Sunday, August 14, 2011


Single transistor flyback driver caused many problems due to it's operating principle. In addition, since it's resonance frequency is determined by every part of the system, when you try to draw an arc from the transformer, it changes dramatically in most of the cases. Just because the operating frequency is important for the safety criteria, (both for mine and power transistor's), I decided to make it run on a constant frequency and built up another simple circuit, trying to stay in the specified limits of the 555 timer.

Setting the operating frequency with an integrated timer is easy and practical. Following schematic is nothing more than the standard astable mode circuit design with a classic 555. It requires only two resistors and a capacitor to set frequency (with duty cycle of course) and another resistor to determine power transistor's base current, which you can find it's optimal value experimentally. I used 1K for R1, 2.2K for R2, and 10nF for C which made circuit to run nearly at 27 kHz theoretically, at %60 high to %40 low duty cycle.

Power transistor is not critical and any other may be used as long as it's characteristics are equivalent or better. Here are the technical datas for BD243C for comparison:

Bipolar NPN transistor : BD243C
Casing : TO220
Max. collector current : 6 Amperes
Max. total power : 65 Watts, while case is at 25 degrees Celsius
Transition frequency : 3 MHz
hFE (current gain) : 30 at 300mA (minimum value)


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