Current limiting schematic

[chiken]

So i found this schematic circuit of cheap 30V 3A power supply, and could anyone tell me which part is in charge of regulating constant current (shunt and opamp??)
https://drive.google.com/file/d/0B7Bk3JifFLbSU255VzE2ZHJJVU0/view

FInished board looks like this
https://img.staticbg.com/thumb/large/oaupload/banggood/images/94/40/5a8647d5-1287-468f-ac31-75f6f9b2ae7a.jpg

[Benta]

R7 and Q1.

[chiken]

Well, it seems like thats it,
i thought that i would be different,

Does anyone know where could i find a bit of explanation of "mechanism" like this?

[Jwillis]

Q1 is for short circuit protection. Under normal operation  Q1 is nonconductive because the resistor R14 through the base of Q1 is held at negative voltage. But when negative voltage starts to fall, the transistor Q1 becomes conductive (due to the positive voltage on its base through resistor R13), and thus connects the output of U2 to the ground and locks the transistor Q2, so it shut down the output voltage through Q4 .U3 and R7 are for holding constant current based on the adjustment of  U1 .

[floobydust]

Just so you know, that 0-30V 2mA-3A PCB layout is shit because the traces to the current-sense resistor are not properly done. People cut the trace and run a wire to the correct spot. Otherwise the current-limit is not so stable. That PSU has been discussed at great length here in many threads.

[Jwillis]

Damn floobydust I was going to say that.

[Audioguru]

That old circuit has many problems and is not reliable because many parts are overloaded. I helped fix it years ago but lately the Chinese are copying the old defective circuit.

Q1 has nothing to do with short circuit protection. When the AC power is turned off then the negative supply voltage drops to a low voltage much faster than the positive voltage. Then the problem with the TL081 opamp U2 called Opamp Phase Inversion causes its output to try to go as high as it can which would cause the output voltage of this project to suddenly go to +25V or more and destroy whatever lower voltage load you have.

Opamp Phase inversion occurs in only a few opamps when an input voltage becomes within a few volts from its negative supply which happens then the negative supply voltage drops. When the negative supply voltage drops a little then Q1 turns on and shorts the drive to ground for the output transistors.

Opamp U3 senses the load current with R7 and the voltage across R7 is compared with the voltage set by current adjust pot P2. When the current is higher than the setting then U3 reduces the output voltage through D8.

[chiken]

Thank you for your replies

Would you Audioguru(or anybody else) hive me link or anthing that i could find topic to improve that stingy board