>
> >
> > >          +--------+24
> > >          |         |
> > >          |       Load
> > >          Rc        |
> > >          |         C
> > >          |        /
> > >          +--Rb2--B
> > >          |        \
> > >          C         E
> > >         /          |
> > > PIC-Rb-B         GND
> > >         \
> > >          E
> > >          |
> > >         GND
> >
> >It worked as expected. Rc causes the load transistor to conduct. When the
> >input is pulled to +5, the switch transistor conducts pulling the base of
> >the load transistor low causing the load transistor to turn off.
> >
> >I'm still interested if the switching transitior in the arrangement above
> >is saturated or if it's conducting in the linear region. But since it
> >works, I'm going to roll with it.
>
> The answer is yes. :)  It all depends on the value of the resistors.

Which translates to the amount of current available at the base. These
are generic NPN switching transistors from RatShack. No data available on the
beta. The only value listed is Hfe = 200.

>
> >The Rb2 was already in place from the original circuit. It didn't seem to
> >have any adverse effect.
>
> In this circuit, it dosen't do much. Rc limits the collector current of Q1
> when it is on, and the base current into Q2, when Q1 is off.
>
> Beware though, As designed, Q2 is not guaranteed to be off, when Q1 is on.
> If the C-E voltage is high enough on Q1, Q2 will get some base current, and
> off you go.
> A series diode in place of that second resistor, with a resistor from Q2
> base to ground, would fix that.

I think I see what you're saying. The load is a relay coil and when the input
is +5, the relay isn't on. Is it still possible that Q2 is conducting but
just not enough to trigger the relay?

BAJ

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