controling multiple LM317 chained in series, all with single pot?

Discussion in 'Power Electronics' started by Hamlet, Oct 5, 2016.

  1. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I'd like to control three LM317 power supplies, each with an isolated transformer.
    I want to use one potentiometer, and I want to put their outputs in series. Obviously,
    I can't do that because the outputs are placed in series with each other +-+-+-.

    If I were using only to regulators, I'd use a dual pot. Since triple pots are unavailable,
    is there some other way I can do this? My aim is to create a 1A, 4v to 90v dc supply.

    I'd rather not use two LM317HV with a dual 7.5k pot. That would be too easy.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,291
    1,255
    I would look for a different way to do it instead of using 317's.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,291
    1,255
    Think about the power when the voltage is set to 4 volts and the current is 1 amp.
     
  4. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,131
    267
    The problem is that "ground" is offset by the voltage of each output.
    I agree that this is not the best way to achieve your goals, but consider these ideas:

    1) use digital pots- with opto isolation

    2) control the regulators with an adjustable current sink, referenced to the ground of the first supply in the series chain.
    This is a simple way to control something that is not at ground potential.
     
  5. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Sure, it would be considerable. But if I can control three, I can do four or more. This would be for sporadic, bench top use. At 4v, I'm only burning 29 watts each stage.
     
  6. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I've been puzzling on this idea of series connected linear voltage regulators...
    What I am thinking is to create a bench supply that can supply about 5A from
    say 5v thru 120v+, all smoothly controlled with a single pot. Using four individual
    voltage regulators with 24v transformers would limit my dissipation. (I don't want
    a switcher and the attendant noise, I don't care about the heat, and I have plenty
    of transformers and heatsinks and fans to play with.)

    Obviously, I can't use a single pot on the adjust pins on
    serially connected outputs on an LM338 in the traditionally way, as the grounds
    need to be isolated...

    Might I use an optocoupler between each adjust pin & ground, perhaps a 4N28?
    Is there a better way?
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
    1,251
    No, it won't, not the way the problem is stated. If you switch the individual supplies into the output stack as higher and higher voltage is needed, then yes. But if all four supplies are contributing all the time, the total power dissipation in the circuit is the same as if there were a single regulator circuit with a single pass transistor.

    And that is the answer. Not all power problems lend themselves to a 317 or 338. This is a perfect example of what Maslow (below) was talking about. If you want to spread out the power dissipation among four pass devices to keep the temperatures down, that is doable without all of the stacked regulator complications. Plus, the overall regulation will be much better.

    ak
     
  8. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    You can choose this kind of VR.

    [​IMG]
     
  9. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I appreciate this. One regulator with a large pass transistor bank is a good idea. Because the voltage
    range is large, I am limited to a narrow range of available regulators and pass transistors. I already have a bunch
    of LM317, LM338, LM317HV in my parts bins, and dozens of 2N3055, TIP35. I get that the dissipation is the same.
    Hopefully series stacked regulation won't be too poor...

     
  10. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Where did you find that?!
    I was going to make my own, with gears, or nose-to-nose with a thumb wheel between them...
     
  11. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    How about this idea. The bottom 317 use fixed resistors and a pot to adjust from about 2V to about 30V. Top LM317 use one fixed value resistor and a single pole rotary switch to switch adjust terminal to ground or fixed resistors in some reasonable steps.. Do the same with
    the middle LM317. Use the bottom 317 to adjust to the exact voltage. Connect the three isolated inputs in series like putting 3 batteries in
    series. Connect the + of each input to the appropriate 317 input. Connect the - output of the top 317 to the + output of the second 317, and the - output of second output to the + of the bottom 317. The output will be the + out of top 317 and the - out will be the - of bottom 317.
    If you need a schematic I can draw one for you.
     
  12. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    4 jointed type In Taiwan's website.
    6 jointed type in Ebay.
     
  13. hp1729

    Well-Known Member

    Nov 23, 2015
    1,954
    219
    Schematic please. "...each with an isolated transformer" but "connected in series"?
     
  14. crutschow

    Expert

    Mar 14, 2008
    13,020
    3,235
    Note that you want linear pots, not the volume control (log) type pots.
     
  15. crutschow

    Expert

    Mar 14, 2008
    13,020
    3,235
    Easy.
    Since they are isolated, you just connect the positive output of one to the common output of the next one, as many as you want.
     
  16. ScottWang

    Moderator

    Aug 23, 2012
    4,855
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    You just don't thinking too complex, the situation like as you using many stand power supply in series.
     
  17. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Like this.
     
  18. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Thank you Scott!
    I'm still thinking of trying something with an optocoupler... I have a bunch of these,
    and maybe I can learn something new with them. I have built a single LM338 power supply,
    a LM317 power supply with 2N2955 bypass transistors, and a 14 amp ua723 power
    supply. Now I want to try for more voltage.
     
  19. ScottWang

    Moderator

    Aug 23, 2012
    4,855
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    What's this power supply used for?
    Do you have load really need this power or just want to learn the skills?
     
  20. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I want another benchtop supply, one capable of more than 30v, and learn a few more skills.

    I also want to test devices to destruction. Are ratings conservative, or over stated? How many volts can certain components really take?

    I'm not trying to develop a product or do anything with a particular god-damned "goal" or any effort to make money. I'm sick of that. Just pure research and learning, especially analog, coding with solder.
     
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