Homemade Electronic Lamp Ballast
This simple homemade inverter can run a discharge lamp from a low voltage DC supply. The basic circuit diagram is shown in Figure 1. The activity page shows the inverter running a 70W SON/T high pressure sodium lamp from a 12V supply. No additional ignitor circuit was required as the number of turns on the secondary coil of the transformer T1 will produce a sufficiently high open circuit voltage to start the lamp. The picture below shows a xenon strobe lamp being run continuously from the inverter. These normally take very high voltages to strike. The strobe lamp arc was only maintained for a few seconds as the tube would quickly over heat. This type of lamp is designed for pulsed operation only
The unit is not particularly efficient and could be improved by reducing the output voltage. This is possible by reducing the number of turns on the transformer's secondary coil and an additional ignitor circuit across the lamp.
Image 2 shows the transformer construction. Two 'E' shaped ferrite cores are inserted either side of the windings on a plastic bobbin.
The oscilloscope trace in Figure 2 shows the inverter oscillating at 38Khz with a peak to peak output of 310V for a 1V input. With a 10V supply the peak to peak output may be over 3kV. The circuit seems happy to run with an input current of 3 - 4 A making its maximum safe wattage around 40W.
A commercial electronic ballast to run a 70W metal halide lamp from the mains is shown below in image 4.