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Re: Sam's High-Power Car Ignition Coil Driver <= Now running



At 10:43 PM 1/6/99 -0700, you wrote:

>Original Poster: Sam Barros <sambarros-at-yahoo-dot-com> 

>>Original Poster: "B**2" <bensonbd-at-erols-dot-com> 

> Yeah, I got my value for the caps like that tooÖ I still donít get
>why you need those diodesÖ

Any energy not absorbed by the load, i.e. inductive flyback, will be blocked
by the SCR (after its reverse-recovery time, typicaly 5 - 50 uSeconds). This
flyback pulse could breakdown the SCR, or just suck more current through the
ballast resistor/inductor. With freewheel/flywheel anti-parallel diode(s),
the flyback can re-charge the capacitor and conserve energy. Your
performance should improve with it, especialy if you have removed the steel
coil core, which decreases K and increases flyback energy.

>The thing troubling me (this question is open to anyone who thinks he
>knows the answer) is that 

OK

> Any attempt to
>increase or decrease any of this capacitances decreases the output. I
>donít know why but it does. 
...
> at 1.1kW there is MORE than enough juice to charge up
>more caps.

>So I donít know why I have to stick to those puny 20uF
>overall. Anybody can think of a reason why changing cap values
>decreases the output voltage? I considered the system going out of
>tune but it canít be thatÖ

RC time constant- The load resistance & limits the capacitor's charging.
Another consideration is heating elements and lamps have non-linear
resistance; they increase resistance with temperature. So there could also
be thermal and thermal time constant effects. Please try an inductive
ballast! I bet your power consumption will drop 400% with no change in your
coil spark length.

Lets see, if your using 220 VAC-RMS & a 1.1KW load, its around 50 ohms. Time
constant at 10uF = RC=.5ms, probably not that.

Capacitor ESR- your 20uF capacitors have too much resistance.