Test Injector Inductance


12/08/04


     I was curious to see what the inductance of those two injectors I scrounged from the local Pick-N-Pull were.  The low impedance unit came from a early '90s Chrysler V6 and the high impedance unit off of a Toyota I think.  Should have paid more attention.  The test setup I used is very similar to the actual injector driver with no hold mode.  What we are doing here is hitting the inductor with a constant voltage, or as close as possible, and measuring a linear portion of the current flowing through it.
     When calculating the inductance I took the mid-point voltage across Rsense and subtracted it from the voltage across the coil.  Another small source of error is the power supply droop.  Mostly under 100mv so it didn't contribute too much error when calculating the inductance. 
     The following formula to calculate the inductance of the coil.   There is a difference between when the injector was still closed and when it opened.  Makes sense as there had to be some change in the magnetics as the solenoid core, or whatever the mechanical mechanism was, physically moved.  Also there certainly is some measurement uncertainty here in that it was a little tough deciding where a linear portion of the trace was.  In fact the high impedance unit look like there was a some associated capacitance.
     Another point of interest in the plots is when the injector opened.  The Low impedance injectors open about twice as fast as the high impedance units did.  Well that is one of there selling points isn't it...

The number one (lower Yellow) trace is the voltage at the Drain of the driver transistor.
The number two (lower Green) trace is the voltage across Rsense.
The number three (top Cyan) trace is the trigger voltage.

Rsense = .0978 ohms so adjust the current upwards by 2.25%

Test Circuit.........



The driver ET.....


Type:High Impedance
Test:Pre Turn-On
Vg:14.0 volts
Delta I:128 ma
Delta T:300 us
Inductance (Off):32.8 mH
Type:High Impedance
Test:Post Turn-On
Vg:14.0 volts
Delta I:136.7 ma
Delta T:230 us
Inductance (On):23.5 mH
Type:High Impedance
Test:Turn-On Point
Delta T:1.1 ms

Type:Low Impedance
Test:Pre Turn-On
Vg:13.7 volts
Delta I:512 ma
Delta T:400 us
Inductance (off):10.7 mH
Type:Low Impedance
Test:Post Turn-On
Vg:13.7 volts
Delta I:1.175 Amp
Delta T:280 us
Inductance (On):3.2 mH
Type:Low Impedance
Test:Turn-On Point
Delta T:780 uS