555 driven ignition coils
From: Bill Noble[SMTP:william_b_noble-at-email.msn-dot-com]
Sent: Friday, November 21, 1997 11:27 PM
To: Tesla List
Subject: 555 driven ignition coils
years ago I made a Capacitor Discharge ignition for my 56 porsche - it used
an aircraft transformer to step the 6VDC up to about 450 V - I used that
through a diode bridge to charge a 1UF capacitor, and then discharged the
capacitor through an SCR to fire the coil - pair of 3055 type transistors
for the oscillator - I drove it at first with a 555, but found that letting
it self oscillate worked better (I can scan and post a schematic somewhere
if wanted) - anyway, the point here is about ignition coils - the input
voltage is not the 6 oer 12 VDC, rather it's the voltage that develops
across the primary when the points open - that's typically 200 to 250 V.
With the circiut I made, the spark would jump from the HV terminal to ground
consistantly if I pulled the HV lead out and cranked the motor - that's a
spark length of a couple of inches. (and the car ran better). This same
approach might be useful for a battery powered tesla coil.
>>If you want to make better sparks, put in a variable resistor in the
>555 circuit. Tune it until you get the longest spark. The best ignition
>coils are the red or blue ones. These are specially for use w/ transistors.
>Also, TV transistors arent the best. IGBTs are the best, but simply a 3055
>is ok. My ignition coil throws 3/4 inch sparks easily. Or, if you want to
>make the longest and fattest sparks, use a thyristor in series with a 1uf
>cap and the primary. Feed this arrangement with 250 VDC. The cap charges up
>and discharges throught the primary. I used 350 VDC and a 4,7uf cap, that
>made 2 1/4 inch sparks!