Creating a bipolar supply from two singles

Discussion in 'General Electronics Chat' started by PeterSt, Dec 15, 2012.

  1. PeterSt

    Thread Starter New Member

    Dec 15, 2012
    Apologies for another question about creating a bipolar supply out of two +V supplies. I read into quite some related threads, but couldn't find my answer.

    Suppose I have two +V supplies. Like with batteries I'd say that a tap in the middle will create the gnd reference, and that's it. So, +-T+-.
    But when I stack the both supplies like this, it doesn't work out as intended; One shows the correct voltage (plus) and the other just over half what's intended (minus). Besides that, the latter is wobbling (something tries to correct ?) and the stable one heats up its transistor used. So, not good.

    One thing : It would be normal to have the midpoint connected to something, which I didn't do yet (I'd really like to have all acting normally first). This, while I can imagine that this is needed. So, I now just measure the voltage from the tap to either end and no wires are connected to anything.
    But will this be my problem ?

    Both supplies are fed from the same transformer winding. This is not how it works with a full blown bipolar supply. Therefore I suspect the problem to originate there. But how ?
    So, obviously both supplies have their two wire AC input and the one transformer wire goes in parallel to both supplies. I do have two windings available and it would be no problem to use both. But now I don't know how to apply that with some helping sense. Tie the midpoint of the transformer to somewhere ? just connect each winding to its own supply ? if the latter, can phase make a difference and should I or connect both the same or just the opposite from eachother ?

    Or can something else be in order ? Like to some extend I don't understand ? :eek:
    So .. I sure can imagine (and could have tried easily) that when that center at the outputs is bound to some gnd reference (like PE) my problems will seem to be over. But seem, because most probably I will have forced something and now create unnecessary current somewhere (which may be the same in both supplies and I may not see it, implying high noise (which I don't want) in the mean time.
    Also, I guess I am trying to create something like a differential supply by this cheap means, and maybe it just can't work.
    And lastly, my further design is differential with all floating ground, and I like to keep it like that (or otherwise I will create a potential elsewhere).

    If this is explained somewhere in here after all, feel free to just post a link.
    In any event, thanks for helping out ...
  2. panic mode

    Senior Member

    Oct 10, 2011
    you CAN do this with power supplies (just like with batteries) but only IF they are isolated (or at least one of them). if they both feed of same secondary, they are not isolated. in this case it is still possible to make a dual supply but requires circuitry that supports it.
  3. PeterSt

    Thread Starter New Member

    Dec 15, 2012
    Aha ... Yea, I've read sufficiently enough about the isolation needed (logic) but that they would not be because of the one transformer (secundary) just wouldn't go into my mind.

    Thanks !
  4. crutschow


    Mar 14, 2008
    It can be one transformer if the two secondaries are isolated (separate).