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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, if I am correct your voltage feedback circuit compared to the original one looks like this below.

    Only 36V output voltage? Higher not feasible?

    Current limit to 1.5A by increasing the current sense from 0.12R to 0.4R? Then I would keep the 0.12R: my current heatsinks are 5x3x12 in size, about 1.4C/W

    Lots of trimming pots, the -comp input to pin 4 seems fairly complicated: the original setup was not feasible?

    Thanks for the work!
    Erik


    Edit: oeps, my first upload of a drawing is clearly visible :angelic:


    LM723 voltage feedback circuits.jpg
     

    Attached Files:

  2. spec

    spec Well-Known Member Most Helpful Member

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    POST Issue 03 of 2016_12_11

    Hi earckens,

    I have now had a look at the output voltage adjustment for your schematic of post #135

    With the present resistor values the output voltage range would be, 0V to 32.6V

    But the power supply of post #135 should be capable of 0V to 35V.

    With a 5K voltage adjustment potentiometer, and to achieve an output range of 0V to 35V, the two 18K resistors should be changed to 47K and the two 82K resistors should be changed to 230K.

    I have also simplified the circuit around Q4 as shown in post #137. On your schematic of post #135 please delete R12 and replace with a trace. Also change R15 to 1K Ohms.

    spec

    (our posts crossed earckens)

    Addendum (just for the record)

    The relationship between the two voltage controlling resistors and the maximum power supply output voltage is, Voutmax = (7.17 *R2)/R1, where R1 was 18K and R2 was 82K.
     
    Last edited: Dec 11, 2016
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  3. earckens

    earckens Member

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    ... and the two 180K resistors ...: you mean 82k ..?
     
  4. dave

    Dave New Member

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  5. spec

    spec Well-Known Member Most Helpful Member

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    Arrragh- I am getting into a right tangle.:banghead:

    Have a nice Saturday on holiday and I will have it all sorted by the time you come back.:)

    spec
     
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  6. earckens

    earckens Member

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    ..in that case Vout = R2 * 7.15 / R1 = 32.5V which is correct for the original design. Very nice find of you, this formula! Thks!
    Erik
     
  7. earckens

    earckens Member

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    Right, we will, and be thinking of you will sipping our champagne :joyful:
     
  8. spec

    spec Well-Known Member Most Helpful Member

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    :happy: (post #143 now corrected)

    spec
     
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  9. spec

    spec Well-Known Member Most Helpful Member

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    Both circuits need trimming, either by changing resistor values or by trim pots, to cater for the various circuit tolerances and obtain exactly 35v and 0V but you can probably get by without trimming.

    I haven't calculated the maximum output voltage, but I am working on 35V at the moment. We can always maximize the output voltage later.

    The limiting factor with 2N3055s is the safe operating area (SOA). At 46V, when all factors are taken into account (mainly junction temperature), 750mA would be a maximum safe current.

    spec

    2016_12_09_Iss_01_ETO_2N3055_SOA.png
    http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF
     
    Last edited: Dec 9, 2016
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  10. earckens

    earckens Member

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    Good morning spec, just a question on your graph: Vce could be max 46Vdc, however, under full load this would drop a bit, say to 40Vdc. So I see on the graph the intersection for the lowest graphline to be at 3A.
    Another question: how could I get a LED to turn on when the current limit is active?

    Have a great day!
    Erik
     
  11. spec

    spec Well-Known Member Most Helpful Member

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    Good morning earckens.

    Hope you had a good day in Lille.

    That is true at 25 Deg C junction temperature, but at 150 Deg C, which could be the sort of 2N3055 junction temperature when the the power supply is passing a high current with the output set to 0V, that safe current would drop to around 1.5A. There are also other factors that limit the maximum current. So I would suggest that maybe 1A maximum per transistor may be a good compromise.

    But, in addition to the spec sheet limits, from experience with similar power supplies, I would not recommend more than 1A per 2N3055, on reliability grounds.

    By the way, you can set the maximum current limit to whatever you like (the LM723 and transistors will source around 6A worst case).

    The other aspect of increasing the current are
    (1) As the collector current of the 2N3055 goes up so does the VBE and minimum VCE.
    (2) The total current flowing out of the reservoir capacitors not only drops the voltage across the reservoir capacitor but also increases the ripple voltage.

    All three of these factors limit the maximum output voltage from the power supply.

    I will have a look at this. With the LM723 type of current limiting having a LED illuminate during current limit mode is a bit awkward- I think.

    spec
     
    Last edited: Dec 11, 2016
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  12. earckens

    earckens Member

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    Hi spec, yes we did have; a bit crowdy -the X-mas market was totally overcrowded and maximum security at the gates- but we had a very good time regardless. Nice town, beautifull old city section: if you ever get the chance it is really worth the trip; lots of english speaking visitors too.

    Thank you for the explanation on SOA: I learn a lot from you!

    If it gets too complicated for the current limit LED I will look for a software solution; feed the input of -comp to a high impedance analog input of the MCU I use to display V and I and do some calculations.

    Have a great Sunday,
    Erik
     
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  13. Les Jones

    Les Jones Well-Known Member

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    Hi Erik,
    I did not follow the start of this thread fully. My feeling at this point is that the specification has evolved from the original and there seems to have been some constraints put on the design such as it must use 2N3055 pass transistors and it must use an LM723. I feel it would have been better to start with the required specification. and to use components that would allow it to meet the specifications. I can see no easy way of adding an overcurrent LED other than having an external (To the LM723) circuit to do the overcurrent detection and then use a signal from that to force the LM723's overcurrent circuit to shut it down. The existing circuit has evolved with one fudge after the other to get around the constraints. (Fudge is sort of a slang term finding a way to get round a problem but not doing it properly.)

    Les.
     
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  14. earckens

    earckens Member

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    Les, I agree with you.

    I will start a new thread on this subject: describe the initial subject, describe the "wishlist" of requirements and make a resumé of what has been covered in this thread re. relevant items.

    Erik
     
  15. spec

    spec Well-Known Member Most Helpful Member

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    Hi Les and Erik,

    We have come a long way with this power supply and I think we are reaching a workable design, especially withe Erik's EAGLE schematic which makes the circuit analysis much easier, so it would be a shame to abandon it at this stage.

    Incidentally there is a parallel thread about power supplies @ http://www.electro-tech-online.com/threads/2n3055-and-heat.149521/page-7#post-1281612 which covers a 'universal' power supply architecture that can be adapted to any reasonable requirements, including a negative version.

    spec
     
    Last edited: Dec 11, 2016
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  16. Les Jones

    Les Jones Well-Known Member

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    Hi spec & Erik,
    Here is a suggestion for one possible solution for fitting an overcurrent LED.
    C_limit.jpg

    It would involve moving the current sense resistor to the positive rail.

    Les.
     
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  17. earckens

    earckens Member

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    spec, I agree with you too, more even now that I read your comment.

    I think it would indeed not add more benefit to start up a new thread, since we are almost at the end of our story about this PSU.

    And on a personal note: I enjoy immensely the back and forth with spec in this thread :)

    Erik
     
  18. earckens

    earckens Member

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    Hi Les, this means an altogether change in design for this unit: I am sure it will work, but when comparing this with the alternative: feeding the -comp into a high impedance MCU analog input and adding about 15 lines of code to an existing program.

    Thanks for the thinking about my question; I will print out the solution for later referral.

    Erik
     
  19. spec

    spec Well-Known Member Most Helpful Member

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    Pretty damn neat Les- I like it.

    spec
     
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  20. earckens

    earckens Member

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    Hi spec, in one of your diagrams you use the BC546. I have a similar transistor: C1815, but is it as good and as reliable as your proposal? Reason Iask is that I read her and there about low reliability of "chinese clones": I am not sure whether mine is genuine or a clone, can't tell. But would be a shame not to use a -in my opinion- a good equivalent?
    Erik
     
  21. spec

    spec Well-Known Member Most Helpful Member

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    The C1815 should be OK but not advisable: the CEo is only 50V compared to 65V for the BC546.

    The function of the BC546 is only voltage level shifting- no current gain is involved. The same current that is sunk from the emitter, essentially flows into the collector.

    spec
     
    Last edited: Dec 11, 2016
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