Design of a 78xx circuit

Discussion in 'General Electronics Chat' started by Konstabel, Jan 31, 2008.

  1. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    I have used the 78series regulators on multiple occasions but do not know why they use the cap values that is mostly the case.(10n,100n,etc)

    Why are these specific values used? Is there a way to calculate the exact values for a specific situation?
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,575
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    Konstable,

    These are to prevent the 3-terminal regulators from oscillating. The values are not critical, but should wired physically as close to the regulator pins a practical. Look at the manufactures' datasheets for recommended values.

    Ken
     
  3. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    Thanks Ken,

    So the values aren't related to specific filtering frequencies? And why then are there almost always a 10n and 100n together?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, they're usually 0.1uF and 1uF. The 0.1uF is generally a ceramic or tantalum capacitor, where the larger cap is electrolytic. The smaller cap takes care of the high frequency noise component, and the large cap is to smooth out lower-frequency transients.
     
  5. John Luciani

    Active Member

    Apr 3, 2007
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    For general applications the values are not critical. If you have high switching current
    you should look at the application notes and the capacitor specifications.

    I usually use a 0.1uF X7R ceramic capacitor in parallel with a 1-10uF capacitor. For
    capitors <=4.7uF I use an X7R ceramic capacitor otherwise I would use a tantalum capacitor.

    (* jcl *)
     
  6. tian

    New Member

    Sep 23, 2007
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    All i know in practice i use a lot capacitor.
     
  7. nanovate

    Distinguished Member

    May 7, 2007
    665
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    You also want to consult the datasheet for recommended values and types of caps. Some Vregs do not like ceramic caps on the output.
     
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