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Help 555 timer problem

  1. Jan 17, 2008 #1
    [SOLVED] help 555 timer problem

    hi all, i am designing a 555 on astable mode. it is designed to operate at 100kHz with a duty cycle of 95% ( and i get 48Khz and duty cycle 93.5%).

    i calculated all the values necessary:

    Ra = 12.987 kOhm
    Rb = 721.5 Ohm
    C = 1nF
    F = 1.44/ ((Ra + 2Rb)*C)

    i used thes formulas :

    tH = 0.693*(Ra + Rb)*C
    tL = 0.693*(Rb)*C

    T = tH + tL

    i searched everywhere and found out the max frequency is 500Khz so im not sure where the problem is....i need the 100kHz because i am designing an infrared modulator and the central frequency for my filters is 100kHz

    can anyone help me thanks.
    Last edited: Jan 17, 2008
  2. jcsd
  3. Jan 17, 2008 #2


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    1nF is a bit small, capacitors this small aren't very accurate and the effects of stray and gate capacitance start to matter, you might want to redesign it with at least 10nF.
  4. Jan 17, 2008 #3
    just tested with 10n and 100n ...got 40Khz this time....ive double checked everthing.

    i just simulated it on multisim and i got 45.9kHz ....but by my calculations it should be 100kHz

    any ideias? i think this is very odd
    Last edited: Jan 17, 2008
  5. Jan 18, 2008 #4

    the multsim 555 design wizard is not correct. there is a problem with it... in the lab i changed the IC (it was a bit damaged) and i managed to adjust it to my desired frequency / dutycycle.

    btw.... thanks for trying to solve my problem
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