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Old 22nd Jun 2004, 9:21 am   #1
radiogammon
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Default Non-polarised electrolytics.

I recently came accross a non-polarised (reversable) electrolytic while replacing capacitors in a Hacker Mayflower VHF valve set. What is the difference between these (apart from being non-polarised) and normal electrolytics. Can they be used to replace normal electrolytics and if so, why use the polarised variety? Obviously there must be some good reason, but I wondered what it was.
With thanks, John B.
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Old 22nd Jun 2004, 11:48 am   #2
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Default Re: Non-polarised electrolytics.

Hi John.

I don't think I've ever come across non-polarised electrolytics in a radio. They are used more extensively in TV where high pulse waveforms may be encountered (for example in an east/west correction circuit). I suppose they may have a place in a valve set where the ripple across it could be excessive. Was this capacitor in the ratio detector? This is the only place other than an AFC circuit where voltage swings my be too excessive for a standard electrolytic.

Rich.
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Old 22nd Jun 2004, 12:20 pm   #3
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Default Re: Non-polarised electrolytics.

Hi Gents, They are bigger and more costly than polarised e-caps. Sometimes used in high power audio for speaker coupling when split rails are used and even slight offsets are to be avoided.
They are often replaced with 2, back to back e-caps with a balancing resistor across each.
Ed
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Old 22nd Jun 2004, 12:20 pm   #4
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Default Re: Non-polarised electrolytics.

You sometimes come across caps that look like electrolytics, but aren't. Paul describes them in his 'capcitors' section elsewhere on this site.

Best regards, Paul
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Old 22nd Jun 2004, 12:57 pm   #5
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Default Re: Non-polarised electrolytics.

I have a Bi-polar 2.5uf in my Philips that I am working on.

Is Bi-polar the same as non-polarised?

Regards

David
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Old 22nd Jun 2004, 1:58 pm   #6
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Default Re: Non-polarised electrolytics.

Rich,

Yes, Rich, it appears to be connected with the EB91 ratio detector valve circuit and also with the AFC loop from this to the RF amplifier and oscillator mixer front end. In the manual, it (C26, 8uf), is described as "a decoupling condenser providing a low resistance path to AC signals which may become present at this state and defocus the tuning indicator (an EM84)".

I assume they are non-polarised because they use a different type of electrolyte to that used in normal electrolytics. Is that correct?

With thanks, John B.

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Old 22nd Jun 2004, 3:02 pm   #7
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Default Re: Non-polarised electrolytics.

Hi John.

Ahh. The circuit position makes sense.

Not sure how these capacitors are constructed. However you can buy bi-polar caps but of course they are more expensive than standard ones.

If you need to replace it, you can get away with using two capacitors 'back to back', i.e negative to negative in series so to make up an 8uF, you would need two 16uF capacitors connected in series but negative to negative (or positive to positive). If the original capacitor is rated at 50v, then use two 50v capacitors which will give a total of 100v...plenty to play with.

Rich.
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Old 22nd Jun 2004, 9:14 pm   #8
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Default Re: Non-polarised electrolytics.

Hi Rich

Two electrolytics back to back won't give double the voltage. With voltage across the pair one will act as a cap while the other a short. It would be best to add a diode across each cap to prevent reverse powering the caps.

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Old 22nd Jun 2004, 9:32 pm   #9
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Default Re: Non-polarised electrolytics.

Something like this...


Much easier to use the right part if you can get it though!
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Old 23rd Jun 2004, 3:00 am   #10
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Default Re: Non-polarised electrolytics.

Non-polarized electrolytics have the same basic construction as polarized, except they use two anode foils rather than an anode and a cathode. The foils are pre-formed before winding. They have a shorter lifetime than polarized caps since a deteriorated anode cannot be re-formed after manufacture; doing so would un-form the other foil.

One of my reference books mentions a partial oxide film on the cathode foil, and I have also seen non-polarized caps made up of two ordinary electrolytics in series inside an outer casing. So apparently there are several ways to make them, depending on the expected use.
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Old 23rd Jun 2004, 9:08 am   #11
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Default Re: Non-polarised electrolytics.

Alan,
Thanks for the information. I haven't been able to find this elsewhere. As they have a shorter life than normal electrolytics, all the more reason to change them.
Best wishes, John B.
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Old 23rd Jun 2004, 9:35 am   #12
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Default Re: Non-polarised electrolytics.

Hi there,

Those of us who've encountered several variations of video Distribution Amplifiers will know all about non-polarised electrolytics and their failings; they're used in the input stage of the BBC AM4/538 (I think - single card, good for frequencies well past 5.5MHz!!) video DA (not sure if BAL used NP electros, I think Avitel might have). Having said that, most of the AM4/538s in use at work have been there happily passing video for the last eighteen years in my studio without a single failure!
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Old 23rd Jun 2004, 10:49 am   #13
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Default Re: Non-polarised electrolytics.

Hi Norm.

Thanks for that clarification on back to back caps. I suppose it's obvious that electrolytics will perform differently back to back as opposed to normal series. In the present application however where voltages are not too great, 50 volt components will probably be adequate. I've only used this configuration in low voltage applications (like speaker coupling).

Anyway it seems as though John can fit a suitable bi polar replacement. As Paul says better to fit the right part if you can get it!

Rich.
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Old 24th Jun 2004, 1:14 am   #14
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Default Re: Non-polarised electrolytics.

When you put two polarized electrolytics back-to-back, there are differences of opinion whether you'll get the original capacitance or half of it. I believe it depends on the applied voltage: at less than a volt or two, the reversed capacitor still acts normally (so for two back-to-back you would get half the capacitance) while at higher voltages, the reversed cap acts as a short. I've never tried the experiment. Shunt diodes should work, but as far as I know, commercial NP caps are never made that way, and there's probably a reason.
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Old 24th Jun 2004, 8:38 am   #15
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Default Re: Non-polarised electrolytics.

A trick with putting electrolytics back-to-back is to add a bias resistor to the common capacitor connections and take the other end of the resistor to a bias voltage such that both electrolytics appear to have DC of the correct polarity. This will avoid the crossover distortion you'd see using diodes and (for equal cap values) give equiv capacitance of half. However

1) it'd almost certainly require caps with a higher voltage rating.
2) The resistor must be small enough such that anticipated cap leakage currents wouldn't cause too large a DC shift. However the resistor will affect operation at low frequencies.

As others have said though, better to use the correct part in the first place.

TTFN,
Jon
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Old 24th Jun 2004, 11:16 am   #16
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Default Re: Non-polarised electrolytics.

I've never been keen on non-polarised electrolytics myself given that half of the capacitor is always effectively suffering from abuse i.e. it is reverse polarised and that as Jon states, these caps can cause crossover distortion. If a replacement for a low value non-polarised electrolytic was required, I'd be tempted to go for a plastic dielectric cap as long as there is space to fit one. For example, Farnell stock 10uF 63V MKT caps that are not too bulky.

John
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