How this ps unit works??

[walid1]

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

 

[indulis]

Think about it this way... first remove the current limit stuff (Q3, R4, P1). Then what to do with Q1... the collector-base looks like a diode... that "kinda works". I don't see how making Q1 a n-p-n will work at all. Then add the current limit function back in.

 

[Kevin Weddle]

A center tapped transformer is perfect for producing a negative dc supply voltage. This one doesn't have one.

 

[Hero999]

A center tapped transformer is perfect for producing a negative dc supply voltage. This one doesn't have one.

This circuit uses a bi-phase rectifier. Each diode acts as a single phase rectifier but the AC voltage applied to D2 is 180o out of phase, thus full wave rectification is achieved.

 

[Kevin Weddle]

Hello Walid, the circuit you found was I think novice. Q3 is an NPN. Indulis recommends removing the resistors and I agree. The load has to be high impedance. Most versatile regulators need to sink current sometimes.

 

[Kevin Weddle]

Try this circuit. It is good for analogue only.

View attachment 41972

 

[Kevin Weddle]

Here's a better circuit.