Subaru TBI

[John Dammeyer]

>dzorde at erggroup.com writes:
>
>>Just down the road from me, good to see some fellow West Aussies on the list.
>
>That makes four!
>
>>On a different note.  I just picked up a Subaru TBI (centre point injection,
>>think its of a 1.8L import motor), the injector is a 1.6 ohm unit.  Can someone
>>tell me if this has to be driven as P/H or whether it can be driven as saturated
>>(if so does it need a resistor ?)
>
>No resistor required. Injectors are normally driven by sinking
>current through them; no "ballast" required. The duty cycle of
>pulsing determines the flow.  No fancy timing requirements.  It's
>just a cheap way of implementing an electronically-controlled
>carburettor. ;-)


Yikes!  or How to screw up an injector in one easy pulse.

This is a PeakHold type injector.  13.8V / 1.6 Ohms is almost 9 Amps.  Injectors normally
don't take more than about 1A.  You have a choice on how to drive a PH injector.  You can
place a Ballast resistor in series to limit the current to about 1A.  This has the effect
of driving the injector hard until the current through the resistor is high enough to
reduce the voltage across the injector and therefore limit the current.  You still end up
with a fast turn on time.  Alternatively you can use a PH type injector driver or a
circuit with a comparator and a current sensing resistor (0.1 Ohm) that reduces the
current once it reaches 1A or a timer based driver that reduces the current after n
microseconds which is equivalent to the time it takes for the injector current to reach
1A.

Only if the injector resistance is around 12 Ohms can you apply 12 volts directly to it.

John Dammeyer

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