Misc Circuits: Bowden’s Capacitive Discharge Ignition Circuit Demo 1

(no narration)
Here’s a little circuit from Bowden’s Hobby Circuits site:
http://www.bowdenshobbycircuits.info/
This is a great site, full of interesting circuits and clear explanations of how they work. It’s like a full course in electronic tinkering.

Here I demonstrate the Capacitive Discharge Ignition circuit. This isn’t really a practical ignition system yet, but with a few changes it could be. As usual the camera frame rate misses some of the sparks; every time you hear a “snap” there is a good spark in the gap.

I managed to scrounge all the parts I needed from my junk boxes and parts stash so I spent no new money, except for the coil which is the very cheapest they had at the local auto parts store: 12 dollars US. The 74c14 Hex Schmitt Trigger chip is configured to make two oscillators, one at around 1100Hz to drive the stepup transformer and another at around 4Hz to gate the capacitor dump through the SCR into the ignition coil primary. The High Voltage section (green board with caps and diodes) is a voltage doubler and pulse capacitor system, operating at around 350-400 V.

The system runs on 12 VDC at around 200 mA. Nothing heats up, the heatsink on the mosfet is not needed at all if you use the IRF3205 mosfet as I did, instead of the IRF510 specified. I also used BYV26E ultrafast diodes for the diodes in the HV board instead of the 1n4004 specified in the schematic, and I used a BC337-25 transistor instead of the 2n3904. The capacitor bank I had to build up out of odd values so it measures more like 2.2 uF instead of 2 uF. And the transformer I used doesn’t have a center tapped low-voltage winding, so I am actually only operating at about half the step-up ratio that the Bowden original circuit reaches using the Radio Shack filament transformer LV winding at its center tap.

I added the red LED + 1.8K resistor in parallel with the SCR trigger pulse as a pilot light and operational check.

I’ll be winding a custom transformer for it, so that I can get my experiment transformer back from this installation.

Link to original circuit on Bowden’s site:
http://www.bowdenshobbycircuits.info/page2.htm#cdi.gif

Thanks Bill!

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