Patent ID: 6998786

Claim:
A control circuit of an electronic ballast for a fluorescent lamp which comprises a first filament with two ends connected to output terminals P 7 and P 8 of the electronic ballast, and a second filament with two ends connected to output terminals P 3 and P 4 of the electronic ballast, the electronic ballast comprising a capacitor C 10 connected between the output terminals P 7 and P 4 , a filter and rectifier circuit providing a DC voltage positive output and a ground, a half bridge oscillation circuit mainly formed by two transistors Q 1 and Q 2 for generating a high voltage output to the output terminal P 8 , and a driving transformer with a primary winding T 1 A and two secondary windings T 1 B and T 1 C, the primary winding T 1 A having a first end connected to the output terminal P 3 and a second end connected to the ground, the secondary winding T 1 B having a first end driving the transistor Q 1 and a second end connected to a resistor R 1 which is connected to the DC voltage positive output through a resistor R 5 , the secondary winding T 1 C having a first end driving the transistor Q 2 and a second end connected to the ground, the control circuit comprising: a protection circuit for preventing the half bridge oscillation circuit of the electronic ballast from generating an abnormal high AC voltage when the fluorescent lamp is defective, the protection circuit including a capacitor C 11 having a first end connected to the output terminal P 8 and a second end connected to an anode of a diode D 12 and a resistor R 8 which is connected to the ground, a resistor R 9 having a first end connected to a cathode of the diode D 12 and a second end connected to a cathode of a Zener diode D 13 , a capacitor C 12 in parallel with a resistor R 10 connecting the cathode of the Zener diode D 13 to the ground, a capacitor C 13 connecting an anode of the Zener diode D 13 to the ground, a SCR thyristor TH 1 having a gate connected to the anode of the Zener diode D 13 , a cathode connected to the ground and an anode connected to a junction between the resistor R 1 and the resistor R 5 , and a diode D 11 having an anode connected to the output terminal P 3 and a cathode connected to the anode of the SCR thyristor TH 1 ; and an automatic re-lamp circuit for lighting a newly-installed fluorescent lamp without switching off and on a supplying power of the control circuit, the automatic re-lamp circuit including a resistor R 11 having a first end connected to the DC voltage positive output and a second end connected to the output terminal P 4 , a resistor R 12 having a first end connected to the output terminal P 4 and a second end connected to the ground via a capacitor C 14 , a transistor Q 3 having a collector connected to the anode of the SCR thyristor TH 1 , an emitter connected to the ground and a base connected to a cathode of a diode D 14 , and a resistor R 13 connecting the second end of the resistor R 12 to an anode of the diode D 14 .