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Today I found a big electrolytic capacitor in a scrap yard. It is about 9 cm in diameter and 16 cm high. The only thing that is marked on it is polarity but nothing else. How can I measure its capacitance when I even don't know its maximum voltage rating?

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    \$\begingroup\$ You don't even know it is not broken. Leave it on the scrapyard. \$\endgroup\$ – Eugene Sh. Aug 4 '15 at 17:03
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    \$\begingroup\$ Use a variable power supply and slowly increase the voltage. When it explodes, you went past the rated voltage. ;-) \$\endgroup\$ – TimH Aug 4 '15 at 17:14
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    \$\begingroup\$ @TimH After it explodes, back off the voltage a bit. \$\endgroup\$ – Spehro Pefhany Aug 4 '15 at 18:15
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    \$\begingroup\$ If you can get a credible capacitance measurement then you can ballpark the volt rating bacause you know the size and can estimate its vintage .Run it up at very low current waiting for the current to fall .Gradualy increase the voltage .Rate your cap at a voltage where current goes to basicly zero . \$\endgroup\$ – Autistic Sep 30 '15 at 9:20
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To safely discharge a capacitor: After power is removed, connect a 20,000 Ω, 5-watt resistor across the capacitor terminals for five seconds. Use your multimeter to confirm the capacitor is fully discharged.

  1. Use your digital multimeter (DMM) to ensure all power to the circuit is OFF. If the capacitor is used in an ac circuit, set the multimeter to measure ac voltage. If is used in a dc circuit, set the DMM to measure dc voltage.
  2. Visually inspect the capacitor. If leaks, cracks, bulges or other signs of deterioration are evident, replace the capacitor.
  3. Turn the dial to the Capacitance Measurement mode ( Capacitance symbol ). The symbol often shares a spot on the dial with another function. In addition to the dial adjustment, a function button usually needs to be pressed to activate a measurement. Consult your multimeter’s user manual for instructions.
  4. For a correct measurement, the capacitor will need to be removed from the circuit. Discharge the capacitor as described in the warning above.

Note: Some multimeters offer a Relative (REL) mode. When measuring low capacitance values, the Relative mode can be used to remove the capacitance of the test leads. To place a multimeter in Relative mode for capacitance, leave the test leads open and press the REL button. This removes the residual capacitance value of the test leads.

  1. Connect the test leads to the capacitor terminals. Keep test leads connected for a few seconds to allow the multimeter to automatically select the proper range.
  2. Read the measurement displayed. If the capacitance value is within the measurement range, the multimeter will display the capacitor’s value. It will display OL if a) the capacitance value is higher than the measurement range or b) the capacitor is faulty.

From this site.

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