Ignition Coil Drive Circuit

[Tony Bryant]

> Date:          Tue, 17 Jun 1997 16:13:10 -0500
> From:          "David C. Carlson" <dcc. at worldnet.att.net>
> To:            diy_efi at coulomb.eng.ohio-state.edu
> Subject:       Re: Ignition Coil Drive Circuit
> Reply-to:      diy_efi at coulomb.eng.ohio-state.edu

> I have observed that ignitions with transistor or mosfet drivers for the
> coil do not have a capacitor (condenser) in shunt with the driver like
> the old-fashioned points, condenser and coil system. I realize that the
> switching times would be slowed by the addition of the capacitor.  I
> always thought the L-C resonance increased the spark energy and
> duration. Is the coil current collapse sufficient for spark generation?
> 
> I am looking for comments.
> 
> Dave

Well the way I see it is:

The energy in the cap is 1/2*C*V*V = 1/2*.22*10e-6*12*12= 16uJ
Energy in coil        is 1/2*L*I*I = 1/2*.01*3*3=         .45J

Assume coil primary L is 10mH (I can't remember exactly...), but the 
point (no pun intended) is that the cap contributes virtually nothing
energy wise to the system.

Its use in points, as far as I can see is to lower the peak of and 
slow down the inductive kick back, so the points don't arc. It could 
be used to help protect the transistor in a similar manner, but 
zeners do a more accurate job.

Remember the higher you can let your coil primary (-ve terminal) get 
to, the higher the peak secondary voltage will be - they are 
proportinally related (k~=100 on normal coils).

As for multi sparking, well maybe it'll help, but why not ensure it 
happens properly on the first try?

BTW, has anybody experimented with building a say 12V->24V 10A 
dc convertor to power the coil with? Think of all that energy - arc 
weld with your spark plugs :-) 

The coil may need heatsinking though...  10A * 24V * .5 duty cycle = 
120W - ouch!


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bryantt at psc.fp.co.nz