DIY_EFI Digest V2 #328

[eric schumacher]

Hi Jake
Don't think of injectors, particularly low impedance injectors, as operating
on a voltage.  Think of them as operating on a current. Their impedance (the
DC part plus the AC part) varies dramatically with the position of the
armature in the injector. What this tells us is to drive them with a current
generator for maximum speed & predictability under varying environmental
conditions. For a current generator to work properly you need a relatively
high supply voltage (compliance voltage) to guarantee that you can force the
required current thru the coil under all conditions. The simplest of current
generators is a  ON NPN transistor with the base held at a controlled
voltage, a resistor in the emitter to set current and the injector in the
collector.  A typical base voltage will be maybe 3 volts. With a 1 ohm
emitter resistor the "constant current" thru the injector will be
3.0-0.7/1=2.3 amps.  This will hold as things vary around, like battery
voltage.  Motorola has some good stuff on this in their App notes for their
injector drivers.


>speaking of which, does anyone know how I can identify the optimum voltage
>of a given injector based on its impedance, and what are some common
>voltages? i'm dealing strictly with production stuff, for example, bosch
>LH-jetronic injectors. They are 4 VDC, right? what cars use 12V injectors,
>and where can i find a good example circuit of how to trigger one?
>
>Jake
>
>
Lotsa Luck Eric
85 GTI with VR6 power