Subaru TBI

[Mike (Perth, WA)]

At 06:12 AM 7/03/2000 -0800, you wrote:
>>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.

Yeah good point, when I first read the post I assumed he meant no fancy
gear off the end of the injector driver IC...

>This is a PeakHold type injector.  13.8V / 1.6 Ohms is almost 9 Amps.

Aren't all injectors in use in last 20 years or so equivalent,

ie. Pullin current is far more then hold current.

>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.

Sorry can't see that - surely the ballast resistor reduces 'sharpness' of
the initial pull in by limiting it to 1A hence also slower to pull in ?

> You still end up
>with a fast turn on time.

Can't see that at all - thats why we have injector drivers - or do we only
need injector driver ICs for specific types of injectors and not PH ones ?

>  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.

A variation of a infra-pulse circuit comes to mind, charge cap to 12v,
use this to supply current to injector *hard*, but main power comes from
lowered voltage - like 5v to keep injector open after pulse dies away,
bit like a CDI,

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

The bosch injectors I used for my EFI project in 82 were around 8 ohms or
so (IIRC) and pullin time varied wildly unless driven hard with 12v,
anything less - even 11v and the pullin time changed noticeably. Power
consumption and injector reliability (back then) wasn't an issue so I
only used the 12v raw battery rail.

For maximum reliability for any injector I'd look at the precise pullin
current for minimum  time to ensure pullin and the hold current issue etc...

Incidentally, most injectors I've seen don't use back EMF diodes,
you should see the 28v spikes on the *battery* - WTF! - no wonder we
need surge suppresion on heaps of other vehicle electronics... <phew>

Rgds

Mike

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