Injector driver question...

[Teller.John at orbital.com]

Please respond to diy_efi at esl.eng.ohio-state.edu
                                                              
                                                              
                                                              
 To:      "diy_efi at efi332.eng.ohio-state.edu"                 
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 Subject: Injector driver question...                         
                                                              




<SNIP>

>When I tested it with a high impedance injector (14.9 ohms), the peek
>current was about 2.5 A and it took 4mS (seems very long to me...) to
>rise to this value. After that 4 mS period, the timer fonction of the
>LM1949 lowered the current to the hold mode of 1 A, as expected.
>
>My question is : why the current is so high with the high impedance
>injector ? If we divide the battery voltage by the impedance of the
>injector, we get 12 / 14.9 = 0.8 A, which is the value of the current in
>the steady state.
>
>    Hugo Villeneuve.

I am not an injector expert, but I believe you are attempting to apply a DC
based formula to an AC problem (pulsed DC actually, but the same thing as
far as calculations are concerned).

What is happening is that the solenoid coil in the injector looks like an
inductance and a capacitance value in parallel with some resistive values
thrown in everywhere for good measure.  The capacitor here is formed by the
inter-wire capacitance of the coil.  What happens at the beginning of the
DC pulse is this:  The capacitor looks like a short and the inductor looks
like an open circuit.  As the solenoid energizes, the capacitor charges up,
and the magnetic field around the inductor starts building up.  Eventually
the capacitor is fully charged, and the magnetic field is built up to
whatever the current going through the solenoid can put out (the current is
limited by the resistance of the coil).  At this point the capacitor looks
like an open circuit, and the inductor looks like a resistor.

As you can see, this is really complicated, which is why the injector's
coil measurement is impeadance and not resistance.  This impeadance also
varies depending on the frequency of the pulses being applied to it!  What
a headache.

--- John Teller