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I'm measuring an assortment of old electrolytic capacitors ('elcos') to see if they are still okay, using my digital multimeter (a simple Duro AL-205). I noticed that most of the values are higher than the rated capacitance but that's okay since I know tolerances are usually in the -20% to +50% range.

But when I have an elco rated at 320uF, (yes, 320 not 330) 6.4 Volt but it measures as 844uF, I'm sure something's broken. Is there some fault mode in elcos that can cause higher-than-rated values? I'm careful not to touch the terminals with my fingers when measuring, so any parallel resistance does not influence the result.

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    \$\begingroup\$ Don't waste your time. Throw them away and get new ones that will still work fine 10 years later. \$\endgroup\$ – Nils Pipenbrinck Mar 19 '16 at 20:01
  • \$\begingroup\$ It won't be time wasted. He's not trying to fix the capacitor - he's trying to learn about them. He's making progress: see his comment below Steve G's answer. \$\endgroup\$ – Transistor Mar 20 '16 at 10:09
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I have heard of low grade electrolytic caps having a -0% to +100% tolerance but yours appears to be +163%. I'm not greatly surprised. When you design in an elco the main parameter is the minimum capacitance, you know the maximum will be a lot more.

When an elco hasn't been used for a several years you are advised to reform it. You could give that a try, but to be honest I would expect reforming to increase it s capacitance rather than reduce it! However, reforming may reduce the leakage current, and perhaps that is confusing your measurement.

To reform your 6.4V elco connect it to a 5V supply through a 10k resistor for about 10 minutes. Then try measuring its capacitance again.

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  • \$\begingroup\$ I tried reforming, it's capacitance is nearly unchanged. However, I did notice the voltage across the capicator would not rise above 1.5 Volt, while in theory it should go al the way to 5 Volt (though slowly). Conclusion: a large leak current will throw my multimeter off. \$\endgroup\$ – JvO Mar 19 '16 at 20:24
  • \$\begingroup\$ That sounds plausible. Further experiments: (1) Make sure the 5 V supply voltage hasn't collapsed due to capacitor internal short-circuit. (2) Either way, try repeating the test and measure the DC current through the capacitor. Then you'll have a voltage and a current and can work out the resistance. You'll remember it for the rest of your life. \$\endgroup\$ – Transistor Mar 20 '16 at 10:07

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