>Hi All,
>I'm in need of some help.  I have a PIC that produces PMW
>output hooked up to a resistor and a cap.  The problem is that
>I need to control an external power source (a 9v batte
>ry) and
>be able to adjust its voltage.  I thought of using a
>transistor but what part # should I use to get the best
>results?  Also is there anything else I'm missing? The output,
>once filtered, gives 0-5v.  Thanks in advance.  Here is a
>proposed diagram:
>
>
>                                                    +9v
>PWM output (0-5v)   4.7k           (tranistor?)     [
>_[~]_[~]_[~]__-----<><><>--------------------------[
>                            ]                      [
>                            ]                       [
>                           --- 0.47uF               (ground)
>                           ---
>                            ]
>                            ]
>                           ---- (ground)
>                            --
>

What is the frequency of the PWM? The 3dB cutoff frequency of your 4.7K,
.47uF components are 1/(2 PI R C) = 72Hz. So if your rate isn't fast enough,
the transistor (common collector/emitter ?) will just amplify your
pulses/ramp. What resistants/type of load do you have?

Someone posted a SPICE faq on sci.electonics, which may help in simulating
your application.