How To Safely Discharge A Capacitor | Galco

An Electrolytic capacitor is a passive electrical component used to filter voltage in electronic devices such as Power Supplies or VFD’s (Variable Frequency Drive) and can hold a charge. Because capacitors can continue to hold their charge even when these devices are powered off, it may be necessary to manually discharge the capacitor before servicing the device.

There is a quick, do-it-yourself method that can safely discharge the capacitor, requiring no more than a multimeter, a resistor, and a set of alligator clip test leads.

To begin, set up your multimeter for taking a DC volts measurement (black probe in COM port, red probe in VΩ port, standard on most multimeters like the Fluke 87V).

You can now test the capacitor to measure its stored voltage.

Next, you have to select a resistor. A 1000 ohm, 5 watt resistor will suffice for most small capacitors. For larger capacitors, a higher resistance is needed to prevent damage, like a 5000 ohm at 50watts. More resistance also extends the time needed to fully discharge the capacitor to a safe voltage threshold.

Now you can attach the multimeter probes to each lead on opposite ends of the resistor, which will allow you to monitor the voltage drop as the capacitor discharges.

Finally, connect the alligator clip test leads to each end of the resistor, and then the free clip on each wire/terminal of the capacitor.

The voltage reading on the multimeter should immediately begin to drop. When the voltage reading drops into the “mV” range, (or less than 1 volt) the capacitor is safely discharged. You should also be aware that the large, high voltage electrolytic capacitors tend to recharge themselves a little bit once the discharge resistor is removed. After 15 to 30 minutes it may reach 20 to 40 volts just sitting there. This is why resistors are used in Power Supplies and VFD’s to not only balance voltage but to help bleed it off continuously after the Line Supply power is removed.

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