Hi Bob,

Right.  The key part I was missing was that the current can't change=20
instantly.  This little tidbit fixed, well... me.

The time I was referring to was just a freewheeling diode across the=20
coil vs a zener, not a TVS.  I'll look into those as well.

Cheers,
-Neil.


On 10/20/2015 2:14 PM, Bob Blick wrote:
> 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:
>> 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!
>>
>>> Since the time is very short, long term dissipation will be a secondary
>>> consideration.
>> Okay.
>>
>>> 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
>> these, and found that with just the freewheeling diode, it would take
>> several (IIRC around 10 ms) to dissipate the energy. That's significant
>> for an injector, so I like the zener idea better -- either across the
>> transistor or together with the freewheeling diode as Isaac suggested.
>> But I've also been trying to figure out why the zener specified in the
>> example circuit for the LM1949 is *seemingly* so small (power rating).

--=20
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