> So today's question is how to use an NPN transistor to switch 24V
> (arbitrarily picked) with a PIC output pin. From what I've read if you
> do the standard:
>
>         +24
>          |
>        Load
>          |
>          C
>         /
> PIC-R--B
>         \
>          E
>          |
>         GND
>
> Then the transisor won't be saturated because the 5V output from the PIC
> isn't high enough relative to the collector voltage.

If I read this ascii schematic correctly, you have a resistor in series with
the base to a PIC pin, the emitter is grounded and the collector is
connected thru' the load to +24v.

This is a simple 'open collector' output and works very well, as long as the
transistor can be taken into saturation.

Since the base of the transistor will only be Vbe above ground (a few tenths
of a volt), the 5V PIC output is more than enough to saturate the transistor
(as long as the beta is high enough and the R is low enough and the PIC
output can source the required current).

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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