March 2014
I pulled some excellent quality electrolytic capacitors from the power supply of an amplifier. The capacitors are rated at 40 VDC but were used in a 10V circuit. I wanted to use the capacitors in a 24 VDC circuit, but I was told that the capacitors "formed" at 10V and wouldn't work at 24 VDC, regardless of the original rating. Is this true?
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True, aluminum capacitors will deform with years of operation at reduced voltage (or no voltage). The typical aluminum capacitor will have a leakage current that increases with capacitance and voltage. A 40 µF 40V cap would have leakage in the order of 2 µA.
To test your capacitor, connect it through a 10K resistor to 24 volts (or 40 volts if it is available); if the voltage rises to 24 volts or more, it is okay. If not, leave it connected until the voltage does rise above 24 volts. You can use a smaller resistor to speed up the process, but don't let the cap get hotter than 50 degrees C.
Short answer... maybe.
You can often reform electrolytic capacitors by slowly ramping the voltage up, or above, your expected voltage. Keep a current meter in series and measure the charge current (after you have reached your top voltage. It should be in the microamp to low milliamp range for a good capacitor. If its higher than a milliamp or two you have a cap that is breaking down (high leakage).
Remember, low voltage electrolytic caps are pretty cheap so don't get caught by false economy.
