series RLC circuit

Discussion in 'Homework Help' started by amilton542, Dec 29, 2010.

  1. amilton542

    Thread Starter Active Member

    Nov 13, 2010
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    allaboutcircuits.com (VOL II - AC) series RLC page 2. im stuck on finding the total impedance for series RLC circuits, theres now three impedances to add together. ive nailed complex numbers, polar and rectangular form with RL and RC in series, but when i add up the values (250 + j0Ω) + (0+ j245.04Ω) + (0 - j1.7684Ω) ive got 250 + j243.2716, where theyve got 250 - j1.5233. And when i convert the rectangular answer of 250 -j1.5233 into polar using pythagarums theory i get 250.004 and for the vector angle i divide 1.5233 by 250 (arctan )i get 6.0932 x10 - 3, where theyve got 1.5437 with a vector angle of minus 80.680 degrees. i got all of RC and RL write,but now its RLC i dont know what im doing wrong with this one????:confused:
     
  2. edgetrigger

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    Dec 19, 2010
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    Impedance of capacitor is wrongly taken by you it is (0 - j1768.4Ω) or (0 - j1.7684KΩ) and not (0 - j1.7684Ω). You have missed the Kilo ohms value. Please correct this and calculate. You will get the desired answer.
     
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  3. amilton542

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    Nov 13, 2010
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    lol! i think i need to look at the values more carefully i missed that one
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    This video covers it well: MIT 6.002 (EE) Impedance Model

    The complete course list is Here.

    He does a great job of helping you "think in impedance" so Ohms Law rules can be used.
     
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  5. edgetrigger

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    Dec 19, 2010
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    still not clear or what? missing K is the only mistake that you have done.
     
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  6. amilton542

    Thread Starter Active Member

    Nov 13, 2010
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    yes thanks problem solved, it was just the K i had missed, all values are present and correct :) .thanks
     
  7. amilton542

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    Nov 13, 2010
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    i was quite shocked there was more voltage across the capacitor than the supply voltage! i made my own values up and gave 230v 50hz supply, and there was 244 volts across the capacitor!
     
  8. edgetrigger

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    Dec 19, 2010
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    Have to be a bit careful, thats it. Anyways i will call it a day(or night)!
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But you have to remember that 230V is RMS value.
    The peak voltage is equal 230V * √2 = 325V
     
  10. amilton542

    Thread Starter Active Member

    Nov 13, 2010
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    could the voltage across the capacitor go over 325v with a 230v supply?
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Yes. 230V is the RMS value (Equivalent amount of "Work" that a DC voltage of the same level, i.e. 230VDC would produce). Since Wave AC crosses zero often, the peaks need to be 1.414 higher in a sine wave to get the same level of "work" from an AC source to match a DC source. The other measurement for AC is p-p, which means Peak-Peak, it is mostly used when trying to boost specifications for electronic equipment like amplifiers. ;)

    You should be using 400V caps or higher rated in the event of surges.
     
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