Hello SgtWookie
I am building power supply
i problem face
first

i am building this cct

Note the addition of R3, R4, and the two relays.
R3 is the same value as R2. If K1 is closed, then R3 and R2 are in parallel. Output from the regulator should then be 2.5 Amperes.

R4 is twice the value of R2. If K2 alone is closed, then R4 and R2 are in parallel, and current output from the regulator is 1.25 Amperes.

If both K1 and K2 are closed, then the three resistors are in parallel, and the output from the regulator is 3.75 Amperes

If K1 is closed, then R3 and R2 are in parallel. Output from the regulator should then be 2.5 Amperes

if K1 closed the output is equal to 2.50A

If K2 alone is closed, then R4 and R2 are in parallel, and current output from the regulator is 1.25 Amperes.

if K2 closed the output is equal to 1.60A not 1.25A

If both K1 and K2 are closed, then the three resistors are in parallel, and the output from the regulator is 3.75 Amperes

If both K1 and K2 are closed the output is equal to 3.04A not 3.75A

 

OK.

1) What is the voltage that you measured for U1's Vref, using a 100 Ohm resistor from IN to ADJ?
2) What is the value of resistance that you are using for R2?
3) What is the value of resistance that you are using for R3?
4) What is the value of resistance that you are using for R4?

 

1) What is the voltage that you measured for U1's Vref, using a 100 Ohm resistor from IN to ADJ?

Vref = 1.2503V

R1 = 0.2495 or 0.24Ω

2) What is the value of resistance that you are using for R2?

R2 = 124.7

3) What is the value of resistance that you are using for R3?

R3 = 124.7

4) What is the value of resistance that you are using for R4?

R3 = 249.4

if K1 closed the output is equal to 2.416A
if K2 closed the output is equal to 1.546A not 1.25A
If both K1 and K2 are closed the output is equal to 2.937A not 3.75A

 

Vref = 1.2503V

OK. That's almost perfect.

R1 = 0.2495 or 0.24Ω

Which are you using? 0.2495 Ohms or 0.24 Ohms? That can make a difference.

 

Also,
5) What is VREF_U2?
6) What is the value of R21?
7) With the circuit powered up, what do you measure from ADJ to OUT on U2?

 

Which are you using? 0.2495 Ohms or 0.24 Ohms? That can make a difference.

0.25Ω

5) What is VREF_U2?

1.25V

6) What is the value of R21?

120Ω

7) With the circuit powered up, what do you measure from ADJ to OUT on U2?

Mrans Voltage across R21 or Vref_U2 = 1.25V

 

R21 needs to be 125 Ohms. Otherwise, the current flow through the output adjust resistors will not be correct.

 

R21 needs to be 125 Ohms. Otherwise, the current flow through the output adjust resistors will not be correct.

If i change the value of R21 = 125Ω than chnage in current

first if K1 closed the output is equal to 2.416A now 2.52A

if K2 closed the output is equal to 1.546A not 1.25A now 1.686A

If both K1 and K2 are closed the output is equal to 2.937A not 3.75A now 3.021A