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I have the following device - polar electrolytic capacitor (it is/was having stripe on its sleeve):

enter image description here

I know its capacitance and voltage rating - 2.2 uF / 400 V, but accidentally removed the remainders of the sleeve so it is not clear where + and where - are.

I have found this question, and tried with 220 kOhm resistor, tried with 10 kOhm resistor, and can not understand the result. I have Fluke multimeter, and it seems to measure only tens of mAs. In both cases above I see it measures approximately 20 mA in both direction when connecting non-charged cap, and then current quickly goes to 0.

Is it multimeter issue - I need more sensitive device? Is there any other way to identify the polarity?

Are you certain that this is a polarized ("normal") aluminium electrolytic?

Yes.

Did you measure case resistance to -ve lead?

Yes, both pins with case in all directions go to infinity.

Throw it away and buy a new one.

Bad idea, I can not find a replacement of the same size (6x11). I would try to reuse it, as I suspect it is of good condition (but I can not measure it, do not have such measurement equipment).

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  • \$\begingroup\$ Are you certain that this is a polarized ("normal") aluminium electrolytic? \$\endgroup\$ – brhans Jan 29 '18 at 18:21
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    \$\begingroup\$ Did you measure case resistance to -ve lead? \$\endgroup\$ – Tony Stewart EE75 Jan 29 '18 at 18:25
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    \$\begingroup\$ Throw it away and buy a new one. \$\endgroup\$ – Andy aka Jan 29 '18 at 18:30
  • \$\begingroup\$ In your other question, you said the caps in question are "bipolar". Is this the same cap here? \$\endgroup\$ – The Photon Jan 29 '18 at 18:52
  • \$\begingroup\$ No, that post was about audio output of the opamp. This cap in this post and on the picture is (was) having stripe on its side on the sleeve, and it is located in the bypass circuit of buck converter. \$\endgroup\$ – Anonymous Jan 29 '18 at 18:54
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In some cases ( pun intended) the cathode lead is connected to the case.

E-caps have a known reverse voltage breakdown effect that leads to thermal runaway and outgassing or detonation depending on the energy applied. The reverse current is threshold dependent and above 10% of the forward rated voltage. Therefore, you would not have reached that threshold with the DMM.

In order to test it requires looking at the DC voltage with a safe AC current applied that exceeds 40Vp. Using impedance ratio, you may be able to compute this. A safe current limit would result in < 1/4W worst case across the cap with > +/-50Vp. The reverse polarity acts a weak zener diode threshold. The large series R acts as a passive current limiter. The voltage applied is up to you and the power/voltage rating of the series R. ( final note in case of Murphy's Law, wear safety glasses)

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Ok, I took Jamicon 10 uF / 16 V, and none of its pins has connection to its body (exposed at the top of cap). \$\endgroup\$ – Anonymous Jan 29 '18 at 18:43
  • \$\begingroup\$ Do I understand it right that we do not know how to figure out polarity without risking damaging the part? Also not sure I understand and can operate theory you explain. Impedance ratio? Why 40 and 50, not 80 and 100? \$\endgroup\$ – Anonymous Jan 29 '18 at 18:49
  • \$\begingroup\$ @Anonymous I don't think Tony stated a specific impedance ratio. Did I miss something there? I do see Tony saying to wear safety glasses though, so I suspect he anticipates your destruction of the capacitor!! Perhaps not what you wanted to hear. ;) \$\endgroup\$ – jonk Jan 29 '18 at 18:59
  • \$\begingroup\$ @Anonymous , when negative AC current rises, DMM DC Voltage rises in the correct polarity. This can start at xx uA levels and result in low voltage results. \$\endgroup\$ – Tony Stewart EE75 Jan 29 '18 at 19:01

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