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LM723 PSU with 0V lowest voltage

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

  1. spec

    spec Well-Known Member Most Helpful Member

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    By the way, if you are considering building another power supply for the experience that is fine. But if you just want a power supply to use it would be much better, in all respects, to buy a power supply. The prices range from the ridiculously expensive to the ridiculously cheap.

    spec

    http://www.ebay.co.uk/itm/10A-0-30V-Adjustable-DC-Power-Supply-Precision-Variable-Digital-Lab-w-clip-CE/281600624877?_trksid=p2047675.c100005.m1851&_trkparms=aid=222007&algo=SIC.MBE&ao=1&asc=20140122125356&meid=21c22124a2ef42628e715bc687bc148c&pid=100005&rk=4&rkt=6&mehot=ag&sd=111711917619

    (very similar to bounty's PSU circuit)
     
    Last edited: Aug 29, 2016
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  2. spec

    spec Well-Known Member Most Helpful Member

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    Here is a modified schematic of post #12 which includes:
    (1) Four output transistors
    (2) Decoupling
    (3) Current sink

    spec

    Issue 03 04 2016_08_30 OBSOLETE: see post #69
    2016_08_29_Iss1_ETO_LM723_PSU_ver2.png

    ERRATA

    (1) Disconnect R19 top from 0V SUPPLY LINE. Connect R19 to D7 lower (cathode)

    NOTES

    (1) All non electrolytic decoupling capacitors are thru hole (not surface mount) ceramic, X7R dielectric, +- 10% or better.
    (2) All frequency compensation capacitors are metal film type, +- 5% or better
    (3) RV1 can be 5K wire would multi turn potentiometer to give fine adjustment
    (4 RV3 can be 500R wire wound multi turn potentiometer to give fine adjustment
     
    Last edited: Sep 22, 2016
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  3. earckens

    earckens Member

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    Hi spec, I meant to say that certain component values are missing from bountyhunter's circuit diagram. Component replacements if needed should mostly not pose a problem. Thanks for querying.
    But I would be grateful for circuits that are up to date in technology; after refurbishing my current PSU I would probably try something more "modern".
     
  4. dave

    Dave New Member

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  5. earckens

    earckens Member

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    Great, thank you!
    I will look into your post in more detail Tomorrow. Bedtime here now ;)
     
  6. earckens

    earckens Member

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    No shipping to Belgium, and it does look like what I need! :arghh:
     
  7. bountyhunter

    bountyhunter Well-Known Member

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    That's why it's generic. Some of the resistors determine what your output voltage range is. Caps are in uF. The 15V line current draw is probably in the 20 mA ballpark but that's a guess. INA is from the positive unregulated supply. Maximum feasible curent is like 4 - 5A ballpark depends on heatsink.

    If you specify what output voltage and current range the unit must have, we can address component values.
     
    Last edited: Aug 29, 2016
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  8. bountyhunter

    bountyhunter Well-Known Member

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    The NPN transistor I used was a 2N5885, the 2n6124 is the PNP driver. To do 150W ps, you probably woud have to parallel four 2N5885.

    Building a linear over 100W is not trivial. It takes serious heatsink knowledge and also SOA considerations on the power devices. I tend to agree it makess sense to buy one unless the objective is to learn in detail. That's why I did it, lot of time used but I take pride in the feat.

    FYI, a 150VA transformer is only good for about 90W of DC load power using a diode bridge input configuration. The RMS current in the xformer is about 1.6X DC load current.
     
    Last edited: Aug 29, 2016
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  9. earckens

    earckens Member

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    spec, I appreciate your efforts!! Can I ask you, while you are busy focussing on the LM723 layout to add a voltage fine adjust? I like that option on the generic layout from bountyhunter. I tried to do it myself but I am not so sure I get it right: in the generic layout from bountyhunter the voltages from two potmeters are added and forwarded tot the + comp (pin 5) but the pot values are much higher then in the LM723 layout and I am not sos sure that I can substitute the same or similar values.

    This morning I did find the same PSU you linked for me on eBay for shipping to Belgium, from a similar vendor.

    Bountyhunter, thank you for your input. Yes indeed, this is for the challenge of achieving and learning. So I will try and calculate the required components.
     
  10. spec

    spec Well-Known Member Most Helpful Member

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    Last edited: Aug 30, 2016
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  11. earckens

    earckens Member

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    Great! What would I do without you guys? :happy: Thank you.

    Slept like a log, and answers coming in overnight ;)

    Just for info, the pot I also found here: http://nl.aliexpress.com/item/3590S..._7&btsid=71c81141-aba1-4bda-988f-fb05fd528e5c

    It takes several weeks to arrive so for those with patience..
     
  12. spec

    spec Well-Known Member Most Helpful Member

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    :D

    My misus is being demanding: she wants me to take her for lunch at the bistro in town. I expect I will have to force down a couple of pints of Butcombe real ale just to be sociable too.:p

    spec
     
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  13. dr pepper

    dr pepper Well-Known Member

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    I did something on the same lines a while back, see post #17

    http://www.electro-tech-online.com/threads/adding-a-current-limit-led-to-my-lm723-bench-psu.131608/

    The lm723 goes down to zero volts, the circuit uses a - supply to do it, but it needs a trimpot to adjust zero volts.
    Also the current limit tranny in the lm723 tends to be noisy while limiting, so I put together a integrating current limit circuit that seems to work well.
    The pic micro also controls a little fan on the heatink, using a ds18b20 to measure temp, which is also shown on the lcd, watts is also calc's and shown, pretty handy gizmo, shame I havent built a case for it yet.
     
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  14. earckens

    earckens Member

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    I had already seen that post. And in fact your particular post triggered me into posting my first message on this forum :smug:
     
  15. earckens

    earckens Member

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    spec, you write "errata 1. connect R19 top to D7 lower (cathode)": is your drawing already corrected for this? If not I do not understand how it should be. Can you please explain? Thks
     
  16. earckens

    earckens Member

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    Hi spec, for Q7 (BC546) may I assume any NPN low power transistor (BC548, 2N2907A, 2N3904,.. ) would do as long as Vce > 30V?
     
    Last edited: Aug 30, 2016
  17. spec

    spec Well-Known Member Most Helpful Member

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    yes
     
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  18. spec

    spec Well-Known Member Most Helpful Member

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    ERRATA updated to make clearer. :)

    spec
     
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  19. earckens

    earckens Member

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    ok spec, that is clear.
    Why not use ceramic capacitors for frequency compensation? Metalfilm does not go below 1nF, what if you need say 50pF, for example on pin 13 of the LM723?

    Hope you had a good time with misus ;)
     
    Last edited: Aug 30, 2016
  20. spec

    spec Well-Known Member Most Helpful Member

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    X7R dialectic capacitors are good for decoupling because they have a low equivalent series resistance (ESR) and a high capacitance for their size, but their capacitance value varies wildly with both temperature and voltage across them. Small surface mount ceramic capacitors are particularly bad in this respect. Ceramic capacitors distort a signal badly too.

    COG dialectic ceramic capacitors have much more stable characteristics but are much bigger for a given capacitance value and more expensive.

    For frequency compensation, as well as a reasonably low ESR, you want stable capacitance and low distortion so you would go for another type of capacitor: polyester, polycarbonate, silver mica and so on.

    Yes thanks, very nice lunch and pint and the weather was bright sunny and fresh with a light breeze (I live on the coast).:cool:

    spec
     
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  21. bountyhunter

    bountyhunter Well-Known Member

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    Avoid using Y5F or Z5U dielectric they are complete junk. X5R and X7R are best buy and good quality.
     
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