Stupid question about twisted pairs

Discussion in 'DIY Discussion' started by mjp200581, Sep 17, 2013.

  1. mjp200581

    mjp200581

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    I understand it is good practice to twist pairs of wires together to help cancel out the negative effects of EMI.

    This is simple enough when dealing with a live/neutral or positive/negative pair but I've got myself into a bit of a muddle when trying to figure out which wires to twist together when transformers have dual secondaries.

    Here are two examples:

    My phone stage PSU calls for inputs of 22v-gnd-22v and the transformer has dual secondaries as follows:

    First secondary: Red - Black
    Second secondary: Orange - Yellow

    I have the Yellow and Red connected together to give me a centre tap so I now have:

    Orange - AC1

    Yellow/Red - 0v centre tap

    Black - AC2

    So which pairs of wires should I twist together? At the moment I have orange/black twisted together and yellow/red twisted together but is that correct?


    Example 2:

    I have a small 5v regulated PSU in my CDP. The PSU needs two AC inputs.

    The transformer has two secondary outputs labelled as follows:

    First secondary:
    Red 9v
    Blk 0v

    2nd Secondary:
    Yel 9v
    Blk 0v

    I have red/yellow connected to input AC1 and orange/Black connected to input AC2.

    Again which wires should I twist together as a pair?

    I did warn you that it was a stupid question.
     
    mjp200581, Sep 17, 2013
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  2. mjp200581

    mjp200581

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    Well I've been trawling through hundreds of images of the internals of top quality amplifiers/CD players and nobody seems to twist the wires of transformer secondaries together.

    Naim seem to group all of the wires together tightly into a bunch with the wires all running parallel and then secure them together with zip ties.
    Like this:[​IMG]
    image upload no size limit

    I did find a couple of images of some old Arcam amps where they had just twisted all of the secondary wires together into one big twisted bunch.

    Like this:[​IMG]
    image hosting 15mb
     
    mjp200581, Sep 18, 2013
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  3. mjp200581

    Tenson Moderator

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    I think you should twist all 3 together as at some point current will flow in all wires. When presented with this situation I braid them.
     
    Tenson, Sep 18, 2013
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  4. mjp200581

    felix part-time Horta

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    The reason is to keep the 'loop - area' described by the current flowing to a minimum to minimise radiation - rectification is very noisy process.

    It doesn't much matter whether you twist or braid or just run tighly parallel and together as the Naim approach illustrated - use whatever is easiest and reliable for you to do.
     
    felix, Sep 18, 2013
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  5. mjp200581

    Tenson Moderator

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    So the best solution is to have the rectification and main smoothing caps right next to the TX, then lead DC over to the circuit to be powered and have some additional smaller caps there.
     
    Tenson, Sep 18, 2013
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  6. mjp200581

    mjp200581

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    Thanks for the replies. I'll have another go at tidying the wiring in my DIY projects.
     
    mjp200581, Sep 19, 2013
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