piclist 1998\10\01\195710a >
Thread: Possibility of using the PWM output as a proportional DC output??
face BY : James Cameron email (remove spam text)

Craig Lee wrote:
> I am attempting to use the PWM output on a '73A through a
> capacitor to ground to get a proportional DC voltage out.

Well, um, presumably the output is oscillating from supply to ground,
right?  So I would expect to get the capacitor charged when the output
is at supply, and then discharged when the output is at ground.  If you
have no resistor in that, then the chip drivers will be pulling and
pushing Real Hard (tm), and so of course the voltage at the capacitor
will look like a saw tooth triangular wave approaching a square wave,
depending on the capacitor value versus the PCB trace resistance.

If you stick a diode between the output pin and the top of the
capacitor, all that will happen is that the capacitor will reach supply
voltage, unless you are draining it some other way.

As someone else said, I think there is some way with appropriate sizing
of resistor and capacitor to reduce the amount of movement of the
voltage at the capacitor.  But there would be the drawback of reduced
responsiveness to a change in the PWM duty cycle.

I don't know a thing about the '73A though.  ;-)

James Cameron                                    (TakeThisOuTcameronSTOPspamspamstl.dec.com)
Digital Equipment Corporation (Australia) Pty. Ltd. A.C.N. 000 446 800

<36141441.60837BC4@stl.dec.com> 7bit

See also: www.piclist.com/techref/io/pwm/index.htm?key=pwm
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