LM338 Power Supply

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

  1. Umer_Farooq

    Thread Starter Member

    Mar 6, 2010
    104
    3
    Hello SgtWookie
    I am building power supply
    i problem face
    first
    [​IMG]
    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
     
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  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    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 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
    22,182
    1,728
    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
    22,182
    1,728
    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 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
    22,182
    1,728
    R21 needs to be 125 Ohms. Otherwise, the current flow through the output adjust resistors will not be correct.
     
  8. Umer_Farooq

    Thread Starter 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
     
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