Regulated power supply question - on schematic

Discussion in 'Homework Help' started by mishu.daniel, Nov 22, 2010.

  1. mishu.daniel

    Thread Starter Member

    May 13, 2009
    Hi all,
    I want to build a power supply but I would like to understand how it works before. I have simulated the attached schematic and seems to be fine. I did some math to check if I know how to calculate all currents and voltages. But, even if my results matched the simulator's, I don't "see" how this circuit works. In fact, I don't understand how this circuit works with the feedback from output.
    Short description of what I see:
    - When the voltage across C-E of Q3 is modified => the output voltage is modified.
    - R2 limits the current on Q3 C-E to aprox 1,8 mA
    - Considering Q3 beta = 100, => the maximum current on Q3 base for saturation is 18uA.
    - The maximum voltage on Q1 collector should be (Q3)Vbe + 18uA*10k ~= 0,7V
    - Maximum current across R6 could be 18V/10k = 1,8mA, so the maximum current across Q1 collector could be aprox 1,8mA + 18uA ~= 1,8mA
    - If Q1 Beta=100 => the maximum current needed by Q1 will be 1,8uA.

    Now, the equation for the current across R3 would be: Voutput - Vbe(Q1) - R3*I3 - (Rpot+R5)* Ipot

    ... where Rpot is the part from the slider down to R5.
    But Voutput is not constant! So I don't understand the mechanism of this circuit :(

    I really appreciate if somebody can make some light for me in this matter.

    Many thanks.