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Basic power supply design question

Discussion in 'Electronic Design' started by Steve, Jun 5, 2007.

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  1. Steve

    Steve Guest

    Let me start off by clarifying that I'm not a design engineer, so I'm
    just seeking some help, and apologize in advance if I'm unclear.

    What I want to build is a regulated 0-30v 0-3a DC supply, adjustable
    current and voltage. I want to build it for personal gratification, I
    know I could just purchase a HY3003, but what's the fun in that?

    Looking at the schematic on Bowden's Hobby Circuits,
    http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#ps6.gif
    it looks like this supply has no line regulation. Am I wrong? I
    think it has load regulation, as the input for the voltage portion is
    fed directly off the output, but there is no input reference. Please
    correct me where I am wrong, I'm sure I am.

    Looking at the LM317 datasheet,
    http://www.national.com/ds/LM/LM117.pdf
    page 17, they show a regulated adjustable voltage and current supply.
    But this will not go to 0 V, only to 1.2V. On page 16, they show the
    negative supply to allow the output to adjust down to 0v, but how do
    you incorporate this into the adjustable current schematic shown?
    Just bring the voltage adjust pot down to -1.2V? How will this affect
    the rest of the circuit, specifically the current limiter? Am I
    making this too complicated?

    Thanks for your help, all thoghts appreciated.

    Steve
     
  2. D from BC

    D from BC Guest

    I always wanted to try turning a 68W amp module into a variable power
    supply. Anybody done that?
    http://www.national.com/ds/LM/LM3886.pdf
    68Watts mono AB amp about $8.00CAD
    Benefits
    Neg and Pos output
    Output down to 0V
    Low part count
    Various internal protection such as overload, shorts and thermal
    runaway.
    Low noise
    +/-35V output
    Output current limited to 7A

    Sound good?
    D from BC
     
  3. Eeyore

    Eeyore Guest

    Most ppl who make their living from electronics would probably *BUY* a lab
    supply rather than make their own !

    Decision ONE !

    Is it going to be linear regulated or not ?

    And don't forget the centre tap. You'll kick yourself otherwise. I'd want 0-36V
    CT actually.

    Graham
     
  4. Eeyore

    Eeyore Guest

    It could make a lot of sense actually.

    Graham
     
  5. Steve

    Steve Guest

    Sorry I didn't specify. It will be linear. Part of the reason I'm
    building it is I have the components. I have a spare 241-7-24, so I
    won't get to 30V w/ full load, or get continuous 3a out, but that's
    ok. I don't mind overdesigning the regulator and putting in a beffier
    xfmr later. I already have enclosure w/ panel meters (30V & 3A),
    heatsink, pass transistor (2n3055), caps, rectifier, line filter, etc.

    And, as a side note, I already have a HY1802, which is ok, and a Power
    Mate Corporation BPA-10E, which I LOVE. I just have the parts around,
    and there's something about building power supplies that is just fun.

    Also, as another side note on the audio amp subject, one of the
    ongoing projects I have is building a 400Hz power supply for powering
    avionics. I have a spare P3A pcb from Elliot Sound Products that I'm
    going to use as the driver. The transistors I'm using can each
    dissipate 200W, and the output I need is max 29VAC RMS, so it'll be
    good for a few amps. The only part I still haven't figured out is
    some good short circuit protection. If I'm using it as a supply,
    sooner or later the leads are going to touch, and I don't know if just
    a fast blow fuse will save the transistors. I plan on just using some
    molex plugs for sockets and making them easily replaceable. Not the
    best solution, but hey..

    Steve
     
  6. Steve

    Steve Guest

    Right after I posted, I started thinking about internal short circuit
    protection. Why would I build a complicated circuit when they already
    have it incorporated in a single chip? Great idea D from BC! I was
    on the same track as you, just going about it in a much more
    complicated manner. P3A is really a great amp, but overkill for a
    simple AC supply where less than .5% distortion is probably fine. Not
    to mention, all discrete components. Takes care of the driver and
    short circuit secion of the supply. There is a great clipping
    indicator circuit on ESP that I plan on using.

    I found an old app note about creating amplitude stabilized sine waves
    using clipped squares. Simple circuit, single pot frequency
    adjustment, less than .2% distortion, no amplitude stabilization time
    like with a lamp, really neat.

    Now the DC supply I'm working on doesn't seem nearly as fun anymore, I
    may push the AC one to the front.

    Steve
     
  7. Traver

    Traver Guest

    It looks like a regulated output with the 2N3055 as a pass element
    controlled
    by an op amp fed back from the ouput of the supply - a typical linear
    regulator.
    Power supplies need a reference voltage to compare the output to and
    tell if it is
    in regulation or not. The LM317 reference is a set 1.25 volts
    developed between the output
    and the ADJ pin. Once you drop below the voltage reference it can't
    track it to
    regulate the ouput and the supply goes "open loop".

    All you really need is is an OP amp, a bipolar to level shift the amp
    so it can control
    the pass element sitting up at 30V, and the pass element itself. This
    setup gives you
    control over the voltage reference itself and you can bring it right
    down to 0V.

    The supply you mentioned above works this way with the added feature
    of a current
    control amplifier. It looks like everything you need.

    Traver
     
  8. D from BC

    D from BC Guest

    Current feedback is often done with a sensing resistor + some
    amplifying components that steer the power electronics to regulate the
    current.

    D from BC
     
  9. Phil Allison

    Phil Allison Guest

    "D from BC"

    ** No.

    ** Total disaster to have the polarity of a DC supply change in use -
    destroy nearly any cct.

    Two devices could be made to produce dual tracking, one following and one
    inverting a reference DC input.


    ** Agreed.


    ** Not true.

    Read the specs properly - that figure is the *minimum* short cct current
    for * 10 mS * with a +/- 20 volt supply.

    The current into a load can be dangerously higher - until the chip
    overheats.

    Come up with adjustable output current limiting and you may have a usable
    idea.

    Stability with capacitive and awkward loads like DC motors cannot be
    certain.



    ........ Phil
     
  10. D from BC

    D from BC Guest

    Oops..Yeah..Those Ispecs can with some fine print.. :)

    I'll be easy for me to add a current limiting circuit to the amp
    module..
    Also..
    I have some notes around here concerning feedback networks and
    reactive loads..It might help with stability.
    I've been tempted to do this project.. Mostly because it's a
    "crazyass" use of an audio amp module..
    But I'm more into 100khz switchmode.
    D from BC
     
  11. Phil Allison

    Phil Allison Guest

    "D from BC"

    ** What the large print giveth - the fine print taketh away.

    ( Old Biblical quote ...... )



    ** Do tell.



    ** Nothing like a big electro across the output for PSU stability.


    ** LOL.

    I have long dreamt of using a "spare" Crown DC300A as a classy dual
    tracking PSU !!


    ** Whatever turns you on - buddy .....




    ....... Phil
     
  12. D from BC

    D from BC Guest

    Ok...I just have many one difficulty when it comes to current
    sensing..
    Let's say I use a 100mohm sense resistor to monitor the PS current.
    I'm not sure if it matters if I do high side current measurement or I
    do low side current measurement..??
    I have design experience with both but I haven't realized the
    difference in benefits from a performance point of view..
    D from BC
     
  13. Rich Grise

    Rich Grise Guest

    "Line regulation" isn't a component, or separately adjustable input or
    anything - it's more like a phenomenon - it's just a measure of the
    regulation relative to the input, where "load regulation" is a measure of
    the regulation relative to the output. i.e., they're not independently
    controllable, unless by circuit architecture or some such.

    I have no opinion on the LM317, having never used one - but I think
    that they return the adj input to -1.2 relative to the output "ground",
    so the circuit itself doesn't really know the difference - it thinks
    it's putting out 1.2V, but the user sees 0V.

    Cheers!
    Rich
     
  14. The difference is involved with not only what the current
    limit is measuring, but with what it is trying to ignore
    (the output voltage, in this case). If the pass device
    control circuit is referenced to the negative rail, but the
    current sensing is taking place on the positive rail, then
    the sensing circuit has a lot more to do in ignoring the
    large possible voltage swing than it has to do measuring the
    small current sense voltage drop, accurately. It is a
    signal to noise problem, and a level shifting problem.

    By the way, it may not be obvious, but it is not necessary
    for the pass element and / or its control circuit to be
    referenced to the negative rail. Many lab supplies are
    designed to operate completely floating, so the negative
    rail has no particular ground or common concept associated
    with it.
     
  15. Rich Grise

    Rich Grise Guest

    .
    To me, the only time high side sensing makes sense (pun unintended,
    but noted) is if the load absolutely, positively has to have its
    negative side grounded to system ground.

    The other arguments are things like level shifting, CMRR, and all
    that stuff - well, you know the drill. :)

    Hope This Helps!
    Rich
     
  16. D from BC

    D from BC Guest

    Yup... Low side sensing can be as simple as a transistor BE across a
    sense resistor..
    Whereas..high side sensing ..ugh :p
    My ugliest high side solution was a battery powered op amp that drove
    an optocoupler.

    I'm wondering if low side current sensing is typically done in high
    end lab supplies..
    D from BC
     
  17. Guest

    The sensing (on lab supplies) is generally done on whichever side of
    the supply is the signal common for the error amplifier. Integrated
    designs more often use the negative rail, but discrete designs use
    either, without prejudice. This makes some of their schematics look
    strange for people who are used to always seeing the negative rail as
    common.


    John Popelish
     

  18. Specifically, the reference is the 4.7V Zener diode. Pin 2 is driven
    to 0V, so the output voltage is Vo= 0V ~ 25V. The op-amps are supplied
    from unregulated voltage, and the line regulation will be pretty
    crappy, maybe 3%, because the current through the zener is not very
    constant and 4.7V zeners at a few mA are not great regulators. DC load
    regulation should be fine because there is plenty of gain.

    This is a pretty cruddy hobby-level 'bench supply'. I'd expect it
    would overshoot a fair bit too if you suddenly remove part of the
    load.
    He could tie the lower end of R8 and C6 to the -1.2V reference (rather
    than ground) with 680R to -10V. The current limit should still work
    with -10V on the LM301A pin4, but I have not analyzed it in detail.


    Best regards,
    Spehro Pefhany
     
  19. Steve

    Steve Guest

    Thanks for the replies. I had already started building the first
    circuit, so I may finish and see what sort of regulation I get.
    I appreciate the input,
    Steve
     
  20. Steve

    Steve Guest

    Just curious, how hard would it be to add an LED for CV/CC indication
    to the above circuit? Any ideas?

    Thanks,
    Steve
     
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