Capacitor Polarity?

[poppydog]

Can anybody please tell me which is positive and which is negative on this electrolytic capacitor? The only clue is a tab as seen in picture.

Regards
Poppydog

[Radiocruncher]

If you look underneath right by the pins then sometimes you can make out the positive and negative embossed in the plastic. Some have it and some don’t but they are hard to make out.

Good luck

Graham

[poppydog]

There's nothing on these at all Graham, ive got 1 or 2 to change like these. They are AEL capacitors, I thought if I could identify polarity just by looking at the tab it would save me a bit of brain ache.
regards poppydog

[Argus25]

The easy way is to just check it on both polarities on an ohm meter. There will be more leakage one way around than the other. (But some meters have different polarity probes so I check that with a diode) but to save any headaches or mistakes; the super easy & quick thing to do is just to grab another labelled capacitor of similar uF and voltage rating and compare them on the same meter for leakage on each polarity, you will quickly see which one is the + or - terminal, then put a small dot of red nail varnish or paint near the + terminal on the side of the unlabeled capacitor body.

[Hartley118]

If I'm replacing electrolytics in a (more or less) working circuit, I sometimes make sure of the polarity of the voltage on the cap by firing up the circuit and checking it with a DVM.

Martin

[paulsherwin]

The polarity is usually (though not always) marked on the PCB. This was to reduce assembly errors in the factory.

[G8UWM-MildMartin]

Another way is by "reverse engineering" (or analysing) the relevant part of the circuit including whether the common rail is negative or positive with respect to the supply rail. Details of the equipment (however obscure!) may well find someone here familiar with it.

[sp10mk11]

I have a Toshiba Circuit Diagram for a tape deck, The polarity is not shown for any electrolytic caps nor is it marked on the board, obviously I marked the board with a (-) before removing any but that was the first time I came across such a lack off information

Gary

[Diabolical Artificer]

Might it be a tantalum? Still, it should be labelled if it were.

I have some of similar caps to that one, there's nothing to indicate polarity. I tend not to trust PCB info 100%. I've often had components mis-labelled and on one occasion all the electrolytic caps were in backwards according to the PCB.

Try putting a non polarised cap in it's place, any value will do, sw on and check with a DMM. That will tell you where + volts is. Out with the non polarised, new electrolytic in.

Andy.

[Bookman]

Talk about coincidence, I removed one of those yellow banded Plessey electrolytics yesterday from a speaker crossover circuit thinking the negative would be so marked as normal with the black plastic end cap but alas; not so. Neither end is marked and nor do I have a diagram.
However, in my case one end is connected to the speaker uninsulated metal case connection so I am presuming this would be negative?

[julie_m]

Capacitors in loudspeaker crossovers are not polarised, because they have to pass alternating currents.

[barrymagrec]

Crossover circuits normally use unpolarised electrolytics - there should be no DC involved....

[GeoffK]

A follow on from the polarity of a capacitor is its working voltage and leakage. Electrolytic capacitors have high leakage values but what is an acceptable leakage in per cent if the capacitor still has its stated capacitance. If a capacitor has a maximum voltage of,say,50V will it still work normally in with a much lower polarising voltage, 6 or 9 Volts as found in transistor radios?

[Bookman]

Originally Posted by julie_m

Capacitors in loudspeaker crossovers are not polarised, because they have to pass alternating currents.

Thanks Julie,
That's what I thought but I presented the facts as the attached pic refers.

[Argus25]

Yes, capacitors used in crossovers are not polarity dependent as it is an AC scenario. The uF values often need to be moderately large and the cheap & nasty way its done by a lot of speaker designers is to use a bipolar (non polar) electrolytic, which is a very poor excuse for a proper poly cap.

Also in light of the fact there are no size constraints in a speaker enclosure normally. . I always replace crossover caps with poly caps and throw the bipolar electrolytics in the rubbish bin where they belong. Sounds harsh, but if you take the time to check the ESR of bipolar electrolytic caps, you will find they are alarmingly high and being electros their values are often +/- 50% and they dry out with time, unlike poly caps.

[Bookman]

Good response and much appreciated and yet again this forum shows that it always pays to ask as there will most likely be someone out there that knows.

[poppydog]

Thanks for all the replies, it took a little while to work out what was what. There was even a 30uf that didn't have a tab on it, I did wonder for a minute if it wasn't electrolytic. The caps in question came from a old benkson transistor radio.

With regard to Geoffks comment, I have replaced 1 or 2 of the caps with 50v rated ones, the old ones are 10v rated and the radio runs off of 6v, would this be ok or would it be wise to get caps closer to the original voltage ratings?
regards poppydog

[GeoffK]

It would seem that the general opinion is to de-rate a capacitor by anything up to a half, so a 5 Volt supply would use a capacitor with a 10 Volt rating. But if it is rated at 10 Volts then surely it is designed to have a maximum of 10 Volts not de-rated to 5 Volts. So a 50 Volt capacitor should still work on a 5 Volt supply and be super safe if this is the case.

[Radio Wrangler]

Originally Posted by GeoffK

It would seem that the general opinion is to de-rate a capacitor by anything up to a half, so a 5 Volt supply would use a capacitor with a 10 Volt rating. But if it is rated at 10 Volts then surely it is designed to have a maximum of 10 Volts not de-rated to 5 Volts. So a 50 Volt capacitor should still work on a 5 Volt supply and be super safe if this is the case.

Not quite, I'm afraid.

Electrolytic capacitors run with bias voltages much lower than their rating tend to depolarise over time. De-rating up to a point makes reliability better, and then beyond that actually makes it worse.

Try to run them with the max voltage across them up to 80% of rating and the minimum of about 20%.

Tantalum electrolytics will survive down to zero bias without any problem, but they have their own nasty foibles. You have to limit current peaks and surges, or else they go off with a smelly bang and stain everything around them.

David

[GeoffK]

Thanks Dave, This is what I thought as well, otherwise a capacitor could have just one voltage rating that would fit all.
Tantalums have their place but I am not keen on using them, having seen what damage they can do when they go bang without warning.