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Non-polarised electrolytics (audio)

PostPosted: Mon May 02, 2016 12:04 pm
by crackle
Is it possible / should you, reform reversible electrolytics, the types used in audio cross overs. I have some 80uf ones which were in some NOS ITT-KB speakers from 1972. the speakers have never been used, we had to cut the tape on the boxes. The capacitors have a lot of leakage just over 5mA which dropped to about 4mA after 5 min. when supplied with 100v DC. from my cap reformer.
Should I expect a non-polarised electrolytic to pass zero DC current.

Mike

Re: Non-polarised electrolytics (audio)

PostPosted: Mon May 02, 2016 12:29 pm
by Michael Watterson
No.
Also DC will only "reform" them in one direction. You'd need to do it the other way too, which will damage the reforming.

Also they are rubbish anyway. Plastic film types are now not much larger and work far better.
I replace them with film types on sight in cross over networks.

I have a load in stock (NOS), I should really dump them :)

Non-polar Electrolyitcs are never meant to have DC on them ever, they will eventually pass current if there is DC, and then it will fall as it becomes a polarised type.

Re: Non-polarised electrolytics (audio)

PostPosted: Mon May 02, 2016 12:58 pm
by crackle
So it looks like these are knackered then. as they are passing current.
OK thanks
Mike

Re: Non-polarised electrolytics (audio)

PostPosted: Mon May 02, 2016 1:05 pm
by Michael Watterson
Well, even "good" ones will start to pass current if you put DC on them for any time.

Only a higher voltage 50Hz AC only capacitor tester is useful and ESR tester. If ESR is poor they are drying out (common problem). If the capacitance is high, then they are leaky. I'd test them with an old 50Hz mains based tester rather than 1KHz or higher modern tester as then AC leakage shows up as "extra capacitance" more than at 1KHz or 10KHz etc.

But really I've given up testing them and just replace with plastic film.

Re: Non-polarised electrolytics (audio)

PostPosted: Mon May 02, 2016 2:45 pm
by Terrykc
Mike, as a last resort, you could always try connecting them to an AC supply and see what happens!

Re: Non-polarised electrolytics (audio)

PostPosted: Mon May 02, 2016 3:34 pm
by crackle
These are 80uF capacitors rated at 150v.
Connected to a 25v AC supply they make a 7 watt 22 ohm resistor hot (in series).
At 40 volts they pass about .2mA of DC.
Mike

Re: Non-polarised electrolytics (audio)

PostPosted: Mon May 02, 2016 7:06 pm
by Rebel Rafter
Hi from RR. Does anyone know where I can get a film crossover capacitor at 68uF? I need one for a jukebox and I've searched the suppliers I know without much success. Thanks, RR.

Re: Non-polarised electrolytics (audio)

PostPosted: Mon May 02, 2016 10:21 pm
by crackle

Re: Non-polarised electrolytics (audio)

PostPosted: Tue May 03, 2016 9:03 am
by Michael Watterson
Terrykc wrote:Mike, as a last resort, you could always try connecting them to an AC supply and see what happens!

Bang?

Well depends on the voltage!

80uF sounds mental in a crossover network! 8uF is more normal. I doubt that voltage rating is needed either!
V = Sqrt (Power * resistance)