Isolation transformer: what's the purpose of this lead?

Discussion in 'General Electronics Chat' started by Involute, Dec 31, 2012.

  1. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    In this diagram, what is the purpose of the GRN/YEL lead? Thanks, and happy New Year.
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    That would be a metal shield between primary and secondary by the looks of the diagram and should be connected to protection earth. It si used to make sure that should the primary loose its isolation, the voltage from the mains would short through the shield and not touch the secondary winding.
     
  3. #12

    Expert

    Nov 30, 2010
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    Tiny point here....that extra shield also reduces capacitive coupling between the windings. Suppose you are building a high speed switching circuit and the high frequency load pulses create noise that tries to travel back into the power line...or the other way..somebody is using the power lines to carry a high frequency control signal. The extra shield will stop the information from getting into your circuit.

    Most of the time, this is irrelevant. That's why I call it a tiny point.
     
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Electrostatic shield is a grounded single turn of conductive nonferrous foil placed between coils to divert primary noise to ground.

    Inter-winding capacitance for power transformers can be a major contributor to noise at the output. This is especially irksome when the transformer is supplying a hi-fi system, and mains borne noise gets through and makes horrid clicks, electronic "farts", electric motor whine, and various other undesirable noises in the music. An electrostatic shield will all but eliminate such noises.
    This problem always exists when the capacitance between primary and secondary is high - electrical noise on the primary is capacitively coupled from the primary to the secondary. As noted above, this can lead to mains noise getting through the entire power supply and into the amplifier in extreme cases. The electrostatic shield is very effective, and this is connected to earth. Note that the shield cannot be joined in a complete circle around the winding, as this would create a shorted turn that would draw a tremendous current and burn out the transformer

    See also http://www.rexpowermagnetics.com/products/product_electrostat.html
     
    PackratKing and #12 like this.
  5. #12

    Expert

    Nov 30, 2010
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    If you want to experience the difference, properly ground the shield then use a single pole, single throw switch to connect a nanofarad of capacitance (400 volt rated if you're starting with 250 VAC) between any wire of the primary and any wire of the secondary.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Note this one turn of copper foil has isolated ends, so it doens´t act like a shorted winding.
     
  7. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    Thanks everybody. Can someone also tell me why there are two separate primary windings and how they might be used? I thought it might be related to 230 VAC usage, but the data sheet doesn't indicate anything other than 115 VAC.
     
  8. BReeves

    Member

    Nov 24, 2012
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    That would be for 220 data sheet just neglected to mention it.
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Likely so you can wire it for double the input voltage (series) OR double the current (parallel). Note that proper polarity must be observed when using both sets of leads so you don't negate the two coils and get zero output to the secondary.
     
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