Ignition coil charge time

[Peter Wales]

Last week Ed Lansinger wrote 14 pages of waffle repeating what every one
else had written and telling us about the magic of an oscilloscope and a
resistor :)

Its a long time since I went to school and things may have changed since
then due to inflation but they taught me that inductors and capacitors have
phase lead and lag. They also taught me that in an inductor, the magnetic
flux was proportional to the current in the inductor. Therefore, for any
given coil, the greatest flux in it will come when the current is at its
maximum, and that won't be until it has achieved a steady state, ie the
voltage change and the current change in the coil is zero. Now, according to
muy simplistic mind, if you put 12v across a coil, instantaneously, there is
no current flow. Then the current increases as the coil "charges up" Then
the coil has full current passing through it and the coil can absorb no more
energy and the transistor saturates causing the voltage drop across the
emitter collector to appear. Hence you know the coil is charged.

Ed' graphics (so much *better* than mine) and the explanation :----

  You get nifty pictures like:
>       
>            _____
>           /     !
>          /      !
>         /       !
>        /        !
>       /         !
>      /          !
>_____/           !_____
>
>     ^     ^     ^
>   coil   peak   spark fires
> begins   current
>     to
> charge
>
>If you measure the slope of the rising part you know how fast your coil is
charging.  If 
>you know that N amps of current is required for reliable ignition, you can
then figure 
>out the correct charge time.  This should be very close to the time
predicted by the 
>equation provided by Matt Franklin and John Stein (given that you know the
inductance of 
>the coil).  Increasing the battery voltage will increase the charging rate
and lower the 
>required charge time.
>

---really don't help a lot. The original question was trying to find out how
long the coil should be charged for to get reliable ignition. If he knew
that he wouldn't have asked the question! Increasing the battery voltage on
a 12v battery is not easy.



>6. "Saturation" simply means that you've reached a limit of some sort
despite the 
>continued application of a stimulus.  It's not a term specific to inductors
(or 
>electronics, for that matter).


How about waffle saturation?


Peter Wales