How to Discharge Capacitors in a Switched-Mode Power Supply

Here is a short tutorial on how to discharge the capacitors in a power supply so you can safely repair it:

  1. Do not short the terminals of a filter capacitor with a screwdriver. This can be dangerous.
  2. Screw a 100 watt lightbulb into a socket with exposed leads.
  3. Attach one lead to each terminal of the capacitor, the bulb should illuminate.
  4. When the bulb turns off, the capacitor is empty.
  5. Alternatively you can bridge the terminals of the capacitor for a few seconds with a high wattage resistor, something like 2.2k ohms, 10 watts will work.

Switched-mode power supplies have multiple large-filter capacitors that can hold hazardous charges, even if the power supply hasn’t been used in days. These filter capacitors typically have values as high as 220uF/250V and 330uF/400V. You must discharge the capacitors before working on power supply circuits so you won’t get shocked.

There are three different ways to discharge large filter capacitors in a power supply: with a screwdriver, the leads of a socketed 100 watt light bulb, and the leads of a high-wattage resistor.

Using a screwdriver to discharge the capacitor is not recommended because you can generate a spark and damage the printed circuit board or circuitry of the power supply. You can even blow the power section. Keep in mind that if you know a capacitor’s stored voltage is relatively low, you can discharge it with a small screwdriver without undue risk.

If the capacitor is holding a heavier charge, discharging the capacitor can melt the screwdriver’s tip as well as the copper of the printed circuit board. A heavy spark is especially dangerous: it can cause small bits of solder lead or copper to shoot from the circuit board, potentially injuring your eyes.

The second method involves placing the leads of a socketed 100 watt electric light bulb on the capacitor’s lead and is used by technicians around the globe. The light bulb acts as an indicator, showing if the capacitor contains a charge. If there is a charge present the light bulb will illuminate and eventually turn off when the capacitor in the switching power supply is discharged.

The last method consists of placing the leads of a high-wattage resistor on the capacitor’s leads. You can use a 2.2k ohm ten-watt resistor to discharge high voltage capacitors in a switched-mode power supply. It is a very simple and effective process, taking mere seconds to completely discharge the capacitor.

There’s really no reason to discharge a capacitor with a screwdriver when all you need is a light bulb or a resistor, so keep that in mind the next time you need to discharge capacitors in a switched-mode power supply.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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