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linear PS's

Discussion in 'Electronic Basics' started by Abstract Dissonance, Jan 31, 2006.

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  1. I'm building a PSU and I've got a 40VCT @.25A transformer. I want to have
    two outputs of +V1 and -V2 volts... i.e., two outputs that are adjustable.
    I've done it for the +V already using the LM317T and a circuit that they
    have in there data sheet... I get from 1.25 to 30V...

    I'm curious though as when I do it for the negative supply(V2) if I could
    take the ground as V2 and the "hot" as V1 so I'd get V1-V2 = V1+|V2| to get
    a new voltage(which would ~= 2*30)? or, say, if I needed two positive
    "hots"(not sure what to call them) could I just reverse the ground the
    V2(i.e., swaping ground and V2 "behind the scenes"(well, lets say with a
    toggle switch)) or does this cause problems?

    i.e., I'd like to get the most out of my psu but I'm not quite sure what I
    can do... right now I just have some voltage regulators(LM317 and some fixed

    One more thing... I was messing around monitoring the voltage and the
    output(of the regulator) seemed to fluxuate with 1/100th of a volt or 2 and
    at some points it was about 1/2 a volt or so. Is this normal? (I wasn't able
    to monitor the input voltage at the same but it seems to fluxuate too).

  2. I suggest you use the center tap of the secondary as common, with a
    bridge rectifier across the 40 volts. Connect 2 storage capacitors in
    series, with the positive end of one to the + output of the bridge and
    the negative of the other to the - output of the bridge. This should
    give you about positive and negative 28 volts, no load. Your LM317
    regulator is fine for the positive output (with the reference divider
    to the center tap). You need the negative version of the 317, and
    LM337, for a symmetrical negative regulator.
    Do you have a high frequency bypass capacitor (e.g. .1uF 50 V ceramic)
    connected directly from the grounded end of the reference divider to
    the regulator input, and a second one from the common to the regulator
    output? If not, you may have an oscillating regulator.
  3. Mike

    Mike Guest

    and the junction of the 2 caps to the center tap of the transformer
    which will be your common or ground.
    It cuts your max output voltage in half but gets you the + and - you
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