[SOLVED] how this circuit regulate voltage and current ?

[eagle-elec]

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 ?! if there then How ?

 

[KlausST]

Hi,

Voltage limit:
5V supply goes to Q2 base. The emitter is 0.6V less = 4.4V
But when the output voltage is high enough (@ 4.4V + 8.2V = 12.6V) then Q2 emitter will be higher than 4.4V making V_BE less than 0.6V and thus Q2 switches OFF.

Current limit:
Voltage across R2 is 4.4V making 0.94mA of emitter current. Collector current is slightly less.
So if Q1 h-FE is 100 (just as example. Didn´t read the datsheet) then the output current is limited to 100 x 0.94mA = 94mA.

Klaus

 

[eagle-elec]

Hi,

Voltage limit:
5V supply goes to Q2 base. The emitter is 0.6V less = 4.4V
But when the output voltage is high enough (@ 4.4V + 8.2V = 12.6V) then Q2 emitter will be higher than 4.4V making V_BE less than 0.6V and thus Q2 switches OFF.

Current limit:
Voltage across R2 is 4.4V making 0.94mA of emitter current. Collector current is slightly less.
So if Q1 h-FE is 100 (just as example. Didn´t read the datsheet) then the output current is limited to 100 x 0.94mA = 94mA.

Klaus

Dear Klaus . Thanks a lot . how much can i decrease the R2 to get more current . i think so ic of q2 is my limit ? is it correct ?

 

[KlausST]

Hi,

i think so ic of q2 is my limit ? is it correct ?

I_limit = IC_Q2 * h_FE_Q1

... as calculated above....

Klaus

added:

How much decrease of R2:
Either you or I need to read the datasheets of Q1 and Q2. I recommend you do it on your own ;-)
Check: Max current, max power dissipation, calculate max expectable temperature rise.

Then additionally it depends on what situation: Short circuited output or normal output.