[SOLVED] Ignition Circuit Query

betwixt said:

You can leave pin 5 disconnected but it is prone to picking up interference and in your circuit where there are high voltages and large current pulses there is likely to be a LOT of interference!

Any reasonable value will do, 0.1uF will be fine. It you make it too big (> 100uF ?) it will take a long time for the frequency to settle down after applying power.

Brian.

Click to expand...

betwixt said:

Pin 5 is an input to the 555, in astable mode it gives you fine control of the frequency, the resistors and capacitor are the main frequency deciding parts. Leaving it disconnected or fitting a capacitor will not change how the spark is produced. The danger of leaving it without a capacitor is it will pick up some of the interference form the spark which in turn could shift the frequency, instead of getting a rapid 'click click click click' of sparks you could get irregular sparks or even screeching noises.

It is good you now have the blinking LED, it means the pulse that triggers the coil current is now being produced. I think the reason it doesn't work is that the transistors are not being changed from fully non-conducting to fully conducting as the pulse is fed to them. Try the changes I suggested and see if it works. As I said, it isn't how I would design it but it's good you are learning by experimentation.

Brian.

Click to expand...

betwixt said:

You can try this:



Sorry about the quality - it was scribbled and photographed on my phone!

It is completely untested, just drawn while I was working on something else and the values are not optimized but should work. I tried to keep as much as possible to the parts you already have but made these changes:

1. The 555 was producing a 66% on 33% off waveform, I changed the resistor values to make it about 90% / 10% because you only need short pulses of current.
2. (if I got it right) the output of the 555 is high most of the time so I added the bipolar transistor to invert the signal. It's collector should go high for about 10% of each cycle.
3. I changed to a MOSFET output transistor, these have much lower input current requirements than bipolar types so I increased the gate feed resistor to 470 Ohms, this also reduces the collector current in the bipolar transistor making it easier to find a suitable one.

You can use almost any small NPN transistor, and I would suggest easy to find MOSFETS are IRF530, IRF540 or IRF640 but any rated at >100V and >10A should work.

As I pointed out - it isn't tested!!! In a commercial design it would have several additional safety components but I kept it a simple as possible.

Brian.

Click to expand...