Capacitor IRMS

Discussion in 'General Electronics Chat' started by edmundopt, Sep 19, 2013.

  1. edmundopt

    Thread Starter Member

    May 4, 2011
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    hello

    I have found this post http://forum.allaboutcircuits.com/showthread.php?t=21083&highlight=capacitor+irms but it ends with the same question that I have.
    I am using the LM2679 datasheet to a project http://forum.allaboutcircuits.com/showthread.php?t=89463 , on the page 20 it mentions that the capacitor Nichicon PL series with 560uF and WV of 35volts has a IRMS of 1.43Amps.. I have found out that Nichicon PM series is the old PL series, and went to download the datasheet(e-pm-29711.pdf), it mentions the following :

    the capacitor with 15x16mm form factor has a a impediance of 0,040 ohms at 20ºC and 100Khz, it has a ripple current of 1.1 ARMS at 120hz.
    What is the IRMS of this capacitor, if I go for I = V / R, I get 35/0.04, wich is 875 amps, wich is wrong.. please educate me :)
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,015
    1,531
    This is how I understand your problem, but I may be wrong. "A"rms is used when working with DC. "I"rms is used when working with AC.

    The are both measurements of the ampere rating of the capacitor, just the different rating for different types of circuits. AC flows through a cap, so it's rating is "I" or current. DC doesn't flow through a cap it is stored in it, so it's rating is "A" ampere.
     
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  3. edmundopt

    Thread Starter Member

    May 4, 2011
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    ok, nice to know, so I have a switching power supply, 250khz, those being the output capacitor, I have to use the Irms.
    So then this capacitor has 1.1Arms = 1.1 Irms ??
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,041
    3,243
    I believe Irms and Arms are both the same. It is the maximum allowed AC current due to the AC voltage component and frequency of the applied signal and the AC impedance of the capacitor. It's unrelated to the applied DC. The AC impedance is, of course, proportional to frequency.

    The 0.04 ohms is the ESR (series resistance) of the capacitor and is not greatly affected by frequency. The ESR is what dissipates power in the capacitor due to the AC signal component. It is what causes the capacitor to heat, and it is the heat buildup which is the factor that determines the current limit rating of the capacitor.
     
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  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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  6. edmundopt

    Thread Starter Member

    May 4, 2011
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    this is a design question, but it's about capacitors so,
    I have a design with 5volts that the load is about 3,2amps, the rules state that voltage of the capacitor is 1.3 of the design, and at least capacitor should supply half of the current.

    so I have these options(2 capacitors ):
    2 * 1800uF @ 10 volts with 1.665Arms 0.99€ (D12.5xL20mm)
    2 * 1200uf @ 16 volts with 1.690Arms 0.99€ (D10xL25mm)
    2 * 820uf @ 25 volts with 1.665Arms 0.74€ (D12.5xL20mm)
    2 * 560uf @ 35 volts with 1.655Arms 0.74€ (D12.5xL20mm)
    2 * 470uf @ 50volts with 1.610Arms 0.83€ (D10xL30mm)
    2 * 390uf @ 63 volts with 1.720arms 0.83€ (D12.5xL25mm)

    Question is, can I choose anyone of them?
    Between 2*820uF and 2*560uf, wich one is best (they have the same dimension and price)
     
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