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lab power supply

Discussion in 'Electronic Design' started by [email protected], Aug 21, 2007.

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  1. Guest

    hi to all
    i'm an italian electronics hobbyist,
    i'm sorry for my bad english :(
    in this holiday i try to build my first lab fully adjustable power
    supply unit
    i try to hack the scheme at pag.8 of the datasheet of the lm317
    http://www.ortodoxism.ro/datasheets/nationalsemiconductor/DS009063.PDF
    for add the negative section,
    the tracking voltage mode,
    the transistor for increase the output current,
    and a costant current output protection..
    i ' ve implemented 5V reference due to start the regulation from 0V
    instead of 1.2V (normal limit of an LM317)
    i would like to use an lm705 for the 5 volt reference using this
    voltage for an ausiliary power supply for TTL boards
    i obtained this basic principle scheme , not yet dimensionated
    i will have an opinion about eventual problem or improvements
    http://dunsscoto2.googlepages.com/SCHEMATIC1_PAGE7B.pdf
    thanks to all for the patience
     
  2. MooseFET

    MooseFET Guest


    ASCII ART:
    Sense
    ! !
    ---------- --+-/\/-+--
    ! \ /e R2 !
    ! --- !
    ! R1 ! R3 !
    +-/\/\- --+--/\/\------+
    ! e\ / !
    ! --- !
    ! ! !
    ----+-/\/\/--+--! LM317 !-----+------------
    R4


    R1 makes the transistor's E-B drop less important so temperature has
    less effect on the performance.

    Moving the R3 connection does the same for the other transistor at no
    cost in parts.

    You have Q24 flipped.

    What are ypou trying to do with Q26?
     
  3. Guest

    when r108*imax it becomes approximately equal to the Q26 Vbe,
    Q26 is turned on ,it short circuit R109 and then switch off Q25 and it
    limits
    therefore the output current
    http://www.ortodoxism.ro/datasheets/fairchild/LM7805.pdf
    pag.23 fig.13

    what do you think about of the current regulator?
    you have some idea to avoid to use a double potenziometer for the
    current adjust?
    we could be taking advantage of the advanced operational U2A

    this is the updated scheme:
    http://dunsscoto2.googlepages.com/SCHEMATIC1_PAGE7C.pdf
     
  4. Guest

    Sorry MooseFET I have mistaken to copy!
    excuse me! the problem is that in Italy it is late night :D
    and I am distracted of nature
    now I understand why the regolator did not seem correct!
    many thanks!!
    I have corrected the scheme from many errors
    this is the new one link
    http://dunsscoto2.googlepages.com/SCHEMATIC1_PAGE8.pdf
    aspect suggestions in particular for the regulation of the current
    thanks to all!
     
  5. MooseFET

    MooseFET Guest


    I don't like what you have done with R4. Transistors should not be
    uses such that their HFE matters. You want an emitter resistor and
    not a base resistor. The has an indirect effect on the current limit
    issue.

    I worry that the current limit circuit will overshoot, ring and do
    other bad things. When the current limit is not active, the LM358
    will have its output as high as it can manage. This is bad.

    A diode or the like on the LM358 so that it never tries to take its
    output all the way up would help to make the current limit engage more
    quickly.
     
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