Homemade 40W electronic ballast

The circuit is shown in the figure below. In the figure, the mains is connected from A1 and A2, D1-4 forms a full-bridge rectifier, C1 and C2 are connected in series to divide the voltage and filter. The two transistors and inductor L1-3 at the back form a half-bridge high-frequency oscillator. D5-6 protects the transistors from damage. Inductor L4 and C6 form a resonant circuit. After the lamp is ionized and turned on, L4 is used as a current blocker.

Component selection

The resistors, capacitors and transistors can be made as shown in the figure. Inductors L1-3 are wound with 3 single-core PVC wires on a toroidal core with an outer diameter of 10mm, an inner diameter of 6mm and a height of 5mm. L1-2 is wound with 2T, and L3 is wound with three turns. Pay attention to the connection method of the same-name terminals. L4 is made of EI-19 core. Since the author did not disassemble the component, the number of turns of the wire diameter is unknown. According to some information, it is about 200-300T.