Re: ignition coil
From: Jim Fosse[SMTP:jim.fosse-at-bjt-dot-net]
Reply To: jim.fosse-at-bjt-dot-net
Sent: Tuesday, July 22, 1997 8:19 PM
To: Tesla List
Subject: Re: ignition coil
>From: Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
>Sent: Monday, July 21, 1997 4:45 PM
>> 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.
>(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.
For a given I, that the coil can withstand, power can be increased by
increasing the applied frequency. P= 1/2 * L * I^2 * F. You then run
into the problem of I not rising fast enough to reach the max
allowable in the time available. However, I is proportional to V/R,
SO, just increase V and add external resistance. If you match them
correctly ,for the shorter time, you can now get the same maximum I in
the shorter period.
aside: I believe this is the reason that automotive systems went from
6V to 12V, they kept the same coil and added external resistance, to
improve the performance. Likewise, aircraft systems went to 24V;
partially for the increase in RPM that the new spark system could run
at (also lighter wireing for the same power).