555 timer as an astable multivibrator?

Discussion in 'Homework Help' started by pfelectronicstech, Oct 11, 2012.

  1. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
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    I'm trying to calculate the duty cycle of a IC 555 as an astable multivibrator. The Ra is 18K ohms, the Rb is 68K ohms, C is 0.04 and I have come up with 0.56 and that is wrong. Any ideas?
     
  2. mlog

    Member

    Feb 11, 2012
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    What equation are you using? I'd say you're wrong too.

    By the way, it's meaningless to say C=0.04. What are the units?
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Show your work for t1 and t2 ... then calculate the duty cycle
     
  4. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
    4
    Sorry its .04 microfarads, just forgot to add.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The capacitor charges with 18k + 68k= 86k.
    The capacitor discharges with only 68k.
    So calculate the duty cycle.
     
  6. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
    4
    Oh, ok, thanks, I'm coming up with 69.4 now, am I correct, getting closer?
     
  7. mlog

    Member

    Feb 11, 2012
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    Remember what Audioguru said? Discharge time divided by the sum of the charge and discharge times.

    D = \frac{R_b}{(R_a + R_b) + R_b}
     
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  8. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
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    Finally, thank you mlog! Thank you, thank you, I just was not thinking about the problem in the right way. I tend VERY often to over think the situation, which is a bad trait. I suppose a lot of Techs over think situations though? Thanks again for the help.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Page 6 in the datasheet has this equation. Datasheets can be very handy.:)
     
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  10. pfelectronicstech

    Thread Starter Member

    Jan 18, 2012
    178
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    Thanks, I did not think to check the datasheet, my nose was stuck in my text books. Thanks again for that.
     
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