regulated power supply using op amp

[CatLikeBread0]

hello everyone,
can anyone please explain me this circuit's working? and please also tell, why to build a dc to dc power supply at all? i know a few basic things like use of capacitors(bypassing) and zener diode(regulation) here, but the connection of the transistors is quite unfathomable.:bang:
how are the transistors switched by the opamp?
and yes, is the opamp in positive feedback?... or negative?

 

[Dominik Przyborowski]

It is a typical old fashion compensated DC regulator. T1 is a power transistor working as emitter follower controlled by opamp, working in negative feedback to set output voltage at level equal to Vdz·[1+(R3+R4)/R5]. The minimum input voltage is higher than output of around Vbe of T1 + 1.5V caused by limited swing of opamp. Small signal BJT T2 looks like current limitation protection.

 

[CatLikeBread0]

ok:thumbsup: , but i still have a few questions :
1) why is t2 configured....the way its configured?? how will it limit the current and prevent short circuit?:-?
2)and please could you elaborate on " The minimum input voltage is higher than output of around Vbe of T1 + 1.5V caused by limited swing of opamp."
3)why is the collector of the transistor connected to +ve of input and -ve as well??
4)what is the use of the variable resistor?

 

[FvM]

1) why is t2 configured....the way its configured??

Sensing the output current at R3 and cutting T1 control voltage if the current exceeds about 12 mA.

2)and please could you elaborate on (...)

Every voltage regulator has a certain voltage drop. With this circuit, it's relative large.

3)why is the collector of the transistor connected to +ve of input and -ve as well??

The schematic is rather blurry, but you still see that this isn't true.

4)what is the use of the variable resistor?

It has no function related to the voltage regulator itself. Either intended to test the circuit or just misplaced.

 

[CatLikeBread0]

ok, i am starting to get it, but could anyone please explain "Vdz(1+(R3+R4)/R5)". what is Vdz??

and yes the collector of T1 is connected to ground via the capacitor and to the positive of input as well via D1( what is the function of D18-O:?: ). But if it has to work as emitter follower, why is it grounded?

and what is 1.5V? where did it come from?

and what should i do to make this power supply a variable one? like if i supply full 15V, i want it to go from 0 to 12.5v. will any variations be required at pin 3 of 741?

 

[FvM]

could anyone please explain "Vdz(1+(R3+R4)/R5)". what is Vdz??

Vdz is the zener diode voltage. An the correct expresion is Vout = Vdz(1+R4/R5), it does not depend on R3.

and yes the collector of T1 is connected to ground via the capacitor

Following this way of seeing, also the output voltage is "connected" to ground via capacitor C2. In fact both are bypass capacitors. But C1 is drawn in an unfavourable way that hides it's circuit function.

and what should i do to make this power supply a variable one? like if i supply full 15V, i want it to go from 0 to 12.5v. will any variations be required at pin 3 of 741?

Impossible with this circuit, at least for two reasons.
- It only allows for output voltages >= zener diode voltage
- LM741 would need a negative supply to work with input voltages near ground.

 

[Audioguru]

what is 1.5V? where did it come from?

The output of the lousy old 741 opamp cannot go lower than about 2.2V above ground in the circuit. Also the output of some 741 opamps cannot go higher than about 5V less than the supply voltage. The datasheet calls it the Output Voltage Swing.

what is the function of D1?

It is called "an idiot diode" that prevents destruction of the circuit if an idiot wrongly connects the polarity of the input voltage.

 

[CatLikeBread0]

okay,got that.
and here the two transistors are working as current limiters for short circuit protection.
this link also helped
https://www.brighthubengineering.com/diy-electronics-devices/66646-basic-transistor-circuits-for-beginners-explored/#imgn_1