> I've got a simple 2n2222a driving a coil from a pic pin.  When I put
> the coil between +5V and the collector, works fine turns the coil on
> and off.  But when I try to the coil between the emitter and ground it
> doesn't work.  A voltage appears across it, but it's only 3.6V.

An emitter follower (load at the emitter, also called common collector)
does not amplify a signal, do you get *less* voltage swing at the
emitter than you supply at the base (but you get lots of current
amplification, unfortunately that is not what you need). Your first try
(common emitter, load at the collector) gives both voltage and current
amplification, so it worked.

Solutions:
- (best) use a pnp transistor in common emitter (load at the collector)
- (might work) in the npn emitter follower do not use a R between the
PIC and the base, but an R (try 1k) between the base and the +5V. When
your load is sufficiently small (compared to the R and the B of the
transistor) you will get a small volatge drop.

BTW you did not forget the diode over the coil, did you?

Wouter van Ooijen

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Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products

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