# Electronic Circuits and Projects Forum

1. ## PWM Signal to LM317 to adjust Voltage Output

Guys can somebody help me to control voltage of the regulator from a PIC pwm frequency?

Most people say generate a PWM signal feed to a low pass filter & feed into an gained opamp & drive a transistor.is this ok?

Required range
*Low end can be 1v or 1.25V & high end will be 30V with a resolution 0.5V.

I have rail to rail opamps in my hand & LM317.

Those are my questions.

*What frequency to drive?
*Low pass filter values?
*Whats the gain?

This is a problem for me from a long time----> I still cannot understand what will be happen if I feed a PWM signal to a low pass filter??

2. Look at the low-pass filter like an integrator.

As for other ways, remember the usual way is used because it works.

John

3. Originally Posted by Suraj143
This is a problem for me from a long time----> I still cannot understand what will be happen if I feed a PWM signal to a low pass filter??
A low pass filter removes the AC part of the signal and outputs the average, and gives you DC. From the PIC it probably will give a DC voltage of 0 to 5 volts. (if the VCC=5V)

4. If you know the frequency of your pwm from the PIC we can help you calculate the low pass filter.

5. Originally Posted by ronv
If you know the frequency of your pwm from the PIC we can help you calculate the low pass filter.
You are on track

Let say I drive 1Khz pulse with a 50% duty cycle.

6. Here is a neat calculator that you can use. Adjust R & C untill you get the ripple voltage low enough for your needs, then make sure you can live with the response time for a change. Go down the page until you see the PWM calculator.

http://sim.okawa-denshi.jp/en/PWMtool.php

7. Hi ronv this is the first time doing such an experiment.That's a nice calculator.

I did some calculations.from the blow values.

PWM Frequency = 1Khz
R=3.3K,C=0.1uF

Whats your ideas on this?Does it have too much ripple?

My main task is to generate voltage by PWM.

8. I think I would use something like 10K and 10 Ufd. That way only about 12 mv of ripple.

9. Hi ron your values are superb.I calculated the vout from uC from the values you given.

PWM Freq 1Khz
R=10K
C=10uF
Ripple (PP)= 12mV ripple

DC (1-100%) Vs V Out

10% = 0.5V
20% = 1V
30% = 1.5V
40% = 2V
50% = 2.5V
60& = 3V
70% = 3.5V
80% = 4V
90% = 4.5V
100%= 5V

If I give this Vout to a gained opamp let say the gain is set to 2 & the minimum value will show as 1V.& maximum val will be 10V.

I think this range is not enough.Atleast it must have 1V - 20V range.

10. ## 317

You need a gain of about 5.75.

It won't be exact because the output of the 317 is 1.25 volts above the adjust pin. So.... You will have to make this adjustment in your micro program I think.

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