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LM723 PSU with 0V lowest voltage

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by earckens, Aug 26, 2016.

  1. earckens

    earckens Member

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    Hi spec and Les,

    I studied all the ideas of the last few hours and have a few questions, just to confirm before I start building tomorrow (and spec: I promise not to wait until the prototype is ready and finished, I try to start on the Eagle drawing tomorrow):

    I refer to the drawings in post 113 and 119:

    1. I miss the voltage feedback circuit to -comp:
    a. no need to recalculate the voltage divider 18k / 82k?
    b. value for the voltage regulator pot: remains at the original 2k2?
    c. maximum 46V gets fed back to -comp via the 82k resistor: +comp remains unchanged compared to the original circuit; surely this is not right?

    2. 18V zener regulator: currently based on 40mA (both zener bias and total load for 18V).
    a. Is this sufficient?
    b. Why 18V and not for example 30V? 36V?
    c. Why not a series regulator with a transistor? (if it can be solved with a zener, sure why not?)

    Thanks a lot!!
    Erik
     
  2. earckens

    earckens Member

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    Full agree, 39V is too close to the LM723 max ratings.
    What about using 36V zener?

    What is the problem of connecting the collector of the internal pass transistor for the series regulator to the rectifier output?
     
  3. Les Jones

    Les Jones Well-Known Member

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    Transistors have a maximum collector/emitter voltage rating and if it is exceeded they can fail. I think 36 volts should give a safe enough margin.
    spec's design is a much better solution.

    Les.
     
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  4. dave

    Dave New Member

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  5. earckens

    earckens Member

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    Sure, I think too. But what about the voltage feedback circuit (-comp at pin 4, etc...): is there no need to adapt this (resistor values)? And the voltage pot: can/may 5k be used? Changes needed there?

    Thks,
    Erik
     
  6. spec

    spec Well-Known Member Most Helpful Member

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    Yes , a 39V +-5% Zener diode, for example, could have a nominal voltage of 41V and the voltage could be even higher when all factors are taken into account.

    As a general design guide, only use maximum of 80% of the components voltage rating. So in the case of the LM723, 40V *0.8 = 32V

    Not sure what you are saying here John. What is the 'series regulator'?

    spec
     
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  7. earckens

    earckens Member

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    spec, I meant this as a reference to my post where I would have substituted your proposal for a series resistor with 18V zener diode (the 18V regulator of your schematic in post 113), with a series regulator where the transistor collector would be exposed to the full 46Vdc. Les' answer was that it exposes the transistor to the full 46V at the collector-emitter junction.

    Spec, is the voltage feedback circuit (-comp on pin 4) not in need of adjustment (your drawing post 113)? Or will the regular 18k/82k voltage divider with the 2k2 voltage pot remain?

    And can/may I substitute the 2k2 pot with a 5k pot, and what adjustments would be needed if I did?
    (I happen to have a 5k multiturn pot available).

    Thks
    Erik
     
  8. spec

    spec Well-Known Member Most Helpful Member

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    Hi earckens,

    I have not analyses the voltage feedback. I will need to redraw that area to understand what is going on. You can get into a terrible pickle with the LM723 type voltage feedback if you are not careful.:)
    Yes: the LM723 takes 7mA worst case, and the base of the TIP142C requires 7 mA max, which means that there will be 40mA - (7mA + 7mA) = 26mA minimum flowing through the Zener diode.
    The LM723, will be happy at 18V.
    An 18V supply line will have better stability and less ripple than a 36V supply line.
    If you mean a standard three terminal regulator, like an LM317, they will not take 56V.
    The drop-out voltage of a three terminal regulator ia around 3V so that would further limit the PSU output voltage range.
    The circuit would be more complex.

    spec
     
    Last edited: Dec 8, 2016
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  9. earckens

    earckens Member

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    Hi spec, is it then not risky to implement your proposal of post 113 not knowing if the voltage feedback circuit is still adequate and would work?

    What would you propose: the theoretical approach: analyse the feedback circuit to calculate correct component values, or me building the circuit and start with fault analysis and try to resolve from there?
     
  10. spec

    spec Well-Known Member Most Helpful Member

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    I will have a look at the voltage control, but it will need a bit of work to untangle what is going on.:eek:

    Almost certainly the circuit can be configured for your 5K multiturn potentiometer.:)

    spec
     
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  11. earckens

    earckens Member

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    Tell me how I can return the favour!
     
  12. spec

    spec Well-Known Member Most Helpful Member

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    The suggested modification does not affect the voltage feedback.

    I will have a look at the voltage feedback and let you know, but do not worry, it is no big deal.

    spec
     
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  13. spec

    spec Well-Known Member Most Helpful Member

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    Hi Les,

    I started a new thread about graphics editors: http://www.electro-tech-online.com/threads/graphics-editors-which-program.149652/

    You could still use the same approach- just put a resistor between the base and emitter of the new transistor so that the voltage drop across the resistor is not more than about 350mV, so that the additional transistor is not turned on.

    The only problem I can envisage is that a grounded emitter transistor will introduce more current and voltage gain, which could make loop frequency stability more difficult, especially with the PNP driver transistor.

    2016_12_09_Iss1_ETO_LM723_PSU_LES_JONES_MOD.jpg

    It would be much appreciated if you did look for mistakes- my stuff needs to be checked.:D

    spec
     
    Last edited: Dec 9, 2016
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  14. Les Jones

    Les Jones Well-Known Member

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    Hi spec,
    The resistor between base and emitter should work. (Why didn't I think of that.) I think Eric may as well stick with your design to save doing another schematic. I like the way you explained to him about the way to makethe schematic easier to understand. I was mentaly turning the PNP transistor upside down so the emitter was at the top.

    Les.
     
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  15. earckens

    earckens Member

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    The Eagle schematic is almost ready, will be posted the next few hours
     
  16. earckens

    earckens Member

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    Schematic attached; upload of .sch not allowed, so I zipped it.

    Edit: zip edited to include exit capacitors

    Edit 2: I do not understand the drawing in post 132? Where is the output voltage driver?

    Edit 3: added the schematic in .png format
     

    Attached Files:

    Last edited: Dec 9, 2016
  17. spec

    spec Well-Known Member Most Helpful Member

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    The output transistors are the two NPN transistors, as before.

    The output NPN transitor driver is the TIP142C, also as before:)

    spec
     
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  18. spec

    spec Well-Known Member Most Helpful Member

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    POST Issue 03 of 2016_12_09

    Hy earckens,

    Below a complete schematic for the LM723 power supply, with the following features:
    (1) Common base transistor to limit the voltage seen by the LM723 (and not increase loop gain).
    (2) Output voltage adjustment from 0V to 36V (but note my previous comment about getting down to zero Volts and leakage currents).
    (3) Only a simple constant current limit of 1.5 A, which is the maximum current that two 2N3055 transistors will safely provide, as previously stated.

    spec

    2016_12_09_Iss1_ETO_LM723_PSU_WITH_VOLTAGE_CONTROL_VER10.jpg
     
    Last edited: Dec 9, 2016
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  19. Les Jones

    Les Jones Well-Known Member

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    Hi spec,
    Sorry to do this again but I think the base and emitter of the current limit transistor need swapping over.

    Les.
     
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  20. spec

    spec Well-Known Member Most Helpful Member

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    Don't apologize Les; your keen eye is much appreciated.:cool:

    I will update the schematic of post #137

    spec
     
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  21. spec

    spec Well-Known Member Most Helpful Member

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    Hi earckens,

    I just had a look at the .png image of your schematic of the LM723 power supply- nice work. that is much easier to follow.:cool:

    spec
     
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