Here is a short tutorial on how to discharge the capacitors in a power supply so you can safely repair it:
- Do not short the terminals of a filter capacitor with a screwdriver. This can be dangerous.
- Screw a 100 watt lightbulb into a socket with exposed leads.
- Attach one lead to each terminal of the capacitor, the bulb should illuminate.
- When the bulb turns off, the capacitor is empty.
- 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.