# What does this circuit do ?

#### moabdoly

Joined May 26, 2013
3
Hi . i cant understand the logic of this circuit . i just know this is a regulator and Vout is about 12.5 to 13 volt . but what about current controlling ? while there is no sense resistor , is there any control of current ?!

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#### ericgibbs

Joined Jan 29, 2010
10,767
hi m,
Welcome to AAC.
Where did you find the circuit.?

E

#### Alec_t

Joined Sep 17, 2013
11,640
Welcome to AAC!
is there any control of current ?
No proper control, but Q1 collector current will be limited by Q1 beta (which is unknown) and the maximum base current which can be drawn via Q2 and R2.

#### moabdoly

Joined May 26, 2013
3
hi m,
Welcome to AAC.
Where did you find the circuit.?

E
Hi . thank you . from an old d/a circuit .

#### moabdoly

Joined May 26, 2013
3
Welcome to AAC!

No proper control, but Q1 collector current will be limited by Q1 beta (which is unknown) and the maximum base current which can be drawn via Q2 and R2.
Thank you for your answer . so by R2 we can control the current ?! is that correct ?

#### DarthVolta

Joined Jan 27, 2015
509
If you know mesh currents, without an output load , it breaks into 4 meshes, and the current through the 10k + Ib2 equals Beta1*Ib1

And the zener wants to fix Vce2=Vce1+Vbe2-Vz

#### Alec_t

Joined Sep 17, 2013
11,640
so by R2 we can control the current ?! is that correct ?
You can limit the current to some degree, but because the zener diode current also goes through R2, the Q1 collector voltage will change somewhat if you change R2. The current limit will be a bit vague, as transistor beta will vary with temperature and load current.

#### dl324

Joined Mar 30, 2015
11,505
Thank you for your answer . so by R2 we can control the current ?! is that correct ?
No.

R2 sets the current in the zener. The voltage on the emitter of Q2 is set by V1 to about 4.3V. That plus the zener voltage sets the output voltage to about 12.5V.

There is no current limiting.

#### WBahn

Joined Mar 31, 2012
26,143
Hi . i cant understand the logic of this circuit . i just know this is a regulator and Vout is about 12.5 to 13 volt . but what about current controlling ? while there is no sense resistor , is there any control of current ?!

View attachment 216538
A real quick-and-dirty analysis:

Assuming all devices are active.

The base voltage of Q2 is 5 V, making the emitter voltage about 4.3 V, making the cathode of D1 about 12.5 V.

Q2 will adjust up or down in order to maintain a constant current in R2 (that puts the voltage across it at about 4.3 V).

The nominal current in D1 is set by the difference in the currents in R1 and R2. The current in R2 is about 4.3 V / 4.7 kΩ or about 915 μA. The current in R1 is about 0.7 V / 10 kΩ = 70 μA. So the zener current has to make up the difference and is roughly 845 μA. The exact numbers don't matter too much. Whatever it turns out to be the circuit will attempt to keep it constant.

If the collector voltage of Q1 increases that will result in more current flowing in D1, which will cause Q2 to reduce it's collector current which will cause a lower drop in the voltage across R1, which will cause the base voltage of Q1 to rise, which will reduce the collector current in Q1 causing the collector voltage to fall. The opposite happens if the collector voltage drops for some reason.

There is no current limiting feature in the circuit.

#### Alec_t

Joined Sep 17, 2013
11,640
There is no intentional current limiting, but if R2 is increased beyond about 50k then Q1 gets starved of base current and the output voltage at Q1 collector drops.