Re: ignition coil

From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Monday, July 21, 1997 4:45 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: ignition coil

Hello Kevin,

> From:   Kevin[SMTP:wawa-at-spectra-dot-net]
> Reply To:   wawa-at-spectra-dot-net
> Sent:   Monday, July 21, 1997 6:24 PM
> To:     tesla-at-pupman-dot-com
> Subject:    ignition coil
> I got an ignition coil yesterday from a friend and I hooked it up to my
> relay.  With a wire connected to the high voltage terminal I drew sparks
> off of the the other terminals.  The sparks weren't very big though
> (3-5mm).  I can control the speed of my relay now by putting a capacitor
> across the relay terminals, but that just determines the speed of the
> sparks.
> I just don't get why the sparks are so small. If I just have the coil
> and a power supply, everytime I brush the wire from one the coil's
> terminals (not the high voltage one) with a wire from the power supply,
> I get big bright sparks.
> I built a pulse generator (circuit found at
> http://www.geocities-dot-com/CapeCanaveral/Lab/5322/coildrv.htm)
> but the results with it were non-existant.
> What can I do?

OK, how does it work? It works by storing energy in the core via the 
primary and releasing it in both primary and secondary when the power 
supply is cut off from the primary. 

(1) How to maximise core energy? Since E = 0.5LI^2 and L is fixed, 
you must get I as high as the primary winding can stand. Since I is 
limited by Vapplied/Rwinding, the more V applied, the more I.  BUT,
applying V to an inductance means it takes time for I to build up to 
the resistance limited value. So dwell time must be set long enough.
The longer the dwell, the slower the breaks have to be if core energy 
is to be maximised.

(2) How to get _all_ energy from the core to the secondary? Not easy.
You have to break the supply from the primary without a spark across 
the break contacts. Fast parting contacts are the order of the day.
You can slow the rate of rise of core energy release by putting a 
small capacitor across the contacts, the ideal being that the 
contacts part company faster than the voltage across them rises.