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Dual Power Supply

Discussion in 'Electronic Basics' started by Ramendra S Roy, Jan 31, 2006.

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  1. Hi!
    I have been trying to make a +15 0 -15 regulated power supply. I have
    been using a pair of LM 723C with TIP 3055 and TIP 2955 (I think I have
    the correct number here) as the pass transistors (NPN and PNP) for the
    +ve and -ve sides repectively. I am interested in a dual supply which
    can supply up to 2 amps with high regulation and as low ripple as possible.

    The question is - are there resources which explain the use of the 723
    in detail? Can I make sure that the +ve and -ve rails are at the same
    level from 0? Do I really need 2 723s? Lots of questions I know. Many
    thanks to anybody who can help me even on one of the questions here!

    Thanks
    roy
     
  2. The LM723 data sheet provides the basic circuits for several regulator
    configurations. Are the specs on any of these close to what you want
    to build?

    http://cache.national.com/ds/LM/LM723.pdf
     
  3. Chris

    Chris Guest

    Hi, Mr. Roy. The data sheet of the LM723 (referenced in Mr. Popelish's
    post above) shows a lot of what you need. Look at the schematic in
    Fig. 4 for your basic circuit.

    Unfortunately, the current limit and current foldback don't work with a
    negative supply. So your best shot might be to use two 16VAC, 4 amp
    transformers, and have both of them feed bridge rectifiers and mongo
    filter caps (think at least 20,000uF). You can then have two separate
    trimmer pots to trim voltages to be identical, and tie the negative of
    one LM723 15V regulator to the + of the second 15V regulator. You can
    call the junction 0V, and there's your +/-15V supply. Use two 1N5402
    barrel diodes at the outputs to protect against reversing voltage.

    The term describing a dual +/- power supply where the magnitude of the
    negative voltage stays the same as the positive voltage is "tracking
    regulator". A good tracking regulator will have both supplies tracking
    (usually the positive supply is the master, and the negative the
    slave), so the negative supply will even compensate for output
    variations in the positive supply with loading or line power
    variations.

    If you're interested in doing this, It's possible to make a +/-
    tracking regulator with two 723s and several op amps, but it's pretty
    complicated.

    A very simple home-brew tracking regulator can be made with an LM340-5,
    an LM320-5 and an LM1558 dual op amp as shown in Fig. 17 of National
    Semiconductor AN-103

    http://www.national.com/an/AN/AN-103.pdf#page=10

    This is good to the current limit of the LM340 and LM320. You can use
    the tricks in Fig. 6 and Fig. 10 to bump up the maximum current on one
    or both supplies with a pass transistor, and to set short circuit
    current limit with a current sense resistor. It tracks from +/-5V to
    +/-15V. You can use two LM741s instead of an LM1558 if you want, and
    tweak the offset adjust on one of the op amps, or better, introduce a
    DC offset voltage to the feedback to compensate for differences between
    the + and - voltage. Post again if this is of interest.

    A very simple ersatz 1 amp +/- 1.25 to +/-15V regulator can be made
    with an LM317, an LM337 and a dual 1.5K ganged pot. Tweak differences
    in reference voltage by adding a few ohms to one of the 120 ohm
    resistors. Also, you can make an ersatz 2 amp dual regulator with two
    ST Semi L200s and a dual-ganged 10K pot. These aren't technically
    tracking regulators, because the negative supply won't track voltage
    droop caused by line or loading, but it should be OK for most
    applications. The latter is described in Fig. 21 of the ST appnote "A
    Designer's Guide to the L200 Voltage Regulator".

    http://www.st.com/stonline/products/literature/an/1678.pdf

    The L200 can crank 2 amps, and no extra op amps are required. Later in
    the appnote is a more complex circuit that will track for you. This
    circuit goes from +/-2.85V to +/-15V. Obviously, use TO-3 packages for
    all transistors and power ICs.

    The best thing is to make something easy that will do the job, and get
    in the game. And be sure to get massive heat sinks, and get a good
    fan. Two amps on both + and - linear supplies will mean up to 80 watts
    of heat to dissipate.

    Good luck
    Chris
     
  4. Pooh Bear

    Pooh Bear Guest

    20,000uF for 2 amps !

    Good Lord.

    Graham
     
  5. John Fields

    John Fields Guest

    Depends on how much headroom he needs.

    20,000µF will give him about 1.2V of ripple with full-wave rectified
    60Hz feeding a 2 amp load.
     
  6. Chris

    Chris Guest

    Yup. You want the rails to be about +/-20V for a +/-15V supply. If
    you use 16VAC transformers for the + and - power supplies , I'd guess
    you'd have a peak voltage after rectification of 21V or so. A really
    large cap (22,000uF) means there's less than a volt of ripple.

    Big linear power supplies generate a lot of heat. A big cap is cheaper
    than a bigger heat sink.

    But I guess I was talking more about a variable tracking supply -- I
    guess I got a little off track. If the OP chooses a design that
    doesn't have overtemp shutdown (like the 723 designs in the data
    sheet), he may be in a situation with a shorted output where the pass
    element is cranking as much as 40 watts.

    Thanks for the spot. Glad somebody's checking -- we couldn't have
    incorrect information floating around the internets. If that happened,
    it would be just like TV journamalism. ;-)

    Chris
     
  7. Thanks everybody! I have the answers I needed. Unfortunately it means
    that I have to do a bit of redesigning - especially the short cicuit
    protection. I have to make sure both rails are clamped down as soon as
    the current exceeds a maximum of 2 amps on any one of the rails. As per
    Chris's suggestions - I am going to improve on this cicuit later. Looks
    like I need to look around and decide on which regulator (or maybe use
    op amps) is the best for my application.

    Thanks
    Roy
     
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