Why PWM generate circuit can not adjust the duty cycle to 10%?

Hi guys,

first of all, which kind of op amp are you using?

Which is the value of the resistor connected to the anti-parallel diods?

Try to shunt this resistor.

Usually the output resistance of any op amp, is quite low, and uP doesn't need to be drived with high current, so I don't think an emitter follower could be a solution.

In your SW simulation, try to add a scope probe on the capacitor, the real core of the oscillator, and check what happen!

Hi, Alberto

My AMP is LM311 Fairchild, and resistor connected to the anti-parallel diods is 10k Ohm--R7, the Potentiometer RP=0~100K Ohm.

I try to change the value of R7, it only cause the change of output frequency, the duty cycle can not adjust to 10%.

The following images is the photo of capacitor waveform.

1,RP_Potentiometer=0%  Duty=90%

2,RP_Potentiometer=25%  Duty=70%

3,RP_Potentiometer=50%  Duty=51%

4,RP_Potentiometer=75%  Duty=30%

5,RP_Potentiometer=100%  Duty=9%

Hi, Alberto

My AMP is LM311 Fairchild, and resistor connected to the anti-parallel diods is 10k Ohm--R7, the Potentiometer RP=0~100K Ohm.

I try to change the value of R7, it only cause the change of output frequency, the duty cycle can not adjust to 10%.

The following images is the photo of capacitor waveform.

1,RP_Potentiometer=0%  Duty=90%

2,RP_Potentiometer=25%  Duty=70%

3,RP_Potentiometer=50%  Duty=51%

4,RP_Potentiometer=75%  Duty=30%

5,RP_Potentiometer=100%  Duty=9%

The mathematics that rule the frequency of the oscillator, is function of C and the equivalent resistance of R7 and potentiometer.

So in order to regulate the duty around 1% and 99% you have to adjust the time of charge and discharge of the capacitor.

When the potentiometer is at 0 position the charge of the capacitor is ruled only by the R7.

This resistance is just 10% of the potentiometer, and this ratio is just the maximum or the minimum of your duty cycle.

Teorically, in this calculation, we have to take in account also the resitance of the diods.

So, try to modify the capacitor value *10 and the potentiometer value 1/10, then put an R7 around 100 ohm, the frequency should remain the same, and you will be able to regulate from 1% to 99%.

I believe a real circuit with a real scope will be much better instead SW emulation.

Other capacitor is needed on the mid point of the two 100K resistor, and GND.