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lab power supply

Discussion in 'Electronic Basics' started by [email protected], Aug 20, 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
    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
    thanks to all for the patience
  2. ehsjr

    ehsjr Guest

    Your bridge rectifier is wrong.

  3. Guest

    you have reason!! :)
    excuse me
    I have placed it without thinking
    I correct the outline immediately
    thanks ehsjr
    other problems relative the current control?
    that one is the part too much complicated for me
    have you got an idea for join the two current controll into one?
    the updated scheme:
    thanks for the help
  4. neon


    Oct 21, 2006
    Il tuo inglese e molto meglio che il mio italiano. BUONO PER TE. the bridge is wrong as stated and you are really are trowing parts in there for no particular reason. the LM 317 can do contolling by itself see the specs. to get to zero you need a bias adjst of -1.2v. there is more things that you can trow in there like current limiting remote shut off digital preset and so forth. if i was to design a power supply with that complexisity i would go to and look up maxim they have controller for everything in one package. This old dog is good but times are changing. buona fortuna
  5. Guest

    I have found an other error of distraction
    and I have update the schematics
    please criticize it :)
  6. I think something is wrong with the +5 VDC supply. I'm not sure what is the
    purpose of Q5 and Q6, and the bases tie to each other, but nowhere else.
    Also, what is the raw DC voltage of +VCC and -VCC, and what is the
    specification of the supply? It appears to be something like 0-15 VDC at
    maybe 2-5 amperes, and the 7805 will provide 5 VDC at about 1 ampere. But
    if its input is 20 VDC, it will dissipate 15 watts. And the other series
    pass transistors will dissipate more than that. Also, the +VCC and -VCC
    connect to the power supply of the op amps, which are only rated a maximum
    of +/- 16 or 18 VDC.

    It looks like the circuit is overly complicated for what is needed, and I
    think there may be other problems. This could be a good project for
    simulation using LTSpice.

    I'd probably design it using a PIC!

  7. Spend a little time discussing your small +5V design near the bottom.
    In particular, where the more positive side of it connects and I'm
    also curious about the roles of Q5 and Q6 and R24 and R25 in the
    context of the 7805.

  8. 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
    i copy it :)
    pag.23 fig.13
    :) this is a problem.. i have may transforma at home
    but no time for searching the right one to use
    yes i add the 5v regulator for force the zero volt
    transistors hold 5A but i don't have choose the transformer yet
    I suppose that I will use an other transformer
    this is the greater limitation but i don't know how to do otherwise
    for having complete protection against the short circuits voltage
    and currente regulation I do not see other possibilities :(
    i try but seem that not exist a model for lm137,
    and I have problems when I simulate the single part with the
    the simulation of the circuit does not want to converge
    i'm not very expert with spice
    a pic?
    controlling the levels with the dac?
    and the sensing with the adc?
    is not a bad idea
    therefore you could use a lcd
    but you would have to write the software and
    to the end you would only eliminate the three opamp..

    now my big problem it's to group the
    two current regulation at the same one
    I thought to use the opamp advanced
    many thanks for the answer!

    I have corrected the scheme newly,
    led and the diode in the negative part was inverted
    this is the new scheme:
  9. Yes... but... where did you get your value for 1k for R24 (that's waht
    appears on the schematic you posted latest), which corresponds to Rsc
    on the datasheet page? What figure are you using as the short circuit
    current limit? And you show 1k for R25, when the datasheet's
    corresponding R1 is 3 ohms? Why?

  10. Guest

    the necessity of the +5V is given from the problem that introduces the
    two regolators
    to supply 1,2 volt in place of 0 when refered to the ground.
    In the schemeI had not still connected the regolator of the +5
    because I was estimating to connect it on a transformer to part,
    I would have intentional to use the same one but thinking that it
    would have been revealed useful
    to have a local power source for feeding logical TTL (that they
    consume a lot current)
    I have preferred to connect it to a transformer to part
    i copy it from
    pag.23 fig.13
    but I am stupid and have mistaken to copy! lol!
    me of I was not shrewed not even when I have tried to explain the
    outline to Paul!
    excused! the problem is that in Italy it is late night :D
    and I am distracted of nature
    i correct the scheme:
    many thanks!
  11. Guest
    excused me still
  12. I see the tie-in, now. That's a beginning. But what about R24 and
    R25?? Exactly how did you calculate their values? What is the short
    circuit current limit you desire, here?

    I roughly calculate hundreds of microamps and I have to imagine that
    the 7805 can handle that without any outboard BJTs floating about. And
    that if you really do want to limit the current to that tiny level,
    the circuit isn't going to do it because all it does is route the
    excess current via Q6 and then the 7805, neither of which will 'know'
    anything about the limits set up by R24 to limit what bypasses the
    7805 via Q5. It's just shunted aside R25 and R24 + Q5 and back into
    the 7805 to hit its own internal limits, based on temperature and/or
    otherwise, instead. Do you agree?

  13. Guest

    r24 and r25 still they have not been calculates to you
    1K it is the value of default of orcad
    I still do not know that transistor I will use for Q5
    i understood.. you have reason
    this is very very beautifull
    but an other lm317..
    I will try to search an other solution
    wath do you think about the other part?
    the operational can work?
    i think that I will have to adjust some resistore value in the opamp
    schematic part but i do it under final testing
    have you got an idea on how to group the
    two current regulation at the same one?
    thanks for the patienz jon
  14. Guest

    creed that I will use one solution similar to this
    for the 5V protection:
    me he seems good not?
    this other solution is brilliant:
  15. Guest

    I am trying to plan and to simulate my new protection:
    with circuit foldback protection and pass transistor
    i think that I will use that for the +5V
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