[Diy_efi] Re: injector current-feedback driver

[Mark Bowers & Family]

>
> the inductance and supply voltage determines di/dt
> through the injector. Rhisteresis/R2 determines
> the histeresis voltage. This together determines frequency.
> Is it that bad or unpredictable? Or are you afraid of
> the temperature dependance of Qfeedback?


In general, using logic gates as part of an oscillator has some variablity,
and theare are a lot of variables in addtion to the logic gates.
The factors  that could make the frequency unpredictable:
1) The temp coeff of Q feedback, as you said
2) The output current  of the logic gates the drive to the injector drive
transistor
3) Heating in the injector, that changes the injector resistance (this
doesn't change with the actual control circuit)
4) The variations in the car's 12V system.

> What if Q1 is a darlington?

That helps, but I personally have never liked using logic gates to try to
drive a saturated output bipolar device. It can be done if all the
computations of worst case temp and drive currents or voltages are examined.
An HC device and a FET are the best combination to possibly make this work
well. I can help you , but am going on the road for 4 days, stating today,
and will not have my data books handy. There was a fellow on one of the
conversations that had made a recommendation of a FET device during th e
conversation on the feul tank circuit. Maybe he can speak up here and give
his recommendation; he seemd to have good knowledge here.
>
> Maybe one other change would be right:
> To connect Rhisteresis directly to Qfeedback base,
> but the common point of R2 and C2 via a resistor.
> That way C2 could not unwantedly slow the positive
> feedback effect of Rhisteresis...

Let me take a look..

Have you considered using a comparator in place of Q feedback? I think you
would get much more precise trigger level , and could run this in  way that
could elminate some of the variability in the Q feedback threshold, due to
temp.

Also, instead of an oscillating drive system to limit current, why not a
definite cut back to a lower current? Seems simpler, but is this a heating
problem?

Regards,
Mark B.



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