Patent ID: 8406023

Claim:
A discharge loop for a power supply circuit having a filtering capacitor connected to an input side of the power supply circuit for removing electromagnetic interference caused by an input AC voltage, the discharge loop comprising: a first bleeder resistor connected to the filtering capacitor for allowing energy stored in the filtering capacitor to be discharged therethrough when the input AC voltage is interrupted; and an active control circuit connected to the first bleeder resistor and the input side of the power supply circuit for manipulating the discharge loop according to a status of the input AC voltage to regulate the discharging operation of the energy stored in the filtering capacitor through the first bleeder resistor, the active control circuit comprising: a switch connected in series with the first bleeder resistor; and a voltage sensing circuit connected to the input side of the power supply circuit and a control terminal of the switch for manipulating the switching operation of the switch, the voltage sensing circuit comprising: a differentiator connected to the input side of the power supply circuit for detecting a variation of the input AC voltage; and a half-wave rectifier connected to the differentiator and the control terminal of the switch for performing half-wave rectification to the input AC voltage to generate a half-wave rectified DC voltage, thereby driving the switch.