INVERTER 4W by TL494
The inverter is based on the TL494 integrated circuit. This is a switching regulator that will operate with an input voltage of between 7 and 40 volts DC. This input voltage is supplied to the IC through a reed relay. The reed in normally open. When filament current from a valve radio passes through a coil, the magnetic field makes the reed close and energises the inverter. The oscillator starts slowly (over several milliseconds) as C1 charges through R4. If this is not required, omit C1 and replace R4 with a wire link. The oscillator frequency is set by C2 and R5 and runs at 100 khz, for efficiency. The oscillator saw tooth waveform can be seen at pin 5 on the IC using an oscilloscope. The 2 internal switching transistors drive 2 external switching transistors which switch the battery voltage into a transformer. Link
