Poly caps - typical leakage current? Testing snafu.

[Diabolical Artificer]

Watched a video this morning on Utube that recommended testing capacitor leakage current before using said cap. We're talking new caps here.

So I stuck 600v on a Poly 220n 630v axial whilst measuring current. At 100v it was 100uA rising to 600uA at 600v. Whilst typing this I'm thinking that's odd, 100uA at 100v and 600uA at
600v, too much of a coincidence.

I had a DMM across/in parallel with the cap to measure voltage, this was obviously somehow altering the current reading.

Anyhoo, with the DMM off, leakage I fell to 1uA ish, so the caps ok. But back to my OQ, what is the normal leakage I of a poly cap? I found this Vishay datasheet but couldn't find any real figure.

Andy.

The voltage measuring DMM would load the circuit a bit, 1 megohm by the look of it. Poly caps are pretty good, at 1uA (100V) simply breathing on the capacitor will make this rise so it is hard to measure. When I worked at a capacitor factory we had to employ femto amp measuring gear for ceramic capacitors, they are worse than poly types for leakage. These where capable of measuring a few electrons a second.

To answer the OP's question, the leakage for a poly type capacitor is unmeasurable using less than a £1000 bit of kit, i.e. near enough to zero for everyday use.

For an experiment, charge one up to a voltage and measure the voltage with a meter straight away, do the same but leaving it for an hour. Using a 'scope may be better as you can then see the instantaneous voltage.

[ms660]

Originally Posted by Dr Wobble

Anyhoo, with the DMM off, leakage I fell to 1uA ish, so the caps ok. But back to my OQ, what is the normal leakage I of a poly cap? I found this Vishay datasheet but couldn't find any real figure.

It would depend on the insulation resistance and the voltage applied, too many variables.

Lawrence.

[paulsherwin]

I would certainly expect the leakage current to be unmeasurable without specialist lab equipment.

You can think poly (whatever) caps as set oil capacitors, polystyrene are the best, a solid crunchy plastic, very stable and very little voltage to capacitance variation. Great for tuned circuits.

Polyester, a bit more flexible and can be made in thinner films than polystyrene, more capacity for your buck/unit volume. Not quite as good for voltage vs. capacitance*.

Polypropylene, even more bendy and can be thinner, even more cap per buck.

In the end any poly cap is way ahead of the wax/paper/oil paper types we replace.

*any competent design will not have a voltage vs. capacitance issue.

[GMB]

The spec. sheet you linked shows that the leakage would be expected to be of the order of 1/1000 of a microamp, but at this point as mentioned above you have to consider factors like how much you handled it and whether you were breathing at the time.

You don't need to spend thousands on equipment to measure this, I have a couple of vintage electrometers that can easily do it.

If in doubt, try heating the capacitor up to near 100C. If you are getting leakage then this will increase massively with temperature thus making it easier to spot (and for valve circuits, more realistic of its actual operating conditions).

[G6Tanuki]

Strangely, some of these polyester/polypropylene capacitors can show a piezoelectric effect!

Short the terminals on one together for a few hours to make sure it's really, fully discharged.

Then connect a really-high-impedance millivoltmeter across the terminals and remove the short.

Then squeeze the capacitor *hard* between your thumb and forefinger.

Thye effect is small - but repeateably-present nonetheless; it's nowhere near as bad as some ceramic capacitors that can generate millivolts of audio-signal if tapped!

[bikerhifinut]

I'm sorry but all I can think is one can watch too many youtube vids and start getting paranoid about irrelevancies. You've enough on your plate without worrying about modern excellent plastic film caps.
Just my pennyworth Andy and please dont take my observation the wrong way, I'm thoroughly impressed with your "proper diy" amp project.

The other Andy.

[bikerhifinut]

Originally Posted by G6Tanuki

Strangely, some of these polyester/polypropylene capacitors can show a piezoelectric effect!

I seem to remember reading about this in the Morgan Jones valve amplifiers book. Basically to the effect that all capacitors will show a piezoelectric effect if distorted, and vice versa. I guess it could be important in some sensitive applications.

A.

[kalee20]

Originally Posted by merlinmaxwell

You can think poly (whatever) caps as set oil capacitors, polystyrene are the best, a solid crunchy plastic, very stable and very little voltage to capacitance variation. Great for tuned circuits.

Polyester, a bit more flexible and can be made in thinner films than polystyrene, more capacity for your buck/unit volume. Not quite as good for voltage vs. capacitance.

Polypropylene, even more bendy and can be thinner, even more cap per buck.

Are you sure about that, MM? Polyester has a higher permittivity than polypropylene so you get more μF's for the same geometry. And it's better at high temperatures, it's easier to metallise, and it's tough! Whereas polypropylene softens at lower temperatures, it's really difficult to get a metal deposit to stick to it (though some companies have cracked that one), and it's non-polar so has a low dielectric constant thus capacitors are bigger. Against that, it does have really low dielectric losses so is super for pulse duty and high AC currents. Price wise, I've generally found that same C, same V, polyester is cheapest!

kalee20, I think I may have it the wrong way round, still the modern poly caps are very good indeed.

[kalee20]

Absolutely! Whenever I see adverts against dropping litter, "That empty crisp packet you dropped will take 50,000 years to biodegrade," I think, Great! The capacitors I'm using are made with that stuff, they'll see me out!

[Radio Wrangler]

Some plastic films exhibit dielectric absorption effects. Chargea capacitor, discharge it, remove the discharge load/short and watch it charge itself part way back up.Probably OK formany uses, but can cause trouble in dual-slope DVM circuits. Bob Pease wrote acolumn on 'soakage'

Polystyrene capacitors do a permanent change in value of several percent if you take them over 60-70C. Not a lot of people know that!

David

[FrankB]

I'll put in my 2 cents worth here also.

There are some circuits that I am aware of in US radios and even at least 1 TV set that used the leakage as sort of a "feedback" for circuit stability. Now those used the old wax covered caps. Great design engineering.

But worth noting if you beat your head to a pulp on a wall trying to find the problem after you replaced the cap. (Don't ask how I found this out!).

There were some US sets that also used the inductance of the wax caps foil as part of the circuit. (Again, don't ask....).

Then there are the US made El Menco mica caps.... The NOS ones...They test 100% good on voltage and resistance and won't work in the circuit. I lost a week of wasted time figuring that out. The 40 year old NOS spares were bad out of the box.
One of the TV techs at the shop also found this out after I told him they were the problem. "Kids"...never listen. He lost 2 weeks of work before he followed my advice.

[Diabolical Artificer]

So not really worth bothering testing the leakage. However I think there is a point in testing whether a cap your going to use will stand up it's stated working voltage.

Your probably right Andy ( no worries I wouldn't take owt you said the wrong way) however you do pick up some good tips off Utube video's, as well as worrying about inconsequential stuff.

I've watched a few video's now that pointed out that the poster's had experienced problem's with polypropylene and polyester caps when tested near to their stated wkg voltage. In the last vid I saw, the poly axial cap with a stated max voltage of 650v failed at 500v. It failed as a dead short so could definately cause problem's if used as a coupling cap, especially at switch on when voltages are high.

The other video I watched ( sorry no links) was casting aspertions on some of the cheaper new poly dipped caps which the bloke found also failed near to their max V. He found Suntan caps were ok despite having a bit of a rep as being cheap and orrible caps.

The forum has also had a few threads about NOS caps failing under test, so I guess it pays to take a few minutes to test new or old caps before fitting.

I was aware of soakage as regards to capacitors, as the chap's over the water call it, but not the piezo electric properties. Out of the different components used in electronics, we seem to talk about capacitors the most; fascinating little beasties.

Andy # 6.

[Radio Wrangler]

You can use certain plastic films as piezo electric microphones. Some people have printed electrode structures on film, where the electrodes are designed to correlate with certain sounds (breaking glass, or a specific coin falling onto a steel block in a slot machine.

Some ceramics are microphonic and I've seen synthesisers have noise sidebands from fan motor vibrations being picked up by PLL loop timeconstant capacitors.

David