On 2013-08-28 10:57 PM, IVP wrote:
>> 1N5818 which is small, cheap, and readily available
>
> We had a lengthy discussion about this some years ago
>
> The outcome, ISTR, was that a 1N4001 or 1N4148 is better
> than a Schottky

Why is that?

> I might be inclined to have a diode for each coil and perhaps look
> at something at the driver end too, considering the length of cable
>
> Maybe a reverse-biased Schottky to catch sub-0V transitions,
> and a zener to clamp > Vcc transitions.

I found that TVS diodes are extremely fast and are a perfect fit for
this application. I would stick a reverse-biased low-voltage TVS in
parallel with the power supply to the control circuit. Also a diode in
series with the power supply to prevent reverse current. Perhaps a SiC
Schottky for fast switching and very low leakage. Which reminds me, I
now strongly suspect reverse current to be the cause of my mysterious
chip-frying problem. Sticking a Schottky in series with my power supply
seems to have stopped incidents of chips dying for now, but time will
tell...

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