GM ECM injector driver capacity

[mrvette]

actually, the devices GM used have a reverse bias diode built in,  or one is in
the circuit board....most switching drivers have this feature already in the
package.....GENE

goflo at pacbell.net wrote:
> 
> 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

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