# LM338 Power Supply

Discussion in 'The Projects Forum' started by Umer_Farooq, Apr 8, 2010.

1. ### Umer_Farooq Thread Starter Senior Member

Mar 6, 2010
104
3
Hello SgtWookie
I am building power supply
i problem face
first

i am building this cct

if K1 closed the output is equal to 2.50A

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

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

File size:
18.6 KB
Views:
63
2. ### SgtWookie Expert

Jul 17, 2007
22,202
1,792
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?

3. ### Umer_Farooq Thread Starter Senior Member

Mar 6, 2010
104
3
Vref = 1.2503V

R1 = 0.2495 or 0.24Ω

R2 = 124.7

R3 = 124.7

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

4. ### SgtWookie Expert

Jul 17, 2007
22,202
1,792
OK. That's almost perfect.

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

5. ### SgtWookie Expert

Jul 17, 2007
22,202
1,792
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?

6. ### Umer_Farooq Thread Starter Senior Member

Mar 6, 2010
104
3
0.25Ω

1.25V

120Ω

Mrans Voltage across R21 or Vref_U2 = 1.25V

7. ### SgtWookie Expert

Jul 17, 2007
22,202
1,792
R21 needs to be 125 Ohms. Otherwise, the current flow through the output adjust resistors will not be correct.

8. ### Umer_Farooq Thread Starter Senior Member

Mar 6, 2010
104
3
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