Modifyng a LM338k based power supply

Discussion in 'General Electronics Chat' started by flyer.gio, Jan 5, 2009.

  1. flyer.gio

    Thread Starter Member

    Dec 16, 2008
    11
    0
    Hi to everybody

    I have this power supply

    (see before.pdf)

    the output voltage is variable by a resistor ladder and 3 binary thumb wheel switches (contraves).
    It can provide very low voltages (below 1,2 volts to nearly 0 volts) as it use a negative reference voltage (see IC1 and IC2).

    now I would like to modify this PSU removing the resistor ladder and the switches, and putting a simple potentiometer:

    (see after.pdf)

    The problem is that the resistor ladder was not linear :
    800Kohm to have 0,1 V.
    400Kohm to have 1,0 V.
    ........
    2,8Kohm to have 28 V.

    And using the potentiometer the output is not linear too...

    Anybody as an idea to get a linear variable output using a potentiomenter?

    Thanks a lot in advance
    Gianni - Italy
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Try to connect the 800K VR like this.

    [​IMG]
     
  3. flyer.gio

    Thread Starter Member

    Dec 16, 2008
    11
    0
    Hi Chung,

    thanks for your help
    I let you know

    Gianni
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Keep in mind that the LM338 requires a minimum load of 10mA to provide guaranteed regulation.

    As designed, the supply by itself does not have sufficient internal loading to provide that load.
     
  5. italo

    New Member

    Nov 20, 2005
    205
    1
    do this from adj to out put 120Ω put 3k pot to an lm113 zener tied to gnd THAT GIVE YOU -1.2V. add a 680 to -12v to supply zener current. this circuit will give you 0-22v output. like the sarge say the min load got to be >10ma otherwise you will get some voltage but no regulation.
     
  6. flyer.gio

    Thread Starter Member

    Dec 16, 2008
    11
    0

    Hi SgtWookie,

    How I can modify the circuit to do that?

    Thanks a lot
    Gianni
     
    Last edited: Jan 5, 2009
  7. flyer.gio

    Thread Starter Member

    Dec 16, 2008
    11
    0

    Hi Italo, thanks for your answer.

    I cannot open your attached file... "lm338.sch"

    could you give me a jpg or pdf ?

    Gianni
     
  8. italo

    New Member

    Nov 20, 2005
    205
    1
    I use express schematic and layout. so when i store it as xxx.sch that is why you cannot open it. I KNEW THAT SO . i wrote the instructions on how to modefy your circuit. read again and apply. you need a neg 1.25v and those resistors as explained. It is simple to inplement. read again and make your own schemastic.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I converted it for you.
    [​IMG]
     
  10. flyer.gio

    Thread Starter Member

    Dec 16, 2008
    11
    0
    Italo,

    thanks for your answer,
    but don't get angry please :)

    I made a polite question, I only requested a different file format of your attachment...
    Sorry if my question was so stupid for you!:)

    Anyway, thanks a lot again
    Gianni
     
    Last edited: Jan 6, 2009
  11. flyer.gio

    Thread Starter Member

    Dec 16, 2008
    11
    0
    Thank you very much
    Gianni
     
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