Injector driver question...

[Hugo Villeneuve]

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.