1uF 250VW capacitors in series do not make a 0.05uF 500VW unit. True, they make a total of 0.05uF in capacitance: but you can't claim that they'll work properly at 500V. It's all down to how the available voltage distributes itself between the two units - in fact, any leakage currents present in the capacitors can entirely kipper the desired effect.

Leakage current can be thought of as being produced by a high-value resistance in parallel with each capacitor. Imagine, for example, that one unit has an apparent parallel resistance of 100megohm (not bad), whereas the other unit is better at 500megohm (much better).

In DC terms, what we have here is a potential divider of 100M and 500M: total 600M, but if we put 500V across the combination, you get 500*(500/600) volts across the high-resistance unit, and 500*(100/600) volts across the other. My calculator says that these work out to 416.7volts and 83.3 volts, which add up to 500volts. Good.

So the 'low-leakage' capacitor is being heavily over-stressed and is likely to fail: if it does, and goes short-circuit as is quite likely, the other capacitor will soon follow it.

These are only example figures, of course: modern capacitors are likely to be much better than this, but the point is that the leakage resistances of the two will likely be different, so the voltages will not split evenly.

The usual 'fix' for this problem is to swamp any leakage resistance by connecting equal, but much lower, values of fixed resistor across each. There are places in which you can get away with this sort of trick, and other places where you can't - coupling capacitors between valve stages being an obvious example.
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Old 30th Aug 2007, 10:00 pm   #3
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Default Re: capacitors in series - pitfalls?

Excellent answer Ray.... similar to imbalance problems in charging series connected batteries
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Old 31st Aug 2007, 2:02 pm   #4
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Default Re: capacitors in series - pitfalls?

Thanks for the answer guys, I know the theory about differing leakage causing the series caps to form an unequal potential divider.

Being the pragmatist I am I can only say I've done this on quite a few occasions with no problems. My question was perhaps not presented as accurately as it could have been. I was really querying problems with ac conditions.
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Old 1st Sep 2007, 10:14 am   #5
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Default Re: capacitors in series - pitfalls?

Quote:
Originally Posted by Ray Cooper View Post
People keep asking variants of this question, don't they? Perhaps the answer needs archiving. Try this for size:-
Ray's comprehensive answer deserves not to be lost.

I'll add "stickyness" - especially appropriate for waxies - so it won't get forgotten!

Regards,
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Old 11th Apr 2008, 11:48 am   #6
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Default Re: capacitors in series - pitfalls?

Its always a good idea to bridge each cap in a series network with a very high value resistor, It of course will depend on what and where the cap is doing. I would beware though doing this to a coupling cap in valve circuits.
In other circuits where the voltage is between 200 - 500v about 6.8 meg per cap.
Trevor
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Old 17th Dec 2008, 5:44 pm   #7
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Default Re: capacitors in series - pitfalls?

Just a warning that electrolytics in series for HV smoothing need a much lower resistance across each one. Certainly not more than 100k and watch the wattage!
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Old 17th Dec 2008, 10:53 pm   #8
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Default Re: capacitors in series - pitfalls?

Something interesting to consider when thinking about the risks / desirability of using two high capacity electrolytics in series for reservoir / smoothing at mains and above voltages is that most switch mode PSUs for computers etc use just such an arrangement after the bridge rectification input . Why that is , I could not say for certain . However the manufacturers of these devices seem happy enough with the safety and performance of such a configuration .

Food for thought at least ....

Cheers ...
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Old 18th Dec 2008, 1:07 am   #9
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Default Re: capacitors in series - pitfalls?

This balancing with parallel resistors is used amateur radio linear amplifiers where you have perhaps 2,500 volts being smoothed by a series of say 350 volt capacitors, imagine the efect of the voltages being very unequal!
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Old 18th Dec 2008, 10:31 am   #10
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Default Re: capacitors in series - pitfalls?

Quote:
Originally Posted by enquirium View Post
. . . most switch mode PSUs for computers etc use just such an arrangement after the bridge rectification input . Why that is . . .
An SMPSU - as used in PCs - are frequently designed to be powered from 230v a.c. or 115v a.c. Often a switch is fitted to change from one to the other. This switch changes the rectifier / capacitor configuration: when on 230v setting, it becomes a conventional bridge rectifier with the caps. in parallel; when on 115v, it becomes a voltage doubler, with the caps. in series.

Al / Skywave.
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Old 31st Mar 2010, 10:21 pm   #11
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Default Re: capacitors in series - pitfalls?

Ray's description of the voltage ratio across series connected capacitors is pretty well spot on. It is not just the leakage current that decides the voltage split. The charge(Q) on each cap is also controlled by the actual capacitance of each individual device. The formula Q=CV dictates the actual votage each capacitor. I won't bore you with lots of calculations here but if you connect 2 identical 100 uF caps in series each cap will have exactly half the applied voltage across it, if you connect a 100 uF +20% in series with a 100 uF -20% the voltage across the lower value will be greater.
An added advantage of using resistors to equalise the voltages is that the resistors also act as a bleed to discharge the supply at switch off. In the back of my mind there is a way of connecting a neon to act as a high voltage on and confirm that the bleed chain is intact.
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Old 10th Apr 2010, 6:53 pm   #12
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Default Re: capacitors in series - pitfalls?

Ummm As a field TV tech who was graded on productivity, I connected capacitors in series for years and never had a problem... Now they were usually electrolytics and I would not stick a pair of 250v in a 500v circuit, but no qualms at all about using a pair on 400-450v...

Tom
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Old 10th Apr 2010, 8:12 pm   #13
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Default Re: capacitors in series - pitfalls?

Very large electrolytics are routinely used in 3-phase inverters to get a 580V dc bus for the transistor bridges. Normally, they use series-connected pairs of 400V rated caps, they always have so-called 'voltage-equalizing' resistors across them, sometimes as large as 27W or so on caps of 4700uF 400V or more each. I've not seen many large-capacity electrolytics used on their own (or readily available) at this sort of voltage.

I suppose modern film capacitors at 0.1uF or so have very little leakage at all.
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Old 21st May 2010, 11:16 am   #14
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Default Re: capacitors in series - pitfalls?

Quote:
Originally Posted by McMurdo View Post
I suppose modern film capacitors at 0.1uF or so have very little leakage at all.............. That would be true, even at 1000volts from a modern megger they show zero leakage.
I agreee with the above, and personally I think that there is much more fuss being made of this than necessary. If you are restoring vintage equipment, it will have had waxed paper caps or other types that all leaked a little from new, and the circuit impedances/resistances were such that no problems arose unless one cap developed a significant leak, i.e. was actually faulty.

Realistically, using modern caps there is no problem at all as there would barely be nanoamps of leakage and I can't think of any vintage circuit that would be that sensitive, its designers had to cope with far greater leakage from new.

Just use good quality caps, keep the voltage rating suitably higher than the working DC voltage and go higher again for dynamic circuits such as output stages that may generate voltage peaks, using 500 volt types if you need to. If you don't have an 'equal value' situation, look at the application and if it is fundamentally DC then don't worry. I'd be a bit more careful if it was off a loptx or the switching stage of a switchmode power supply, but all I'd do is make sure the values were equal and the quality appropriate. The same applies to electrolytics, just use common sense and replace a crook 450v with two 350v in series (but double the capacitance value) if you don't have the right value to hand (say 8uF 450v) but do have two 16uF 350v.

I've been servicing for nearly 50 years on radios, TVs, transmitters big and small, and industrial electronics and I've never met a series cap problem, but I've used a few when I needed the voltage rating or a cap value I didn't have in my kit at the time.

Most of the caps fitted during restorations are probably only going to see a few hundred hours of use at the most anyway. Only a small percentage will see much more than that.

Cheers

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Last edited by Billy T; 21st May 2010 at 11:23 am.
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Old 21st May 2010, 2:46 pm   #15
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Default Re: capacitors in series - pitfalls?

Hi,

An example of taking series (well, series/parallel) connection of capacitors to something of an extreme is the MMC often employed by 'coilers'.

Cheers, Kat
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Old 7th Nov 2010, 12:15 pm   #16
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Default Re: capacitors in series - pitfalls?

Just a small point on the above subject - If you connect electrolytics in series, you will also have to bear in mind that the E.S.R. will probably be materially larger than that of a single unit rated to do the same job. If you use two identical caps , then the E.S.R. would be twice that of one of them. This probably wont matter in valve equipment, but would need to be borne in mind in the case of a S.M.P.S. or similar application. Tony
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Old 25th Nov 2010, 4:55 am   #17
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Default Re: capacitors in series - pitfalls?

For what its worth, I recently read (via Douglas Self) that the distortion caused by capacitors is a function of the voltage across them - its generally a very small figure, but can be significant at low frequencies in power amp coupling, or below the corner frequency in a low pass filter, for example. One way to reduce the distortion is to use series-connected capacitors.
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Old 18th Jan 2011, 9:03 am   #18
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Default Re: capacitors in series - pitfalls?

Well - I was about to use two pairs of 8uF 500V electrolytics in series to replace the 4uF HT caps in my Mullard Valve Tester, but I'm not so sure now!!!!

I'm having difficulty obtaining 4uF 500V eelctrolytics - but I see that they are readily available for dishwasher motors!! Look:

http://cgi.ebay.co.uk/UNIVERSAL-DISH...item48352a0f15

Can I use those


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Old 18th Jan 2011, 9:19 am   #19
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Default Re: capacitors in series - pitfalls?

Not sure what capacitor you are replacing but could you replace it with the higher value 8uf .

Mike

Last edited by MichaelR; 18th Jan 2011 at 9:20 am. Reason: spelling
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Old 18th Jan 2011, 11:31 am   #20
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Default Re: capacitors in series - pitfalls?

Normally plenty of 4.7uF 500V available.

David
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