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'Please help with basic problem!'
1995\12\31@112704 by Gerry Smith

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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)
                           --

1995\12\31@151832 by Scott Stephens

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{Quote hidden}

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.

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