Similar problem in relays, the common diode across the coil slows down the
release which causes arcing problems.
\We use zeners from the bottom of the coil to ground. The zener voltage is
a bit lower than the VCE of the transistor.
Speeds up the release by about 50% on our relays.

On Tue, Oct 13, 2015 at 3:20 PM, Bob Blick <bobblick@ftml.net> wrote:

> Hi Neil,
>
> As with any inductor, when the transistor turns off, current continues
> to flow through the inductor into any path there is, until the energy
> stored in it is gone. That energy is dissipated in the snubber device
> and the coil resistance. The effect in an injector is that the fuel
> shutoff is delayed. By using a zener diode instead of a regular diode,
> the stored power is released at a higher voltage and the stored energy
> is dissipated faster.
>
> Remember, the coil current will be the same, so the higher the snubber
> voltage the quicker the coil's field collapses.
>
> Cheers, Bob
>
>
> On Tue, Oct 13, 2015, at 02:02 PM, Neil wrote:
> > Anyone here a pro on these?  Specifically...
> >
> > I usually see regular diodes across the inductive load itself, but I
> > notice the LM1949 fuel injector driver puts a zener across the switchin=
g
> > transistor.  Makes sense to protect the transistor, but wondering if
> > this was just for the ease of putting it internally to the controller,
> > or if there's something other better about this arrangement.
> >
> > Secondarily, how does one calculate the value for these?  I can
> > calculate an estimate of the energy held in coil once energized, but
> > with the field collapses, what determines the voltage and current
> > produced?  It should be the resistance of the load, but that would be a
> > diode in this case.
> >
> > My end goal here is to determine how to calculate the correct size
> > protection diode.
> >
> > Cheers,
> > -Neil.
>
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