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
    3,676
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    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

    AAC Fanatic!

    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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