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  • Capacitor Leakage Adaptor for DMMs

  • Created: Sep 08, 2014

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Summary

Supercapacitors, also known as electrochemical capacitors, store energy electrostatically like other capacitors. To quantify the internal electrochemical activity of a capacitor using this circuit, simply measure the capacitor's leakage with the test voltage set to zero. If the reading is negative, the capacitor is self-charging with its plus terminal becoming positive with respect to its minus terminal. If the reading is greater than zero, the capacitor is self-charging with its minus terminal becoming positive with respect to its plus terminal. In the circuit, the 10kΩ potentiometer (VR1) adjusts the test voltage.


The circuit is using a Digital Multimeter (DMM) on the 20V range; this circuit gives a rapid and direct measure of the leakage current of capacitors. There are two ranges, with maximum readings of about 20µA and 2mA, and the test voltage can be varied. This will test the leakage at or near the capacitor's rated voltage. In addition, the circuit can help determine the amount of internal electrochemical activity, which reduces the capacitor's lifespan. For example, one 0.33F 5.5V supercapacitor is tested on an open-circuit voltage that rises exponentially to about 0.8V over a period of 10 days.


The Zener diode ZD1 limits the maximum test voltage to ensure that the output of pin1 can swing to at least 2V above the test voltage while; pin7 and associated components derive the ground rail from the single-ended supply. The negative supply voltage is fixed at -3.3V by ZD2 to give more range to the test voltage, which is derived from the positive supply. The circuit will operate from any voltage in the range 9-36V but keep in mind that the maximum test voltage is 8.4V less than the supply voltage. With S1 in position 1, the IC is configured as a unity gain buffer and the DMM reads its output voltage. Without a capacitor under test (CUT) connected, the DMM will display the test voltage. When a CUT is connected, it will be rapidly charged to the test voltage via S1a. The 100kΩ resistor, in series with the inverting input of the IC, protects the op amp in case the capacitor charged to a high voltage, particularly when the circuit is OFF.

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