NI 9758 Manual
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© National Instruments
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5
The NI 9758 has two phases of current control. The maximum phase current is 4 A. Control can
be set to transition from the first to the second phase after a set time or immediately when the
first current level is reached. To drive low impedance injectors, set the first phase peak current
to 4 A and the second phase hold current to 1 A.
If for the entire duration of the injection event, neither peak nor hold current levels are reached,
then an open circuit will be reported for the channel. It is possible that an open condition may be
falsely reported under certain conditions, even though a fuel injector is properly connected.
Examples of this include very low battery voltage, poor connections which introduce added
resistance, or total injection duration, which is less than the requested peak time and the peak
current is never reached.
Each PFI driver channel is protected from accidental short circuit to battery. At the beginning of
each injection event, the current rise rate is monitored in order to detect a short circuit. If the
current rises faster than expected for a typical port fuel injector solenoid, then a short circuit error
flag will be reported for that channel and the channel will be disabled until software clears the
error flag. Short circuit detection is also performed continuously at a later time within each
injection event. The short circuit error flag for each channel will not be cleared while an injection
event is being commanded for that channel. It is strongly recommended that the user not
continually clear the short circuit error flags, as this may lead to driver circuit damage in the
event of an actual short circuit. Even though the short circuit detection and disable feature is
present, it is strongly recommended that the wiring harness include a 5 A automotive fuse for
four low impedance injectors as shown below.
Figure 2 is a representation of a typical peak/hold current trace if one of the injector leads were
monitored with a current probe and oscilloscope.
Figure 2.
Peak/Hold Current Profile
