Injector driver question...

[Walter Sherwin]

>Hi all,
>
>    I have designed and tested an injector driver based on the LM1949. I
>want to be able to run low impedance as well as high impedance type
>injectors. I tested it with low resistance (2.4 ohms) injector and the
>operation was as expected (at least on the scope screen!) with a peak
>current of 4 A and hold at 1.0 A. The rise time is 0.8 mS.
>
>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.




Your P&H values sound like they are right on the money for an  LM1949  with
4/1  P&H ( did your  4-1 timer shift  take place at roughly 3.9 ms?).  On
the other hand, your saturated values sound a bit off.  Did you change
anything between the two experiments (except for the injector)?  I assume
you had only one high impedance injector hooked-up during your saturated
tests?  Seems kinda strange.  I'd be really interested in knowing more about
your test setup.  Injectors have an equivalent circuit that can be modelled
as an RL circuit (no capacitance).  From what I've seen/tested, the current
response usually follows the classical "RL" equivalent circuit model?????


Walt.