Transformer power supply - how to wire secondaries?

Discussion in 'The Projects Forum' started by pete_l, Sep 15, 2009.

  1. pete_l

    Thread Starter New Member

    Dec 9, 2008
    4
    0
    I'm building a dual power supply with outputs of 0 - 20V. Each supply comes from it's own secondary, see the barebones circuit below
    [​IMG]
    The regulators (LM338 ed. PJL) and associated gubbins come from points A and C for each channel. However I want the outputs to have a common zero volts - as opposed to the separate points B and D. I need this as the power supply has a microprocessor controlled Voltage/Current display, which is run from just one of the channels, so 0V must be a common reference point for the A-D converters.
    I'm apprehensive about just connecting these two points together as I have no information about the relative phases of the secondaries (i.e. whether they are connected as shown, with P1A - P1B and p2A - P2B, or with one secondary "reversed", relative to the other).
    Measuring the voltage across B - D shows about 5 Vols A.C, but with a 1k resistor, this drops to zero. The transformer is spec'd at 100W, so if I get this wrong, big smoke is a definite possibility.
    Simple question: can I safely connect the points B and D to form a common 0V for the two supplies?
    Answered by sgtwookie - yes you can.
     
    Last edited: Sep 17, 2009
  2. mkbutan

    Senior Member

    Sep 30, 2008
    270
    16
    I'm building a dual power supply with outputs of 0 - 20V. Each supply comes from it's own secondary, see the barebones circuit below


    SORRY
    but could not find any circuit below

    [​IMG]
     
  3. pete_l

    Thread Starter New Member

    Dec 9, 2008
    4
    0
    Thanks for the alert. There's something odd going on here. I posted this from Firefox and the image shows up - it even shows in your reply !. However it doesn't show up in Opera, but I can see it in I.E. Something browser specific seems to have happened.
    I've added the circuit as an attachment and it is visible in all 3 browsers (hopefully :))
    Sorry for the inconvenience.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The LM337 is an adjustable negative regulator.

    If you are attempting to output two negative voltages with a common 0v reference point, connecting just A and C together should be fine - as long as the secondary windings are truly separate.

    However, if you are attempting to output POSITIVE voltages using the LM337, you will have problems. In that case, modify your design to use LM317 regulators for positive outputs, and LM337 for negative outputs.
     
  5. mkbutan

    Senior Member

    Sep 30, 2008
    270
    16
    hi
    the ckt is visible now
    it is possible to connect P1B & P2A
    it will be as 3tap winding transformer (P1A;(P1B+P2A);P2B)
    it will rate as 6-0-6 V AC or 12-0-12 V AC or 06-0-12 V AC
    you can use with two BR. Diodes
    and the o/p will be X-GND-X
    where X is point A & C and GND will be B & D.
     
  6. pete_l

    Thread Starter New Member

    Dec 9, 2008
    4
    0
    Ooopsie, typo alert. That should be LM338 3 terminal positive regulator
     
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