The Right WayTo Discharge Capacitor In Switch Mode Power Supplies 

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, 150uf 400volt and 330uf 450volt! 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 blow 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.

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Author By : Jestine Yong