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...
>=20
> I usually see regular diodes across the inductive load itself, but I=20
> notice the LM1949 fuel injector driver puts a zener across the switching=
=20
> transistor.  Makes sense to protect the transistor, but wondering if=20
> this was just for the ease of putting it internally to the controller,=20
> or if there's something other better about this arrangement.
>=20
> Secondarily, how does one calculate the value for these?  I can=20
> calculate an estimate of the energy held in coil once energized, but=20
> with the field collapses, what determines the voltage and current=20
> produced?  It should be the resistance of the load, but that would be a=20
> diode in this case.
>=20
> My end goal here is to determine how to calculate the correct size=20
> protection diode.
>=20
> Cheers,
> -Neil.

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