>
> >        +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.
>
> Actually this is the best way to do it. From base to emitter there's a
> diode junction(0.6 volt) and as long as you have current flowing through
> it, the transistor will conduct collector-emitter.

Well this would be ideal simply because it takes the least amount of
components.

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. And unfortunately
I don't have time to study Horowitz and Hill ;-)

>
>
> >Now Bob Blick has an HBridge motor drive that uses a darlington configuration
> >with no pullup between the high voltage and the base of the drive transistor.
>
> Look at the lead-in page to the schematic, I've shown the internal
> schematic of the darlington transistors I used. They have built-in
> pinch-off resistors and also collector-emitter diodes, saves external parts.

I see how the integrated parts help. Maybe I'll start using those TIP parts.

BAJ

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