Pulse width control with passive components

Discussion in 'General Electronics Chat' started by bisonriver, Jun 10, 2010.

  1. bisonriver

    Thread Starter New Member

    Jun 10, 2010
    I need to adjust pulse width with passive components (R, L, C and D only). The input is a 5V, 100KHz, 50% duty cycle clock. How do I adjust the duty cycle using only RLCDs? My target is to be able to adjust the duty cycle from 20% to 80%. The output will be used to drive a TTL load (2.4V is considered high, 0.8V is considered low). The attached is the circuit I came up with. It "kind of" works, but not great. As seen in the attached file, by adjusting R1, I can change the charge/discharge time constant and also adjust the amplitude. Without R2, the output may remain higher than 0.8V (never turns off).
    • PWM.doc
      File size:
      22.5 KB
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    That is an odd way of doing it. Try applying the signal to a Schmitt trigger input gate. The Schmitt input is voltage rather than rate sensitive, and may work more consistently.
  3. bisonriver

    Thread Starter New Member

    Jun 10, 2010
    Yes, it's a special application and we can only use R, L, C and D. Can you make a schmitt trigger out of these components? If other devices were available, things can be much easier. By the way, from my original design, I can put a zener diode at the output so the shape looks more square-ish.