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LM7805 - Linear Voltage Regulator And DC-DC Converter Analysis

Discussion in 'General Electronics Discussion' started by Integrator741, Dec 6, 2013.

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  1. Integrator741


    Jun 16, 2013

    I am trying to find some information about "Voltage Regulator" and "DC-DC converter" circuits. Things like how do they work? Component analysis and purpose of the LM7805 IC in the circuit.

    Voltage Regulator

    DC-DC Converter

    If somebody would give me some bits and peaces I would be really thankful :)

  2. BobK


    Jan 5, 2010
    The 7805 is a linear voltage regulator. It samples the output voltage, compares it to a reference voltage and then adjusts a pass transistor to keep the output voltage at 5V.

  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    This almost sounds like homework...

    If you google for LM7805 you wull find links to various manufacturers datasheets which will describe all you need in eye-watering detail.

    Have a look and see what you can understand. If you have more questions, tell us what you're stuck on and we'll try to point you in the right direction.
  4. OLIVE2222


    Oct 2, 2011
  5. Integrator741


    Jun 16, 2013
    OLIVE2222 - Thank you for the data sheet, that was very helpful :)

    Okay so I got that C (input) is required to prevent oscillation and reduce power supply ripple;

    C (out) is for phase compensation of the internal error amplifier.

    So are those capacitors doing the same job in two different circuits?
  6. davenn

    davenn Moderator

    Sep 5, 2009
    sort of

    For whatever reason the low value cap has been left out of the input to the regulator in the second circuit
    there should still be a cap of ~ 0.1 TO 0.47Uf in parallel with the 470uF
    The 470uF will only help with ripple reduction, it wont have much effect on the oscillation stability issue as its value is too big

    Also ... if running off a battery supply, there not a lot of requirement for a high value cap on the input, as the ripple will be 0V

  7. JimW


    Oct 22, 2010
    From what the original poster put in the title and the labels of the two circuits, there is a bigger misunderstanding here. Both circuits use the same IC, both are linear voltage regulator circuits. The values of the caps are generally not differentiating the two from each other.

    A DC-DC converter circuit is a whole different beastie. You did not include a drawing of one. What is does is take a DC input, change it to a high(-ish) frequency AC signal, increase or decrease it using an inductor or transformer, and then filter it back to a DC voltage (where the final voltage is generally controlled by the pulse width and separation of the AC part of the circuit). Google "DC Converter IC" and you will get more information and some sample circuits.

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