# Tesla Coil using 555

Posted on Sep 2, 2012

Single transistor flyback driver caused many problems due to it's operating principle. I received e-mails from people who were unable to get it functional even when they are sure that their flyback and transistor is OK. 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. You can quickly calculate operating parameters from the resistance and capacitor values with a small program that I've written. Values given for R1, R2 and C in this diagram are the ones used on my prototype. You may change R1 and use a trimmer instead of R2 to find an optimum frequency / duty cycle combination for your flyback. By changing C, you'll have the ability to use higher or lower resistor values, but do not prefer too low resistances (especially for R1) for not to overload 555. 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) :...

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