[Diy_efi] injector current-feedback driver

[Mark Bowers & Family]

> The circuit works by masking the master injection signal
> when injector current exceeds a threshold. It has a histeresis
> comparator. The histeresis can be set with Rhisteresis.
> The frequency will not be very stable.
> The limiting current is basically wired in by Rcurrent (so
> that voltage drop is ~0.7V), but you do not want to change
> that except when also changing the injector.
> Do you have comments on the circuit?

The Qfeedback needs to be a PNP transistor to work with the Rcurrent
connected as shown. You are right; the frequency will not very stable at
all! The Schottky diode is not connected in the right place; it will allow
current to flow in the injector all the time. There are a few other nuances
that could be made better, and the dependence of frequency on a lot of
variables in the circuit components would make this circuit pretty hard to
predict in exactly how it would perform, and I would wonder if it would be
consistent between each implementation.


> Can 4 paralleled HC7404 inverters drive an injector driving
> NPN transistor?
The ability of logic gates to drive current out of the output can vary a lot
from devices within a batch and over temperature, so using logic gates to
drive injector drive transistors is not a good method in general. I would
prefer to design a drive that has very repeatable and tightly determined
characteristics. If you are trying to drive amps or many 0.1 amps through
the injector, then 4 paralleled gates do not have nearly enough output to
drive Q1 as shown. (I assume you want to saturate Q1, and use the pulsing
feature to provide the current limiting.) Since FET's do not need actual
current to keep them on, they are easier to drive with logic gates; that
would be a better choice for this design.

Regards,
Mark B.






_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi