Darren Gibbs wrote:

> I was thinking that having +50V on the wiring out to the solenoids and
> back all the time was in some sense less safe.  Switching the +V side
> would mean the network of wires is dead except for the few milliseconds
> during which they're energized to activate the solenoid.  

Yes, that's one of the reasons why one might use high-side drivers. 

> I just wondered if there were other reasons for using NPN.

Usually the circuit is simpler, and often the layout: you have the high
voltage only at the driver output. You may not even have to connect it to
the board. With high-side drivers, part of your circuit has to operate at
the high voltage.

Gerhard

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