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LM338 ouput voltage is decreasing with an increasing laod - why?

Discussion in 'Electronic Basics' started by bg, Nov 29, 2003.

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  1. bg

    bg Guest

    You are trying to dissapate 90 watts in the regulator. Thats alot of power
    even for a TO3 case. I don't have the specs but my gut feel is that you have
    to lower the 32 volt input to about 20 volts otherwise the regulator will go
    into a protection mode.
    bg
     
  2. bg

    bg Guest

     
  3. Jamie

    Jamie Guest

    Hi all,

    I have just made a regulator using LM338 TO-3 Reg.
    I have 32V DC 5 amps as the input to the reg, and the reg is reducing this
    to around 14 volts.
    I have a 240 ohm resistor from the adj to the out of the reg, and a 5k 10
    turn trimpot from the adj to the circuit ground.
    The 240 ohm is connected directly to the reg case.

    Protection diodes are installed as per the lm338 datasheet, and an 0.1uf
    ripple cap, from the adj pin to ground parallel with the 5k variable res.

    The problem is if i test the unit by connecting a 6 ohm 10 watt wirewound
    resistor from the output of the reg to ground, my output voltage drops from
    14v down to around 6-7 volts. The 6 ohm resistor represents a ~2 amp load,
    The reg should be stable up to 5 amps??

    The input voltage does not drop when the load is applied, only the output
    voltage.- any ideas why??

    If anyone has any suggestions, please email me


    thanks in advance
    jamie
     
  4. Ahhh, It FAR more important to have bypass capacitors shown in the
    spec.sheet
    on the INPUT and OUTPUT than any "ripple" cap on the adjustment pin!
    I NEVER bypass the adjustment, it slows the response of the regulator!
    1. Oscillations! (lack of proper bypassing.)
    2. Improper pin connections.
    3. Not enough heat sink.
    --
    *
    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
    (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

    http://hamradio.cmcorp.com/inventory/Inventory.html
    http://www.harvardrepeater.org
     
  5. Ahhh, It FAR more important to have bypass capacitors shown in the
    spec.sheet
    on the INPUT and OUTPUT than any "ripple" cap on the adjustment pin!
    I NEVER bypass the adjustment, it slows the response of the regulator!
    1. Oscillations! (lack of proper bypassing.)
    2. Improper pin connections.
    3. Not enough heat sink.
    --
    *
    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
    (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

    http://hamradio.cmcorp.com/inventory/Inventory.html
    http://www.harvardrepeater.org
     
  6. Ahhh, It FAR more important to have bypass capacitors shown in the
    spec.sheet
    on the INPUT and OUTPUT than any "ripple" cap on the adjustment pin!
    I NEVER bypass the adjustment, it slows the response of the regulator!
    1. Oscillations! (lack of proper bypassing.)
    2. Improper pin connections.
    3. Not enough heat sink.
    --
    *
    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
    (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

    http://hamradio.cmcorp.com/inventory/Inventory.html
    http://www.harvardrepeater.org
     
  7. Jamie

    Jamie Guest

    Thanks for the info,

    The pin connections are fine, I can draw 500 - 800ma and its no problem, but
    with a 2 amp load ,the output voltage is reduced.
    I have 2200uf caps on the input and on the output, and also 0.1uf on the
    input, that should be sufficient?

    I have tried lower input voltages (15vac into the diode bridge instead of
    24vac) and the same occurs...
     
  8. The input capacitor may be part of the problem. With a 2 amp load, it
    is going to have a pretty large ripple voltage, especially if it is
    not in particularly good condition. Do you have another similar cap
    you could parallel on it, to see if the symptoms change?
     
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