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How to Test Electrolytic Capacitors for Dielectric Absorption

A metal electrode made from either etched aluminum foil or tantalum powder, anodized with a very thin insulating oxide layer, is the definitive element of an electrolytic capacitor. The large surface area of the electrode provides high capacitance values in relatively small packages and electrolytics that can store large amounts of energy. A charged electrolytic capacitor can hold a significant voltage across its terminals, but once discharged, the voltage should remain at zero. Dielectric absorption, a little-understood phenomenon, results in a small fraction of the original voltage regenerating a few seconds after discharge.

Things You'll Need

Electrolytic capacitor
Wire-wound resistor
Connecting wire
Single-pole switch
DC power supply
Digital volt meter with alligator clip leads

Instructions

- 1
  Check the working voltage of the electrolytic capacitor that you are testing. Divide the working voltage by two and note this value. Select a wire-wound resistor that will limit the peak current to no more than 250 milliamps when connected across a DC voltage source of this value. For example, if the working voltage of the capacitor is 24 volts, the resistor value should be at least 48 ohms.
- 2
  Use wire to connect one terminal of a single-pole switch to the positive terminal of a DC power supply, checking that the switch is in the open position. Connect the other switch terminal to one end of the resistor.
- 3
  Connect the positive terminal of the capacitor to the free end of the resistor. Connect the negative capacitor terminal to the negative terminal of the power supply.
- 4
  Clip a digital volt meter across the capacitor, with the positive lead to the positive capacitor terminal and the negative lead to the negative capacitor terminal. Switch on the meter.
- 5
  Switch on the power supply, and set the output voltage to half the working voltage of the capacitor. Close the switch, and observe the voltage indicated on the voltmeter, which should rise. When the indicated voltage stabilizes, open the switch. Turn off the power supply and disconnect it from the circuit, leaving the remaining components connected.
- 6
  Note the voltage of the charged capacitor indicated on the voltmeter. This should be approximately the same value that you set on the power supply. Use wire to connect the free terminal of the switch to the negative terminal of the capacitor.
- 7
  Close the switch, and observe the voltage indicated on the volt meter, which should decay to zero. Open the switch and continue to monitor the indicated voltage. If it remains at zero, the capacitor is not exhibiting dielectric absorption.
- 8
  Record the indicated voltage if it increases again, waiting until it has peaked. In this event, the capacitor is exhibiting dielectric absorption.

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