Ignition coil charge time

[Peter Wales]

At 04:58 PM 3/23/95 EST, DIY_EFI at coulomb.eng.ohio-state.edu wrote:
>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?
>
The answer for your coil can be found by trial and error. Build a variable
width pulse generator and fire it into your coil. You will get a waveform as
below on the primary.

               |
---|           |
   |           |
   |           |
   |_____|-----|
A  B     C     D

A is the before switch on time
B is the driver transistor switched on but not drawing a lot of current so
it is not saturated
C is the coil saturated and the transistor drawing full current so the
slight drop in voltage across the coil is from the Vce sat of the transistor
D is the switch off of the transistor and the coil voltage decaying to start
generating back EMF

You should size your transistor to be able to drive the coil continuously at
DC and have a Vce big enough to cover the back EMF. I use 450v 10amp devices
and never have failures. The minimum pulse width for the full spark is B-C
and remains a constant for that coil. C-D is overkill as the coil is
magnetically saturated and no more energy will go into it.

Peter