GM ECM injector driver capacity

[goflo at pacbell.net]

Bruce Plecan wrote:

>  why would increaseing the load raise
> the back EMF,  I can understand having a higher current load to deal
> with, since the higher switching currents, but the voltages should be
> close eh?.

Sorry, I just realized all that blah blah I just sent did'nt answer the
question you asked.

When the driver is "on" the current path from the battery thru the
injector thru the driver to ground is a low resistance - Even though a
fairly substantial current flows (several amps) the voltage developed
across that resistance is low. 

When the driver turns off (becomes a high resistance) the  magnetic 
field of the solenoid collapses, producing a current across what is now 
a high resistance path - Thru the driver, which is now high resistance,
and thru the battery the wrong way - Also high resistance.

The energy of the mag field is released in a millisecond or so - You 
now have (briefly) a substantial current across a high resistance. 

Voltage = Current x Resistance, so driver-on you have (approx)
4 amps x 3 ohms = 12 volts. 

Driver-off: The driver resistance is now kilohms (it's off!), the
battery's resistance is at least hundreds of ohms, lets say 300 ohms. 

4 amps x 300 ohms = 1200 volts. Not bad for a 12V circuit. Of course
this spike is just a few milliseconds, but its enough to smoke a
semiconductor junction or gate, or erode switch and relay contacts.

Finally, since Gar is no longer available to chew my ass, I should
mention that nearly everywhere above I say resistance I should be
saying impedance - There are a few other simplifications as well, but 
we can worry about them another day.

Hope this makes some sense.

Regards, Jack