555 Timer Duty Cycle

[Michael]

Hi,

I'm trying to use a 555 timer to produce a PWM waveform however can't find
any reference as to how the value of the resistor* can be used to calcuate
the duty cycle. Is there a formula that says something like when you use
Xohms it gives 2X% duty cycle?

Cheers,

Michael

*http://www.scienceprog.com/control-motor-pwm-schematic/

 

[Tom Biasi]

Hi,

I'm trying to use a 555 timer to produce a PWM waveform however can't find
any reference as to how the value of the resistor* can be used to calcuate
the duty cycle. Is there a formula that says something like when you use
Xohms it gives 2X% duty cycle?

Cheers,

Michael

*http://www.scienceprog.com/control-motor-pwm-schematic/

Bill has a calculator.

http://ourworld.compuserve.com/homepages/Bill_Bowden/555.htm

 

[Michael]

Bill has a calculator.

http://ourworld.compuserve.com/homepages/Bill_Bowden/555.htm

Hi,

Thanks, I've put the formula into Excel and everytime I change of the values
(R1,R2,C1) it changes both the frequency and duty cycle. Is there a way to
change only the duty cycle once you have the frequency you want?

Cheers,

Michael

 

[Lord Garth]

Look to alt.binaries,schematics.electronic for an attached file...

 

[Phil Allison]

"Michael"

Thanks, I've put the formula into Excel and everytime I change of the
values (R1,R2,C1) it changes both the frequency and duty cycle. Is there a
way to change only the duty cycle once you have the frequency you want?

** Not possible by varying only one resistor.

A pot will do it:

http://www.cpemma.co.uk/555pwm.html




.......... Phil

 

[Michael]

"Michael"


** Not possible by varying only one resistor.

A pot will do it:

http://www.cpemma.co.uk/555pwm.html




......... Phil

Thanks guys,

Michael