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Designing a variable dual rail supply (tracking)

Discussion in 'Electronic Design' started by Googler, Sep 9, 2004.

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  1. Googler

    Googler Guest

    I want to build a power supply (basically) for powering up op-amps.
    Specs :
    variable voltage from +/-1.5 to +/-15 V.
    current -> 1 A
    (i also want to power other ckts when op-amps are out of "fashion" ;-)).

    my intention is to "gang" the positive and negetive supply and to be able to
    control both +ve and -ve supply with a *single* pot( no ganged pots pls).

    can we rig up a power supply with this requirement with the LM317 type of
    regulators ?
    is there any type of "digital pot" ( not rotary switches) instead of usual
    analog ones?( i hope digital pots would be repeatable and less drifty)
    i dont mind putting a microprocessor to control the digital pot if that is

    any ideas are welcome.

  2. Issue 144 July 2002

    an 8031 controlled psu


    Serious error.
    All shortcuts have disappeared.
    Screen. Mind. Both are blank.
  3. I think you can make an ordinary 317 type variable regulator and make
    a negative one track it with the addition of a single opamp. The
    trick is to sum the two outputs and force that sum to equal zero.

    So you need a pair of equal resistors, one to each of the supply
    outputs, with the other end of each going to the - input of an opamp
    (powered by the unregulated + and - supplies). The + input goes to
    ground (the equals zero reference point). The output goes to the
    reference input of the negative regulator. You will also probably
    need a series RC network from opamp output to - input to stabilize the
    control of the negative regulator (cause the opamp output to move no
    faster than the negative regulator can respond).
  4. CFoley1064

    CFoley1064 Guest

    Subject: Re: Designing a variable dual rail supply (tracking)
    Life sure is easy with Google. I punched in

    lm317 +lm337 +"Dual tracking regulator"

    and the first result is:

    At the bottom of the page, it gives the circuit Mr. Popelish is talking about.
    Gosh, it even gives you a PC board layout. Can life get any better?

  5. mike

    mike Guest

    The answer is yes, but do you really want to?

    If you do much breadboarding, you'll soon find many places where you
    need a supply that can be adjusted to 0V. 1.5Vmin is just too high.

    About the third time your probe slips and you smoke the circuit,
    you'll be wishing you had a variable current limit. You can't charge
    a (small) battery with a 1A limit or a 1.5V minimum.

    You're also gonna be wanting floating outputs. You'll find two floating
    supplies much more useful than one dual tracking one. I've been doing
    this for 40 years and I've NEVER needed a tracking supply. Simultaneous
    +/-15V yes, simultaneous +5V +15V yes, tracking no.

    If you really want a useful supply, build one that can source or sink
    current from +20V to -20V.

    The hardest part of a power supply is the mechanical part putting
    together all the transformers, heat sinks, caps, pots, meters,
    connectors etc.
    Add another 10% to the project effort and build a really useful one.


    Return address is VALID.
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  6. Love it.
    I would either eliminate the 10 uf between the opamp output and ground
    or put a 10 to 47 ohm resistor in series with it, to be sure of not
    driving the opamp crazy.
  7. Googler

    Googler Guest

    Thanks all ,
    i will give a try to the ckt from ckt cellar.
    did google before posting here though.
    looks like i need some help in creating proper google query.

    Thanks & Rgs.
  8. CFoley1064

    CFoley1064 Guest

    Subject: Re: Designing a variable dual rail supply (tracking)
    Sorrry -- I didn't mean to sound too harsh. It must be the season or
    something. We're getting a lot of that on s.e.d. these days. After the
    "Housewife" in a prior post, you would think we eat our dead.

    Mr. Popelish and I have seen that circuit before, and knew what to look for.
    Unless you had known, you would not have added "+LM337" for the negative
    adjustable regulator, which was the key to getting the answer fast. Proper
    google queries come with knowledge, which is what you're looking for.

    I hope we were of help.

    Good luck
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