Hi Neil,

Since the amount of energy stored in the inductor is fixed, and the
current is always the same at shutoff, the amount of time it takes to
discharge depends on the voltage during discharge. So the higher the
voltage during discharge, the quicker the discharge. And the voltage
will be as high as you let it go, sort of.

I don't know why a TVS would take longer than a zener, they both react
in about the same way, except the TVS is rated for very high peak
current and the zener is rated for a relatively low current since it has
a high dynamic resistance.

Cheers, Bob

On Tue, Oct 20, 2015, at 11:04 AM, Neil wrote:
>=20
> On 10/14/2015 2:06 PM, Bob Blick wrote:
> > ...
> > The current will start at exactly what it was right before turning off,
> > and drop off quickly.
> Bam!  This is what I have been unaware of.   Enlightening!
>=20
> > Since the time is very short, long term dissipation will be a secondary
> > consideration.
> Okay.
>=20
> > You might look at using TVS diodes if you are pushing high currents.
> > Even tiny ones have a peak power rating of 300 watts.
> >
> > But thinking back in this thread, you were originally talking about PWM
> > on the injectors after the initial opening. In that case, you'd probabl=
y
> > be back to using a freewheel diode, not one of these higher voltage
> > snubbing methods.
> I read something online where someone was doing some experiments with=20
> these, and found that with just the freewheeling diode, it would take=20
> several (IIRC around 10 ms) to dissipate the energy. That's significant=20
> for an injector, so I like the zener idea better -- either across the=20
> transistor or together with the freewheeling diode as Isaac suggested. =20
> But I've also been trying to figure out why the zener specified in the=20
> example circuit for the LM1949 is *seemingly* so small (power rating).

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