[Diy_efi] Re: new EFI controller

[Bevan Weiss]

> Hi Klaus,
>
> >> to make sure they switch off fast put a zener into the back EMF diode
path
> >> (A higher voltage during the injector energy discharge makes it faster)
> >
> > Yep, the consensus seems to be zener or shottky...
> > I'd probably say shottky, though I'd have to look into it slightly
more...
>
> Alexei suggested rectifying diode (fast or ultrafast).
> Not as sensitive, as a schottky.
> Or using a controlled flyback, as yourself used in
> the PWM limiting app.
>
> The AVR megasquirt is great and here: (thanx for the hint)
> http://caffrey.dk/megasquirt/
> Look at schematic page 6. T3 is a power pnp,
> flyback diode is connected to T3 emitter, collector
> connected to ground (
> (Michael, may be it should connect to power for efficiency
> and cool T3 operation!!!)
> base offset is set by a resistor and zener.
> I would also use a power resistor (~20..47 Ohm)
> parallel with T3 CE. I think this is better than
> a power zener. (Especially if we want to control it
> for PWM limiting)
>
>  Marcell

I'm slightly confused by the actual circuit presented in that schematic...
T3 appears to be a power pnp as stated, but I'm used to that circuit being
an implementation of a shunt voltage regulator.  Resistor R25 is set to
allow current to flow through the pnp and D17 such that D17 regulates to
provide 36V's to the pnp transistor.  The 36V's on the base of T3 then
establishes ~36V's on the collector of T3, with T3 conductor current as
required to maintain that voltage...

That would kind of make sense with D15 and D16, only allowing the flow of
current when the voltage at the injector is greater than ~36V, acting like a
higher power zener diode... providing better stabilisation of the 36V
threshold too :)  I'd think that the reverse avalanche (as used in zener
diodes) wouldn't be the fastest mode of conduction to enter into, and hence
possible damage could occur to the zener...
I guess if it's always in the zener region this isn't a problem, especially
with such a well regulated current through it, and leave the hard current
control job to the power pnp (like it was designed for)...

I maybe off the ball with this, so if I am just call me on it.  I'm not much
good at analog circuits :(

As for the injection scheme it's simple saturated current flow... :(

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