piclist 1998\10\01\184819a >
Thread: Possibility of using the PWM output as a proportional DC output??
www.piclist.com/techref/io/pwm/index.htm?key=pwm
picon face BY : Peter L. Peres email (remove spam text)



On Thu, 1 Oct 1998, 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.
>
> The PWM frequency is 3090 Hz and the duty cycle is varied.
>
> However, I get too much ripple on the DC output.
>
> Any brilliant ideas?  Is my frequency too low? Too high? How
> do I select the proper capacitor? Do I need a diode?

You need a low pass filter. The easiest would be a resistor between the
PIC pin and the capacitor, for now. Choose one such that T = R * C >= 10 /
f0 for starters. This has some tradeoffs (such as settling speed and
output impedance) but it works. Later, you may want to use a coil instead
of the resistor and add a diode or two. This will improve matters
considerably but needs some calculations. If you want really smooth dc
output, use an op-amp low pass filter (active filter) chosen to attenuate
the lowest component in the PWM to your required ripple specs.

hope this helps,

Peter

<Pine.LNX.3.95.981002012412.533X-100000@plp4>

In reply to: <000601bded8b$168390c0$0300a8c0@craiglee.telusplanet.net>
See also: www.piclist.com/techref/io/pwm/index.htm?key=pwm
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