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Load Driving from Transformer with Secondary Center Tap Earthed

Discussion in 'General Electronics Discussion' started by epsolutions, Jan 11, 2020.

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  1. epsolutions

    epsolutions

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    Sep 7, 2019
    I am feeding an audio signal into the primary of an 8 ohm to 1K ohm microphone coupling transformer. The secondary is split, having a center tap.

    I would connect the center tap directly to earth so I can pull inverted signals off the active legs and have each drive a small resistive load to earth. In other words, they would be anti-phase with respect to each other.

    I do not want to overload the signal source, which is a 10W amplifier, so I was planning to connect a 12 ohm ceramic resistor at the common point between the two loads and earth.

    Is this a proper set up, or should I also float the center tap of the transformer above earth?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Floating the output works well if the input of the following stage has a DC path to ground to allow for input bias currents. The input of the following stage also needs to be able to tolerate a common mode signal overlaid on the input signal (various sources for a common mode signal are possible, e.g. capacitive coupling across the transformer, DC bias from input currents etc).

    You can avoid all that trouble by connecting the center tap of the transformer to signal ground (0 V). I strongly prefer this term instead of earth as earth is commonly used for a real earth connection e.g. as protective earth.
    The transformer will create the DC bias current bypass. Also common mode voltage on the signal lines will have a low impedance path to signal ground thus preventing the build-up of high common mode signals.
    The output resistor can either be connected directly between the two outer transformer pins or as you plan to do be split into two resistors, each going to signal ground. The latter will further reduce the impedance of the circuit and even more reduce common mode disturbances.
     
    darren adcock likes this.
  3. Minder

    Minder

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    Apr 24, 2015
    The problem is that in N.A., the term Ground is also used for a Earth referenced conductor as well as chassis common etc.
    Back when I grew up in the UK, we always used the term Earth for a conductor or plane that was actually connected or directly reference to the earth ground.;)
    On the same subject, there is a general misuse here of the correct symbol for earth GND also. Often used for un-grounded power supply commons etc.:(
    M.
     
  4. epsolutions

    epsolutions

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    Sep 7, 2019
    Thank you. I will try this suggestion. Would it work equally well if I join the returns (low side) of the two resistive loads and the transformer's center tap at a single point and connect just one load resistor from there to signal ground?

    We used to say that "earth" was the US term for what in Europe was called "ground". But I like your distinction better.
     
  5. Minder

    Minder

    2,908
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    Apr 24, 2015
    Not sure of your location, but when I trained in the UK, it was always refered to as Earth, as in 'run power with an Earth' as opposed to N.A. where it is ground.
    M.
     

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    darren adcock likes this.
  6. epsolutions

    epsolutions

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    Sep 7, 2019
    Yes. I got it backward.
     
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