PWM using 555

MrSalts

Joined Apr 2, 2020
284
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.
 

Thread Starter

Mariachi

Joined Jan 4, 2021
27
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
 

Thread Starter

Mariachi

Joined Jan 4, 2021
27
If you give cryptic answers to my questions, then you get none from me. :rolleyes:
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.
 

Bernard

Joined Aug 7, 2008
5,788
Calculations will give approximate answers due to mostly component tolerance.
C discharge can be thru part of pot & maybe a fixed R.
 
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Thread Starter

Mariachi

Joined Jan 4, 2021
27
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!
 

sghioto

Joined Dec 31, 2017
2,363
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.
 

MrSalts

Joined Apr 2, 2020
284
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%.
 
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