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Re: 555 timer driver for ignition coil

I too have built a few 555 driver circuits for small coils over the years. 
But, I used a different circuit than the one you found for my most recent 
one. Mine came from a back issue of Radio Electronics magazine. It is the 
1990 Experimenters Handbook. Your local Public Library might have a copy of 
that somewhere on hand if you're interested. In my experience the publisher 
of that magazine is very unreasonable on their back issue prices, especially 
since they are a technology oriented company and doing business in these days 
of the WWW, PDF's, FAX's, etc.

The circuit uses an 18vac 2amp transformer rectified by 4 1N5408 1000V diodes 
and filtered by a 2200uf 50WVDC electrolytic. This rectified current, in my 
project, goes to a barrier strip and branches to the driver circuit board and 
to the ignition coil. the 555 drives a 2N3906 at about 125Hz fixed and that 
in turn drives an AF PNP (I used the TIP42 from radio shack) which drives 2 
parallel 2N3055's into saturation through 2 parallel 100ohm 1w resistors 
each. The Ignition coil has it's positive lead tied directly to the positive 
bus and the coil's negative lead goes to the 3055's and then to the ground. 
And I threw in a .47uf 250v rolled PP cap across the +/- busses on the driver 
circuit board. I mounted the 3055's on a heat sink made of a piece of 
4X5X0.030" Al.

I am in the process of trying to simplify this by using a MOSFET, maybe an 
IRF540, but I am still learning this electronics stuff so I don't know how 
that will work. Maybe if I use a MOV to protect everything......

I have let it run for around 5-10 minutes at a time for a total of about 4 
hours so far with many short runs for tuning in between. 

BTW this is used to energize a primary made of 12g 600v solid copper wire 
wound on 4" PVC for a total of 25 turns with the insulation nicked every foot 
from the 13th to the end for a tap point on every turn from 13 to the end. 
The tank cap is made up of three 500pf 10Kv ceramic disk caps in parallel for 
1500pf. Those little caps (I mean little) came from Mouser and are only about 
3/8" in diameter and less than .250" thick. They do nothing but arc back and 
forth between their leads, which are about .250" apart, while running exposed 
to air, so I potted them in wax. I know that I shouldn't use ceramic caps 
here but the plans called for doorknobs and I didn't want to spend the money 
for those. I tried a home made plate cap using single strength window glass 
and Al foil but didn't get any improvement in the output and the little 
ceramics are so much more portable. And, I find it odd that the 10KV caps 
stay together in this circuit when the Ford type ignition coil I am using is 
supposed to put out several times that voltage when running on 12VDC and I am 
basically doubling the voltage into the coil over that. The gap is just a 
couple of fahnestock clips on 1 inch standoffs with a piece of 12ga wire on 
one side and a piece of #26 on the other.  I know this gap sounds odd, but 
the plans called for it this way and I didn't see any reason to change it 
until the rest of the design proved solid. I am thinking of a small RCQG made 
of 1/4 inch copper couplings. I don't know if it will even work, much less 
improve anything, but this is a hobby after all and will look better if 
nothing else. The secondary is #26 magnet wire wound on inch and a half PVC 
for a total of 22 inches wound. The output electrode is just a #10 machine 
screw threaded rod in a HV pass-through bushing about an inch tall which I 
found, after many hours of fruitless searching, at ACK Electronics in 
Atlanta. All this is mounted on 5/16" acrylic which I cut on from scraps 
using the 4'X8' CNC router we have at the sign shop where I work. The 
coilforms and gap live on a 9" circle suspended 12" above the 11" square 
baseplate using 1" PVC pipe sections held together with threaded rod. It 
looks pretty good in my opinion even if the output is not spectacular.

Not the most powerful coil in existence but it seems to work reliably.
I have built more powerful coils, But I always seem to come back to these low 
powered ones for fun.

Hope this helps,
Marc S.