# Building a DRSSTC Pt. 15 - The Interrupter (Pt. 1)

Blog entry posted in 'Building a Dual-Resonant Solid State Tesla Coil', December 30, 2015.

Hello everyone, welcome back to my Tesla Coil build!

In this entry I will explain the design of a simple interrupter that I will probably be using for the initial build. Originally I had planned on using a microcontroller, but for several reasons I eventually decided to go the analog route. The circuit I have designed (with some help from a friend of mine) is very simple, based around two 555 timers (or, alternatively, a 556 dual timer) and an LM324 quad op-amp.

The criteria for the interrupter design is as follows:

The operator must have the ability to adjust the output signal frequency independent of the duty cycle and on-time limits. The range should be within that of human hearing, preferably between 60Hz and 12kHz (doesn't need to be exact)
The operator must have the ability to adjust the output signal duty cycle independent of the frequency and on-time limits. It should be fully adjustable, with the outer limits as close to 0% duty and 100% duty as possible.
3. Upper duty cycle limit adjustment
The operator must be able to set an upper limit for the duty cycle, which limits the output pulse duty cycle regardless of the duty cycle setting. This would be useful if the duty cycle needs to be limited to, for example, 5% to ensure the transistors in the H-bridge have enough time to cool down during each cycle.
Similar to #3, the operator must be able to set an upper on-time limit for the output pulse. This would be useful if the on-time needs to be limited to, for example, 10uS to ensure the transistors in the H-bridge aren't on for too long at a time.
5. The output pulse width must abide by the limit set in either #3 or #4, whichever is smaller.

The circuit designed to meet these criteria is shown below:

96472

As you can see, the circuit is split up into three main sections: Frequency, Pulse Width, and On-Time. Let's take this piece-by-piece and make sure we understand each part of the circuit individually.

Frequency

96465

If you have ever played around with 555 timers, this circuit will probably look very familiar to you. It is a basic astable multivibrator, whose frequency is adjusted by R2. With 10k resistor for R1, a 10nF value for C2, and a 1M potentiometer for R2, according to the 555 timer formula:

$f=\frac{1.44}{C1(R1+2*R2)}$

where f is the output frequency, we should get a maximum frequency (where R2 is 0 ohms) of:

$f=\frac{1.44}{[10*10^{-9}F]([10k]+2*[0k])} = 14,400 Hz$

and a minimum frequency (where R2 is 1 megohm) of:

$f=\frac{1.44}{[10*10^{-9}F]([10k]+2*[1M])} \approx 71.64 Hz$

The criteria calls for a range between 60 Hz and 12 kHz, so this will be good enough.

The main difference you'll notice with this schematic is that we do not take the signal from the output (pin 3) of the 555 timer. In order to have independent duty cycle adjustment, we need to be a bit more clever. We actually take the signal from the trigger (pin 2) of the 555. The reason why we do this is explained in the next section.

(Continue to Pt. 2)