I want to make a pwm generator with variable frequency. I notice that the capacitor is charged through R1, but the discharge? I don't know with what formula the duty cycle can be calculated in this case. Thanks!

 

Might I suggest that you read the 555 PDF...

 

Hello,

Have a look at page 5 (fig 6) and page 8 of the following application note.

Bertus

 

Hello,

Have a look at page 5 (fig 6) and page 8 of the following application note.

Bertus

The duty cycle is very close to the ratio of resistances on each side of the potentiometer's wiper.
Why would you need a variable frequency PWM? A research project or a personal interest? Something else. I don't see a practical use.

 

I want to make a pwm generator with variable frequency.

Why variable frequency?
Normally you want PWM with a more or less constant frequency with a variable duty-cycle, which is what the typical 555 PWM circuit does.

 

The duty cycle is very close to the ratio of resistances on each side of the potentiometer's wiper.
Why would you need a variable frequency PWM? A research project or a personal interest? Something else. I don't see a practical use.

P

Why variable frequency?
Normally you want PWM with a more or less constant frequency with a variable duty-cycle, which is what the typical 555 PWM circuit does.

personal project

 

personal project

If you give cryptic answers to my questions, then you get none from me.

 

If you give cryptic answers to my questions, then you get none from me.

Sorry. It’s for a personal project. I would like to know if the capacitor is discharged only by potentiometer or not. I encountered problems regarding the duty cycle obtained on the oscilloscope and the one obtained by calculation.

 

Calculations will give approximate answers due to mostly component tolerance.
C discharge can be thru part of pot & maybe a fixed R.

 

Note that the forward drop of the diodes will affect the calculations.

 

I encountered problems regarding the duty cycle obtained on the oscilloscope and the one obtained by calculation.

I can certainly understand why.
You have two charging paths and discharging paths.
Remove R1 and the connection on pin 7.

 

I can certainly understand why.
You have two charging paths and discharging paths.
Remove R1 and the connection on pin 7.
View attachment 226782

Thanks! now what is the charging path if R1 is no more?

 

Hello,

Have a look at the situation when pin 3 is high and when pin 3 is low.

Bertus

 

Hello,

Have a look at the situation when pin 3 is high and when pin 3 is low.

Bertus

Hello. If I select 25% at the potentiometer will the charging be done by 75% of the potentiometer value and by 25% of the potentiometer value will the discharge be performed? Thanks!

 

Hello. If I select 25% at the potentiometer will the charging be done by 75% of the potentiometer value and by 25% of the potentiometer value will the discharge be performed? Thanks!

Depends on which end of the pot is at 25%. To be exact the charging path is through D3, pin3 and pin2 of the pot.
Discharges through pin2 , pin1 and D2.

 

It also depends on the value of any voltage on the control pin (5). You can trim the voltage at pin 5 if the center of your PWM potentiometer is not at 50%.