How this ps unit works??

[samy555]

The circuit in the following figure (from: http://www.hobby-circuits.com/circuits/power-supply/dc-power-supply/891/variable-dc-power-supply) represents a variable power supply Transistor Q1 is npn type

This was corrected in the following figure

I have thought carefully about this circuit, but I could not get to the
mechanism of how it work!
I think that when the transistor Q1 operates, it must turn off Q2.
I try to simulate it using multisim10, but it didn't work, that is when I
change the value of any of P1 or P2 the output is fixed at near the
retectified voltage.
Any help in clarifying the way this circuit work would be welcome, thanks

 

[duke37]

The transformer and D1,D2 provide a rough DC across C1.
Q2 and Q4 are emitter followers so the voltage at Q4 emitter will be 1.2V (two diode drops) less than the collector of Q1. R2 pulls this voltage high.
Output voltage comes through R5 and a proportion of this, set by P2, feeds the base of Q1. When the output voltage rises, Q! is turned on more, so its collector voltage decreases and the output voltage is stabilised.
The output current develops a voltage across R5 and a proportion of this, set by P1 feeds the base of Q3. When the current reaches the set value Q3 turns on which turns on Q1 so reducing the output voltage to limit the current.

Both voltage and current setting depends on the emitter/base voltage of a transistor, this is not very stable but a lot better than nothing.

 

[Rleo6965]

P1 for adjustment of current limit and P2 for adjustment of dc voltage output.

 

[samy555]

Thank you duke37

Is Q1 makes Q2 only ON and OFF, or it take it gradually among OFF, partily ON and fully ON???
thanks

 

[duke37]

It will be gradual to give proportional control.

 

[samy555]

How Q3 controls the current????

 

[(*steve*)]

By robbing base current from Q4