Ignition coil charge time

[Ed Lansinger]

Bill Lewis wrote:

>Does anyone know how to figure out the correct charge time for a given
>ignition coil?
>
>Should you run the coil until the current saturates?  Slightly less, or longer?

I'm not sure what you mean by "until the current saturates", but I'll give a shot at 
a quick-and-dirty answer anyways.

1) Don't use so much charge time that the coil overheats and melts.

2) Don't exceed any other physical or electricals limit to the coil or drive circuit.

3) Use as much charge time as you can so you can build maximum current in the coil, so 
long as you don't run into problems with 1) or 2).

Assuming a coil of reasonably low resistance, your current will build linearly with 
time.  Energy stored in the coil is a function of the current squared.  The more energy 
the merrier (pretty much).  At some point you will saturate the ferrous core of the coil 
so that higher currents will not actually store more energy.  I don't know that you 
really need to care about this (for backyard-mechanic purposes) as long as you aren't 
burning anything up.

Ignition drives that have a current-limiting feature should not stay in the "limiting" 
condition very long.  That's when power dissipation in the drive transistor reaches a 
maximum.  The transistor really wants to be fully on or fully off to run cool.

If there is no current limiting circuitry, any coil will eventually max out on current 
(the maximum level being a simple function of the voltage drop across the coil and the 
coil's resistance).  Perhaps this is what you mean by "current saturation".  There's no 
point in turning the coil on longer than it takes for this to happen, because no 
additional energy will be stored.  Your coil and/or drive circuitry probably couldn't 
take it for long anyways.

Ultimately, the "correct charge time" is the amount of time needed to charge the coil so 
that sufficient energy gets delivered to the spark gap to ensure reliable ignition.  If 
you know the inductance and resistance of the coil in question, and the source voltage, 
you can, of course, figure out how long it will take to charge it to a certain current 
level.  Figuring out what that "correct" current level is can be a simple matter of 
trial and error with a running engine and the coil in question.

-------------------------------------------------------
Ed Lansinger
General Motors Powertrain
Powertrain Control Center
Premium V Software & Calibration Group
Milford Proving Ground, Milford, MI
elansi01 at mpg.gmpt.gmeds.com  8-341-3049  (810) 684-3049
The opinions stated above are mine, not my employer's.
-------------------------------------------------------