Switch mode power supply contain some large filter capacitors that can hold dangerous charges even if the unit has not been used for several days. Typical values of this type of filter capacitor are 220uf 250volt and 330uf 400volt! If you are going to work on the power supply circuits, it's a good idea to discharge that capacitor first.

There are three ways to discharge the big filter capacitor in a power supply.

1)Discharge it with a screw driver (not recommended).

The reason for not using the screw driver to discharge a capacitor is because

-the printed circuit board or circuitry can be damage due to the spark generated while discharging the high voltage capacitor. I once blowed the power section using this method. However, if you know that the capacitor stored voltage is not too large, you can easily discharge it with a small screw driver.

-if the capacitor holds a heavier charge of electricity. Discharging the capacitor may melt the tip of the screw driver and the copper on the printed circuit board.

-Sometimes a heavier spark may cause small disintegrated solder lead or copper to fly out from the circuit board and may injured your eyes.

2)Place the leads of a socketed 100 watt electric bulb on the leads of the capacitor.

This method had been used by many technicians around the world for the light bulb will act as an indicator to see if the capacitor still holds the charge. If there is a charge the light bulb will light and after discharged the light bulb will goes off.

3)Place the leads of a high wattage resistor on the leads of the capacitor. I use a 2.2k ohm 10 watt resistor to discharge the high voltage capacitor in a switch mode power supplies. It is simple to use and very effective .It takes only couple of seconds to fully discharge the capacitor.

Conclusion

I strongly suggest for those who use screw driver to discharge a capacitor to swift to the second and third method as these is the safest method to protect the circuit and most importantly is yourself.