A repost

[Bruce Bowling]

> 
> Since the injector solenoid is an inductor I imagine 10 kHz 50 % duty
> cycle into the injector does two things.
> 
> 1.  Exceeds the Vce of the transistor unless you provide a path for
> the back EMF

Put in the zener diode, if one is worried. I think that the
Vce of the TIP120 is like 60 volts, and perhaps the injector
could put out spikes which exceed this.

I have run a very similar circuit for hours without the external
back-biased diode/zener. The schematic for this device (on the
blister pack from radio shack) shows an internal diode back-biased
across the collector/emitter, so perhaps there is internal
protection on this device.

> 
> 2  Approaches a steady state current that approximates DC.
> 

Yes. This was the point made earlier, that one can use PWM to
lower average current, as the LM1949 can do (and Motronic ECU's).

- Bruce


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                 Bruce A. Bowling
                 Staff Scientist
   Thomas Jefferson National Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23606
                 (804) 249-7240
bowling at cebaf.gov  http://devserve.cebaf.gov/~bowling
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