> >        +24
    > >         |
    > >       Load
    > >         |
    > >         C
    > >        /
    > >PIC-R--B
    > >        \
    > >         E
    > >         |
    > >        GND

    I guess the question I'm still trying to resolve is how to figure out
    how much current must I provide the base in order for the transisor to
    saturate. Specifically I have a 1K resistor for R which was providing
    12 ma of base current in the original RS-232 config. If I drop the voltage
    to 5V it'll be 5ma (or thereabouts because of the 0.6V BE diode drop).
    What determines if that's enough current to cause saturation.

I(b) * beta should be much greater than I(c)

So if your load current is a 30mA relay coil, and you have 5mA of base
current, you need a transistor with a beta (current gain) of significantly
more than 6 (which is pretty easy.)  You do need to know I(c) to tell
whether you're going to hit saturation; since I(c) has a maximum value for
the transistor, you can calculate the minimum I(b) you'll need to maintain
saturation over the whole operational range, but you really only need to
care about the actual current you'll be using.

Note that beta is not a tightly controlled parameter on transistors; that's
one of the reasons for the "much greater" qualifier...

BillW

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