cap selection & what are deciding parameters

Discussion in 'General Electronics Chat' started by aamirali, Feb 22, 2012.

  1. aamirali

    Thread Starter Member

    Feb 2, 2012
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    In case of voltage regulator let say 7805, there are input & output capacitors. How to select their values. Is there any mathematical formula for that.

    Similarly we place a small capacitors b/w vcc & gnd of any ic. How to calculate its value. I saw when current increases, cap also increases?
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Most data sheets have a drawing called- "typical applications" that show the needed capacitors and other components needed. Such as this on the first page; http://www.ti.com/lit/ds/symlink/lm340-n.pdf

    Always download the data sheet to go with your components/IC, it has all you need.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Most ol' timers like me don't use calculations. We know from experience what works.
    For high frequency bypass, 0.1uF is pretty standard.
    For low frequency bypass, use 10uF.
    For audio frequency and higher current drain, 100uF is preferred.
    Any thing higher than that becomes a reservoir capacitor for which you can do the calculation.
     
  4. @android

    Member

    Dec 15, 2011
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    Vr=I/2fc
    Vr=ripple vtg, I=load current, f=line freq, C=capacitance.
    So ideally to get zero ripple C should be infinite.
     
  5. @android

    Member

    Dec 15, 2011
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  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I agree! And if we do, we use the best type of calculation;
    "Calculation by oscilloscope".
    ;)
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    And your peak diode current becomes almost infinite (limited only by series resistance).
     
  8. aamirali

    Thread Starter Member

    Feb 2, 2012
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    1. So I Cap act as voltage source if voltage falls below.

    2.But capacitors also by pass ripples through it as it provide low resistance to it.

    3. In datasheet of LM7805, 0.1uF is specified at output. Will it be applicable for entire current range upto 1A.
     
  9. aamirali

    Thread Starter Member

    Feb 2, 2012
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    I want 2 ask sum questions regarding dis:

    1. I read that caps acts as power source if suddenly voltage cuts off. Then in Dc ckts, voltage is always present how does it helps anyway.

    2. One reason is if power source is several inches away then due to impedences in path etc, closed decoupling cap acts as power source.? I don't get this point exactly. If voltage source is present even at some inches then how does it helps in overcoming path impedences,

    3. One could say, it provide low resistance to ripples & gnd them & allow only only dc to be available. If for +5v cap gets charged how does it gnd ripples anymore.
    I don't understand above two statements exactly. One it acts as power source, second it gnds ripples.
    Kindly help in clearifying this doubt.

    4. In many volatge regulators input cap of .22uF & o/p cap of 0.1uF is mentioned, Will it be applicable for entire range of current upto 1A limit lets say for LM7805.
    I read if current increases, bypass cap also increases. Do the same applies here also
     
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