Skinned electrolytic caps (in loving memory of Fred D.)

[moamps]

On the picture below there is a measuring setup for determining correct leads polarities on an unmarked electrolytic capacitor. I've started this thread because some questions were left unanswered after the thread http://www.diyaudio.com/forums/showt...threadid=35381 was closed.

You need an audio signal generator and a moderate-quality oscilloscope to perform the measuring.

Procedure:
0. For higher capacity use 100Hz generator frequency, for smaller 1kHz.
1. Mark capacitor leads with numbers 1 and 2.
2. Connect the generator via one resistor (ca 100ohm) on the capacitor; ground to lead 2.
3. Measure voltage V1 on the aluminum cap's body with the 'scope.
4. Reverse leads; ground to lead 1.
5. Measure voltage V2 on the aluminum cap's body with the 'scope.
6. Finally, in the setup with the smaller value of measured V, you have the ground wire connected to the cap's MINUS lead.

When a bipolar cap is measured, the difference in measured voltages is very small but clearly noticeable.
The "minus" lead (which is shorter in length when the cap is new and unused) should be connected to the less sensitive point in your design.

This setup has been successfully tested on the following cap brands (not on all brand variations):
nippon, nec, jamicon, elna, rubycon, panasonic, iskra, lelon, samhwa, jimax, taicon, teapo, elite, roe, giant, yec, jun fu, elecon, suscon, ctc, viva, forever, na, jd, samsung, punsumi, capxon, selcon, rec, marcon, bellcap, sanyo, cornell, philips, bhc aerovox.

This measuring method is IMO safe and the results are very reliable.

Warning for caps skinners: Note that most manufacturers don't recommend (or warn against) connecting aluminum body to any potential (either MINUS or chassis ground).

Using a similar method (sniffing EM field with high input scope probe) it is possible to find the inner and outer lead on almost any foil no-polar cap (for those to whom this may be important).

Regards

Milan

[fdegrove]

Hi,

Well done, Milan.

Cheers,

[jam]

Milan,

Good stuff. We need more of this on the forum.

Regards,
Jam

[scottnixon]

Nice and safe I like it. You can also use that same basic set up on film caps to determine which lead attaches to the outside foil. The direction with largest signal on cap body is usually the one. If that kind of thing keeps you up at night.

[Peter Daniel]

The method for finding outer foil is also described in post #69 here: Black Gate 'super E' caps

[Sunsun22]

I’ve tried to find out the inner and outer foil of capacitor asper “moamps” thread with the following setup: -


The physical setup as follows: -



The small PCB is a sine wave generator and I have tested the result of it by using Jansen caps (with clear marks on the cap) and BG N caps. The set up is able to determine the desired result. I am also using this setup to test some Wima caps and is able to provide results to determine the outer and inner foil leads by placing the probe on the top of the plastic cap until on one occasion that I started to measure a broken Wima cap.

You can see there is a melted area on the cap showing exposed metal plate. I was curious to use the probe to test the metal plate that was exposed and there is a complete reverse result of normal testing procedure as above: -


Probe on PVC Probe on metal plate
Lead 1 to ground 1.8 mV 2.5 mV
Lead 2 to ground 2,5 mV 1.8 mV

I am now confused to determine which should the inner or outer foil. Can anyone help?

Thanks regards,

Sunny
Hong Kong

[moamps]

Originally posted by Sunsun22
......until on one occasion that I started to measure a broken Wima cap.......I am now confused to determine which should the inner or outer foil.....

Hi,
this procedure for determining inner and outer foils can be used only for caps that are not stacked. I'm not sure which type is your Wima cap but if it is stacked (i.e. not wound), there are no inner and outer foils to determine. Honestly, I have never measured broken/melted caps with my circuit so I really don't know what happens in that case.

Regards,
Milan

[Sunsun22]

Thanks Moamps. I don't know what is the type of Wima that I'm measuring but I am trying to use your way to measure Vishay MKP 1837 metalised polypropylene film caps ranging from 0.01 uF to 0.047 uF. Is this workable?

Sunny

[fdegrove]

Hi,

measure Vishay MKP 1837 metalised polypropylene film caps ranging from 0.01 uF to 0.047 uF.

The MKP 1837 series, formerly Roederstein, are also stacked film caps...just like that WIMA cap.

Cheers,

[Sunsun22]

Hi Frank, then is it necessary to find out the inner and outer foil of stacked film caps?

Sunny