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555 Timer Duty Cycle

Discussion in 'Electronic Basics' started by Michael, Feb 15, 2007.

  1. Michael

    Michael Guest

    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/
     
  2. Tom Biasi

    Tom Biasi Guest

  3. Michael

    Michael Guest

    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
     
  4. Lord Garth

    Lord Garth Guest

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

    Phil Allison Guest

    "Michael"

    ** Not possible by varying only one resistor.

    A pot will do it:

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




    .......... Phil
     
  6. Michael

    Michael Guest

    Thanks guys,

    Michael
     
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