7805 regulator

Discussion in 'General Electronics Chat' started by screen1988, May 25, 2013.

  1. screen1988

    Thread Starter Member

    Mar 7, 2013
    310
    3
    I want to ask about the 7805 regulator.
    [​IMG]
    The capacitors C1 and C3 are used to filter out any noise coming from the voltage source. What are the reasons for the use of C2, C4 and D1, D2?
     
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  2. nerdegutta

    Moderator

    Dec 15, 2009
    2,515
    785
    The way I see it, C1 and C3 takes care of the highs, and C2 and c4 takes care of the lows. The diodes protects the IC from revers polarity.

    I could be wrong, thou...:)
     
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  3. screen1988

    Thread Starter Member

    Mar 7, 2013
    310
    3
    Do you mean that the capacitor C2 and C4 are used to filter out low frequencies?
    Can you explain about it? I can't see any differences between C1, C3 and C2, C4 but different type of capacitor.
     
  4. nerdegutta

    Moderator

    Dec 15, 2009
    2,515
    785
    They are the filter to eliminate any noise and reduce voltage fluctuation.

    My guess is that C1 and C3 are ceramic, and C2 and C4 are electrolytic capacitors.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,233
    D2 should not be there. It will just reduce the output voltage by a diode drop and decrease the regulation.

    If you want to protect the regulator from reverse voltage when the input voltage is removed, you can place the diode, reverse biased, between the regulator input and output.

    Where did you get that circuit?
     
    screen1988 likes this.
  6. screen1988

    Thread Starter Member

    Mar 7, 2013
    310
    3
    I just found an circuit on the net to ask questions about functions of capacitors. (can't remember where)
    Can you comment about the capacitor C2 and C4?
     
  7. Shagas

    Active Member

    May 13, 2013
    802
    74
    I read somewhere on the official datasheets of these regulators something like 'if the output is more than 6 inches away from the regulator then a cap after the voltage regulator is required ' . My guess would be that it's for output voltage stability for impulsive current bursts .
    Another thing I read is that lower capacitance caps for example ceramic ones filter higher frequencies (noise) out more efficiently than large electrolytics .
    And the large electrolytics (10uf and more) are there to filter out the output from the rectifier
     
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  8. jerseyguy1996

    Active Member

    Feb 2, 2008
    206
    9
    I can't remember which forum I read this on but there was a great explanation of the reason for combining ceramic and electrolytic caps. In my layman's understanding of it, the ceramic capacitors offer much lower ESR which means that in an instantaneous change in current requirements by the circuit, they can react very quickly to supply the needed current. The electrolytics are much slower to supply the current but they have the advantage of higher capacitances. So combining them means that the ceramic can supply an instantaneous change in current requirements until the electrolytics have a chance to catch up and then the electrolytics can take over and supply the increased current demands over a longer period of time given their higher capacitance.
     
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  9. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    Putting capacitors in parallel across the power pins of ICs is not the same situation as for that at the input and output pins of a linear voltage regulator.

    That 220uF at C4 is too high. You should interchange the values of C2 and C4.

    C2 is the reservoir capacitor. Too large a value for C4 will prevent the regulator from doing its job. C4 is to prevent high frequency oscillation.
     
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  10. DeanHuster

    New Member

    Jul 4, 2012
    3
    0
    The electrolytic caps, because of their construction method, have higher ESR and higher series inductance that the ceramice, which have darned near zero of both. Because of those characteristics, the explanations for the electrolytics handling the lower frequencies (e.g., 120Hz) while the ceramics handle the high-frequency spikes is valid. Throughout the circuits that this regulator supplies, there should be several points where ceramic caps and sometimes smaller electrolytics are added for additional input filtering and for preventing spikes generated by a circuit from making their way back to the power supply and/or other circuits.
     
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