Bleeding off charge

In a high-voltage, direct-current power supply, capacitors are used to smooth out the fluctuations in the output. These capacitors have an electric charge, and they store it for awhile. In some power supplies, these filter capacitors hold full output voltage of supply, something like 750 V, even after the supply has been turned off, and even after it is unplugged from the wall outlet. If you try to repair such power supply, you may get clobbered by this voltage. Bleeder resistors, which are connected across the filter capacitors, drain their stored charge so that servicing the supply is not dangerous.

Figure--   A bleeder resistor is connected across the filter capacitor in a power supply.

It is always a good idea to short out all filter capacitors, using a screwdriver with an insulated handle, before working on the high-voltage direct current power supply. I recall an instance when I was repairing the supply for the radio power amplifier. The capacitors were holding around 2 kV. Even if the supply has bleeder resistors, they take just a while to get rid of the residual charge.