Multiple voltages with single main adjust.

Discussion in 'General Electronics Chat' started by ElectricSpidey, Jan 13, 2018 at 2:51 PM.

  1. ElectricSpidey

    Thread Starter Member

    Dec 2, 2017
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    So I’m looking to get 5 different voltages for fine tuning, and one single control for overall control. Will this work, I have omitted some components for clarity.

    A_Regulator.jpg
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    If you're using the LM317, it is an adjustable regulator which has a constant voltage of 1.25V between the output and adjust pins, and this is used to set its output voltage with resistors.
     
  3. LesJones

    Active Member

    Jan 8, 2017
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    I don't understand how it is supposed to work. The five 317s will be trying to provide an output of + 1.25 volts with respect to ground. The 337 will by trying to output a voltage of -1.25 volts with respect to V+. All the regulators outputs will be being pulled above the output they are trying to give. I think this will probably destroy them.

    Les.
     
  4. ElectricSpidey

    Thread Starter Member

    Dec 2, 2017
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    Ok, so I added the missing components as to avoid confusion.

    A_Regulator.jpg
     
  5. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Still makes no sense.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Your explanation of what you want and your schematics make little sense.
    Describe exactly what you want each regulator to do.
     
  7. ElectricSpidey

    Thread Starter Member

    Dec 2, 2017
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    I want each 317 regulator to act as a variable resistance to each load resistor, and the 337 to act as a variable resistance to the ground.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you want to put the regulators in parallel to have more current?
    This drawing is used in the NI datasheet:

    LM317_multiple_regulator_for_higher_current.png

    Bertus
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Still makes little sense.
    What do you mean "variable resistance to each load resistor"?
    That's not the purpose of a voltage regulator.
    Regulators act to deliver a desired fixed voltage to the load.
    Is that what you want?

    Please state exactly what you are trying to accomplish with this circuit, not what you think it should do.
    What are the loads?
     
  10. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    I see what you are trying to do, and strictly in terms of voltage polarities it might work. You have to be careful about the total circuit current, because the five 317's can easily overpower the 337. Also, you have six control loops determining the voltage of a single common point, so don't be surprised if the whole thing oscillates.

    ak
     
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  11. ElectricSpidey

    Thread Starter Member

    Dec 2, 2017
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    I'm trying to get a master control, but also have trim adjust to the loads.

    How about if I use the 337 to set a new ground reference like this?

    A_Regulator.jpg

    Thank you ak
     
  12. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    True, but it is the effect in a limited situation. The same is true of changing the DC through a diode to alter its effective resistance to an AC signal riding on the DC. This was the basis for the first multiplexer and voltage-variable resistor.

    If V+ = 20 V, an LM317 is set to output +15 V, and the LM337 is set to "output" -15 V (which is +5 V above GND), then there is 10 V across the load resistor. Varying the 317 output voltage varies the current through the load, making the 317 a resistance-controlled power resistor. The 337 holds the common point at a constant voltage so that the five circuits can be varied independently. The 337 is effectively a virtual ground summing node.

    ak
     
    Last edited: Jan 13, 2018 at 4:28 PM
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  13. AnalogKid

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    I like the independence of the first schematic better.

    ak
     
  14. ElectricSpidey

    Thread Starter Member

    Dec 2, 2017
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    So do I, but everyone was poo pooing on it. :)
     
  15. crutschow

    Expert

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    How much voltage adjust range do you want for the master control?
     
  16. AnalogKid

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    It is either a very creative idea, or a disaster that will burst into flames. Jury is still out.

    ak
     
  17. WBahn

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    Mar 31, 2012
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    What are "the loads" and what, exactly, are you trying to accomplish regarding them? Are you trying to control the voltage across them? The current through them? What is it that you want to have happen to when you adjust your master control? What is it you want to have happen when you adjust the trim control?
     
  18. crutschow

    Expert

    Mar 14, 2008
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    Because we did not, (and still don't) know exactly what you are trying to do.
    The first step is doing a design is defining precisely what it needs to do and you haven't done that. :rolleyes:
     
  19. ElectricSpidey

    Thread Starter Member

    Dec 2, 2017
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    What are "the loads"
    They are resistors

    and what, exactly, are you trying to accomplish regarding them?
    See below

    Are you trying to control the voltage across them?
    Yes

    The current through them?
    Yes

    What is it that you want to have happen to when you adjust your master control?
    I want them all to change.

    What is it you want to have happen when you adjust the trim control?
    I want to change the voltage across that load and current thru that load to change

    Now…your turn…
    Will the circuit work?
     
  20. WBahn

    Moderator

    Mar 31, 2012
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    WHAT resistors? Some resistors sitting in a box in Tahiti?

    Are those resistors even shown in your schematic? If so, which ones?

    Are you aware that you can't do both (at least not at the same time)?

    For instance, let's say that I have a 1 kΩ resistor and I want to put 5 V across it and 2 mA through it. How can I do that?

    Answer: I can't. There's no point in trying.

    So if that is really what you are trying to accomplish, then you might as well stop right now.

    Change what? Change resistance? Change color?

    Have the voltage across them and the current through them change?

    If that's all you want, then just put a potentiometer between a voltage supply and the load resistors. Connect one end of all the load resistors together and to the other end of the pot. Then connect all of the other ends of the load resistors to the other side of the supply.

    Now when you change the pot you change both the voltage and the current through them.

    Okay, then take the prior circuit and put a pot in series with each load resistor.

    Nope. You can't control both the voltage across and the current through a resistor, so it won't work.

    But it will probably accomplish all the rest of your goals since you are only looking for something to change and won't say what or how. An almost random circuit will likely accomplish those goals as you have laid them out.
     
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