Darren Gibbs wrote:

> In my case, with a bunch of solenoids spread out over a fairly large
> area, wouldn't it make more sense to use a PNP transistor and switch the
> +V side instead?   

Why do you think it would make more sense? You may think that you have the
ground anyway where the solenoids are, but in reality, wherever you have a
/useful/ ground, you (usually) have a supply rail, too. (The useful wires
usually are wired in pairs, I guess.) So it doesn't really matter, from an
installation POV, whether you use low-side drivers (NPN) or high-side
drivers (PNP).

> Is there any reason not to do it that way?

Increased circuit complexity without a real advantage. Say you have 5V
logic and 48V solenoid supply. You can make a very simple low-side driver
with a single NPN transistor. If you want to make a high-side driver with a
PNP transistor, you need to drive that transistor's base at around 48V --
which requires another circuit, usually an NPN transistor. You might need a
second stage anyway, in which case there is not much difference in effort.
But you might not...

There may be cases where a high-side driver has advantages (like wall
switches for lighting: when switched off, the socket is actually without
power -- which is safer). But they are not simpler, neither in circuit nor
in wiring.

Gerhard

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