DIY Electronic Ballast. How it works

 

An electronic ballast (EB) is used to start and maintain the glow of gas-discharge lamps. EB can be found inside energy-saving lamps, in lamps with bactericidal or fluorescent lamps. Before the electronic ballast, choke ballasts were used. The choke ballast consists of a large choke, a starter and a capacitor. Bulky choke ballasts have their drawbacks: the frequency of the voltage supplied to the lamp is 50 Hz, due to which the lamp has a stroboscopic effect, that is, it simply blinks. The service life of lamps with a choke ballast is shorter, and the current consumed from the network is higher. With a lightweight and compact EB, everything is different

I assembled an electronic ballast from separate parts to understand how it works. I took the parts from a 15 W ballast and drew the diagram using the printed conductors of the board

The assembled ballast turned out to be working and here is its circuit. A rectifier is assembled on four diodes and C1, which rectifies the alternating voltage of the 220V network to a constant voltage of approximately 310V. An 8 Ohm resistor serves as a fuse and as a limiter of the charging current C1. A device for starting the converter, which is made on two 13003 transistors, is assembled on the resistor R1, C5 and the dinistor DB3. This is a push-pull half-bridge converter, the transistors of which are switched thanks to two control windings of the transformer, which are wound in antiphase. As soon as the voltage on the capacitor C5 reaches approximately 30 volts, the dinistor breaks down and sends a positive pulse to the transistor T2. This pulse is enough to start the generator, then the dinistor is excluded from operation, for this purpose the diode D5 serves, which shunts the capacitor C5

The choke  and capacitors C6-C7 serve as a series resonant circuit, which increases the voltage to ignite the lamp. As soon as power is supplied to the device, there will be a voltage of hundreds of volts on capacitor C6. This voltage breaks through the gas in the lamp and it begins to shine. Between the spirals of the lamp, when it shines, a small resistance appears, which shunts C6 and the resonance stops. Then the choke limits the high-frequency current through the lamp and the lamp enters the operating mode. Damping diodes are located in the transistors themselves, they protect them from reverse voltage pulses. Capacitor C2 and resistor R2 are used for the same purpose

For ten minutes the device worked properly and no heating of the parts was observed

I connected the oscilloscope probes in parallel with the capacitor C6 to see what happens during the ignition of the lamp and when it enters the operating mode.

During switching on, when the voltage resonance occurs, the signal amplitude is large, it is about 900 volts, this is what I was able to see

A split second after switching on, the resonance disappears and the signal amplitude drops and is 200 volts. The lamp has entered the operating mode and the capacitor C6 is shunted. The signal frequency on the lamp is 25 kHz

In general, it is not a problem to assemble an electronic ballast yourself, the main thing is that the transistors used and the size of the choke correspond to the power of the lamp. Transistors 13003 for a power of 15 W, transistors 13006 for 75 W, 13001 for a power of 7 W, and transistors 13009 for a power of 150 W.

Approximate number of turns on the choke: for a power of 9 W - 330 turns, for a power of 15 W - 250 turns and for a power of 30 W - 109 turns