Secrets To Discharge Capacitor That All Electronic Repairers Should Know


Capacitor discharge is the first thing you should do as an electronic repairer before touching the power supply section. The large filter capacitor in the power supply section can hold dangerous high voltage even if the unit has been off for few days. It is advisable to discharge the filter capacitor first before you start to troubleshoot in order protect yourself from electrocuted. Though the capacitor current stored in the capacitor can’t kill you but it is enough to burn your hand, fingers and your skin when you accidentally touch it and the worst case is while you’re carrying the Monitor or TV, you might just throw off the equipment when you got a shocked from the filter capacitor and the cathode ray tube monitor may break and cause the flying glasses to hit your body or eyes.


Just be cautious when handling the power supply area. You can always use a meter to confirm whether there is still charge in the capacitor. Place your meter’s black probe to the capacitor negative pin (you can see the negative marking at the sleeve of the capacitor) and the red probe to other pin. If there is high voltage registered in your meter then you have to discharge the capacitor otherwise you may proceed to repair the equipment. Typical value for this type of filter capacitor is 220 to 330 microfarad with a 400 to 450 voltage rating.



capacitor discharge


The large filter capacitor in power supply section


There are three ways to discharge the large filter capacitor in a switch mode power supplies.

Discharging the capacitor 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 damaged due to the spark generated while discharging the high voltage in the capacitor. I once have blown the power area using this method. However, if you knew that the capacitor stored voltage is not too large after confirmed it with a meter, you can easily discharge it with a small screw driver.


Just place the screw driver tip to touch the two pin of the capacitor, within seconds the charge will be gone. If the capacitor holds a heavier charge of electricity then discharging the capacitor with a screw driver may melt the tip of the screw driver and the copper on the printed circuit board. Sometimes a big spark may cause small disintegrated solder lead or copper to fly out from the circuit board and might injured your eyes or body.

Another method is that you can use a socketed 100 watt electric light bulb and touch the two wires coming out from the light bulb on the leads of the capacitor. This method had been used by many electronic repairers 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. I still preferred the third method because you have to use both hands to touch on the capacitor lead.



capacitor discharging


Use a resistor to discharge the capacitor leads

Third method and also my favorite method is to place the leads of a high wattage resistor on the leads of the capacitors you want to discharge. You can use either a 1.8 k or a 2.2 kilo ohm 5 to 10 watt resistor to discharge the high voltage capacitor in a switch mode power supplies. It is very simple to use and very effective .It takes only a couple of seconds to fully discharge the capacitor.

Using only one hand, you can do the job while the other hand you still can hold a solder gun or secure the equipment casing.  

I strongly recommend to those who are using the screw driver to discharge a capacitor in power supply to consider the second and third method as these is the safest methods. It not only protects the circuit, it also protects you. If you had discharge the capacitor and you are still not sure or no confidence whether the charge already gone, you can always use a meter to confirm it.