# 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
19
0
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.
Mishu.

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