On Thu, Dec 26, 2002 at 04:24:09PM -0500, Wagner Lipnharski wrote:
> Dave Dribin wrote:
> >> In the meantime, I'm going to go for a simpler, even if less
> >> effecient, circuit, like the following:
> >>
> >>
> >>                   +6V (Battery)
> >>                    |
> >>                    R1
> >>                    |
> >>                    C
> >> PIC 1-----R2-----B    Q1
> >>                    E
> >>                    |
> >>                    +---------+-----+
> >>                    |         |     |
> >>             470uF  C1        -    SOL
> >>                    |         ^     |
> >>                    |         |     |
> >>                   GND        +-----+
> >>                                    |
> >>                                    C
> >> PIC 2-----R3---------------------B    Q2
> >>                                    E
> >>                                    |
> >>                                    |
> >>                                   GND
> >>
>
> You are missing an important point here.
> Name is transistor polarization.
>
> So, at your original circuit, Q1 being a NPN is just messing with your
> life.
> It not only charges C1 with max of 4V (even with 6V as VCC) and a simple
> resistor could do better.
>
>
> +6V o----R1----o--------o------.
>         100    |        |      |
>               _|_      _|_     3
>               === C1    A      3 SOL
>                |        |      |
>                |        '------o
>               _|_              |
>                                C
> PIC o-----------R2---o-------B   Q1 NPN
>                390   |         E
>                      R3        |
>                      |2k2      |
>                     _|_       _|_

Hmmm... the reason I was using Q1 was to only charge C1 when
necessary, since the solenoid fires only occasionally.  Looks like I
got too caught up in the "transistor is a switch" line of thought and
did not pay attention to the details of what was going on.  Thanks for
pointing out what I overlooked.

I don't think a PNP would work well, either, since the collector
voltage (Vc) would still be dependent on the base voltage (Vb), which
is still on the order of 4.6V, right?  It seems the only way to fully
charge C1 to the full +6V battery voltage is to use a resistor like
you have shown.

Which sorta leads me back to the buck regulator Roman showed.  I don't
understand how that circuit can charge the cap to +6V when it's going
through a transistor, too.  But I'm not gonna worry about that for
now. :) At this stage of the game, I'll take a simpler, but not quite
as efficient, circuit and worry about the better version later.

-Dave

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics