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Impedance is the property that limits the electrical current flow in an injector (or any other inductor, like a coil, solenoid, etc.).
Impedance (symbol Z) is defined as the ratio of the voltage phasor to the electric current phasor. It is a measure of opposition to time-varying electric current in an electric circuit. It is the alternating current equivalent of resistance.
where X is the reactance, and for an inductor (like an injector) this is XL = ωL = 2πf×L and the frequency is f, and the inductance is L.
Stated more simply, it is the resistance to a changing electrical signal. For injectors, the electrical signal is the On/Off ground from the MicroSquirt® controller.
For injectors, we can use the DC resistance (measured with a digital multi-meter (DMM)) as a 'proxy' for the impedance. This is because the impedance is formed largely by DC resistance component for injectors. However for other devices, this isn't true at all (a capacitor, for example).
For injectors, a rule of thumb is that low-impedance injectors have a DC resistance of 3 Ohms or less. High-impedance injectors have a DC resistance of 10 Ohms or more. Very few are in between these values.
Low-impedance injectors will flow 4 or more Amps if connected directly to a 12V supply with no current limiting (Ohm's Law: I = V/R; I = 12/3 = 4 Amps), and this will over-heat them very quickly and damaged them permanently. So low-impedance injectors must have some form of current limiting.
The V3 MicroSquirt® fuel injector drivers will clamp the current flow at 5 amps each, enough to drive one low-impedance injector, or up to 4 high-impedance injectors, per bank. Or for low-impedance injectors you can use resistors (see below). For up to 4 high-impedance injectors per bank, the driver will be fine, and high impedance injectors don't require current limiting (their internal resistance, i.e. 'impedance' resistance is high enough to limit the current to safe levels).
Normally, to limit the current for more than one low impedance injectors, injector resistors are placed in series with the injectors (V3 MicroSquirt® does not have PWM to limit current). Click the links to find out more.
High-impedance injectors will only flow an Amp or so if connected to a 12V supply, and do not need current limiting.