Hi, I have build my PCB.I checked the circuit without load so I get 30V at the output.
I have full wave transformer not centre tape 24VAC,5A.(Does it affect the circuit?)
The transformer must be 28V or 30V for the output to be regulated at 30VDC.
At 3A your output will be about 26V and will not be regulated because it will be full of ripple because the voltage from your transformer is too low.
What is current limiting and how it works in this circuit?
The circuit does not have simple current limiting. Instead it has accurate current regulation.
R7 senses the load current and feeds the sense voltage to the (-) input of opamp U3. The current setting pot P2 sends its voltage to the (+) input of U3. Then U3 amplifies the difference to turn on Q3 and the LED to indicate that current regulation is reducing the output voltage through D9. The output voltage is reduced to the amount that accurately produces the current set by the current-setting pot.
I have connected a load of 100ohms 10W so i get 23V and 0.23A at output so Is it correct?
only if the output voltage is set to 23V and the current-setting is set higher than 0.23A, or if the voltage is set higher than 23V and the current-setting is at 0.23A.
I get 11.8V at the output of U2
U2, the driver transistor and the output transistors have a voltage gain of R12/R11 (plus one) that is changed a little by the voltage calibration pot. They amplify the 0V to 11.2V from the voltage setting pot up to 30.0V so the voltage gain should be 2.67857 times.
[/quote]I get 28.2V at the output of U3.[/quote]
When not regulating the current, the output of U3 should be higher than 11.2V. 28.2V is fine.
what voltages should be maintain at the inputs of ICS (U1,U2 and U3)?
IC1 is an amplifier with a voltage gain of 2.0 and it amplifies the voltage of the 5.6V zener diode D8 so the output of U2 should be 11.20V.
IC2 is an amplifier described above.
IC3 is also an amplifier and it reduces the output voltage for current regulation.