The present invention relates to an electronic ballast for a fluorescent lamp with low-voltage power supply comprising a wave filter connected with its output to the input of a rectifier circuit which is connected with its output to the input of a generation and control circuit of high frequency the output end of which is connected to a resonance circuit composed of a choke and a capacitor and an ignition circuit; the generation and control circuit including two solid state switches and a feedback transformer means which is put with its primary circuit into the output of the generation and control circuit. Mainly the present invention relates to a circuit for the ignition and control of a fluorescent lamp.
The electronic ballast of fluorescent lamps is composed of a rectifier circuit, a wave filter circuit, a generation and control circuit of high frequency, an ignition circuit of the lamp tube, etc. In a fluorescent lamp with discharge in low-voltage mercury vapor using the electronic ballast, the direct current voltage that is supplied to the converting transistor of the generation and control circuit will be too low when the voltage of the power supply is 120 V or lower and the circuit of the electronic ballast works under cheap and reliable direct rectification. Thus the voltage after conversion will be also too low. Because of this, the fluorescent lamp cannot work stably and normally. In order to solve the problem of the insuffucient low-voltage of high frequency, one of the methods is to add a resonance circuit to the output end of high frequency so that the voltage of high frequency that is supplied to the lamp tube can be raised.
In FIG. 1 of the drawing there is shown the principle diagram of a circuit of the electronic ballast for a fluorescent lamp. A wave filter comprising capacitor C1 and choke L0 is connected to the low-voltage mains. The output of the wave filter circuit is connected to the input of a rectifier circuit composed of diodes D1-D4. The output of the rectifier circuit is connected to the input of a generation and control circuit of high frequency comprising two transistors V1, V2, resistors R2, R3, R4, R5, capacitors C4, C5, diode D5, and feedback transformer B1. Connected to the output of the rectifier circuit is a filtering capacitor C2. Resistor R1 and capacitor C3 are connected in series to the output of the rectifier circuit. The collector of transistor V1 is connected to the positive output of the rectifier circuit. The emitter of transistor V2 is connected to the negative output of the rectifier circuit. The emitter of transistor V1 is connected to the collector of transistor V2. Connected to the joint point of emitter of transistor V1 and collector of transistor V2 is the primary coil B1-1 of feedback transformer B1. Secondary coil B1-2 of feedback transformer B1 is connected in series with resistor R3 which is connected to the base of transistor V1. Resistor R4 is connected to the base of transistor V2 and the secondary coil B1-3 of feedback transformer B1. One end of the secondary coil B1-3 is connected to the negative output of the rectifier circuit. One end of secondary coil B1-2 is connected to the joint point of emitter of transistor V1 and collector of transistor V2. The second end of capacitor C5 is connected to one tap or contact respectively of the lamp F. The other plug-in pin or contact of the lamp F is connected to one end of a choke L and one end of a capacitor C7 which is connected with its other end to the negative output of the rectifier circuit. One end of the choke L is connected to the primary coil B1-1 of feedback transformer B1. The starting circuit of the fluorescent lamp F includes a capacitor C3 in series with resistor R1 and DIAC DB 3.
In the electronic ballast circuit shown in FIG. 1 capacitor C7 and choke L form a resonant circuit. Because the capacity of C2 and C5 is relatively big, the voltage of high frequency on C7 is mainly supplied to the two ends of the lamp tube so that the lamp tube can work normally. There are great defects in this circuit if the lamp tube is a part that can be changed. Firstly, the resonance circuit C7 and L will work under the idle situation and the transistor will be burnt out quickly if the plug-in pins of the lamp tube have a loose contact and the lamp tube itself does not work normally. Secondly, the safety regulation requirements of the changeable lamp tube cannot be fulfilled.
It is an object of the present invention to overcome the above mentioned defects and to provide an electronic ballast which will not fail if the lamp tube has a loose contact or the lamp tube is removed from its lampholder.
An electronic ballast for a fluorescent lamp with low-voltage power supply according to the invention comprises a wave filter connected with its output to the input of a rectifier circuit, which is connected with its output to the input of a generation and control circuit of high frequency, the output end of which is connected to the resonance circuit composed of a choke and a capacitor, and an ignition circuit, the generation and control circuit including two solid state switches and feedback transformer means which is put with its primary circuits into the output of the generation and control circuit; at least one coil of the primary circuit of the feedback transformer means for supplying feedback to one of the solid state switches is connected between the joint of the choke and the capacitor of the resonance circuit and one contact of the fluorescent lamp. The normal work of the generation and control circuit of high frequency is maintained by the positive feedback that is formed through each group of coils in the feedback transformer. Thus the primary feedback coil will have no feedback current in the feedback transformer if the lamp tube has a loose contact or an open circuit. Thereby the solid state switch, for instance a transistor, stops working and the safety regulation requirements of the changeable lamp tube are fulfilled. Preferably the feedback transformer means comprises two transformers and the primary feedback coil is connected on the two transformers between the joint of the choke and the capacitor of said resonance circuit and said one contact of the fluorescent lamp.
In another preferred embodiment of the present invention the feedback transformer means comprises one primary feedback coil connected with a two holes transformer. In still another preferred embodiment of the present invention the primary feedback means comprises two transformers; one transformer being connected in the front and one at the back of the choke.
In another preferred embodiment of the present invention the primary feedback transformer means comprises one transformer including two primary feedback coils one coil being connected in the front and one coil being connected to the back of the choke. The invention will be further explained hereinafter by reference to the drawings.