Is the Isolation transformer always 1:1 turns ?

Discussion in 'Power Electronics' started by Forum Algérie, Oct 1, 2016.

  1. Forum Algérie

    Thread Starter New Member

    Oct 1, 2016
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    Hello everyone,

    I have a simple question, is the step-up or step-down transformer considered as isolation transformer ? ( isolation to prevent electrical chocks when repairing electrical devices).

    Thanks in advance for your answers.
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    No.

    I have one setup to step down 240 VAC to 120 Vac and it has taps to handle low and high line voltages. This is a true isolation transformer which includes electrostatic shielding between the primary and secondary windings.
     
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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It does provide galvanic isolation where the primary is referenced to earth ground and the secondary, high or low, is now isolated when needed.
    The exception is of course the auto-transformer.
    Max.
     
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  4. ian field

    Distinguished Member

    Oct 27, 2012
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    Mains isolation transformers are most often 1:1, but any transformer other than an auto-transformer should provide isolation.

    You can still go for the coveted Darwin award while using a mains isolating transformer - but it doesn't complete one path to earth ready for you to complete the other arm of the circuit.

    Also handy if you want to connect earthed test instruments to it.
     
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  5. Forum Algérie

    Thread Starter New Member

    Oct 1, 2016
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    there's something that i still don't understand; if i touch the wire of the secondary winding of an isolation transformer, why doesn't the current flaw throw me to the ground of the primary winding of the transformer ?

    [​IMG]
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Because the transformer offers galvanic isolation where the secondary is not referenced to earth ground.
    IOW how would the current flow back to ground?
    Max.
     
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  7. Forum Algérie

    Thread Starter New Member

    Oct 1, 2016
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    Okey, but what came to my mind is that current always goes from high to low potential, ... the secondary winding has high potential compared to the ground of primary winding, isn't this right ?!
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    But again, the secondary is NOT connected to earth so there can be no completion of the current path from secondary to earth.
    Max.
     
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  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    @Forum Algérie if you measure from the top terminal of the primary to earth you would see the primary supply voltage at this point, now measure from either of the secondary terminals to earth and what voltage would you expect to see?, assuming perfect insulation between Pri. & Sec.?
    Max.
     
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  10. DGElder

    Member

    Apr 3, 2016
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    Only if there is a conductive path between the two potentials. If it is isolated there is no path.

    No, at least not necessarily. The top of the secondary has a high potential with respect to the bottom of the secondary due to induction from the primary. With respect to ground or other voltage references the secondary is floating. That means its voltage with respect to ground or other references will depend on stray capacitive coupling to other voltage planes (that capacitance and stored charge is tiny). If you touch the secondary you change that coupling and it will assume the voltage of the probe, your hand or whatever is touching the secondary (Don't test this with body parts!).

    Of course touching the two ends of the secondary could kill you. Or if you grounded one side of the secondary (removing the isolation) then the other side is at high potential with respect to ground and creates a shock hazard.
     
    Last edited: Oct 3, 2016
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  11. ian field

    Distinguished Member

    Oct 27, 2012
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    If you need that explaining - you need a different hobby............................
     
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  12. ian field

    Distinguished Member

    Oct 27, 2012
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    You have to complete the circuit for current to flow.

    Birds perch on the high voltage transmission lines - the supply is earthed at one end, but the birds aren't.

    An isolation transformer is safer than a supply with a ground reference - but you can still kill yourself if you're stupid enough.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    @Forum Algérie If you want a true understanding of grounding I would suggest you get hold of a book by Eustace Soares book on grounding that is used as a reference for NEC etc.
    I managed to get a as new copy from AbeBooks clearing house for a $1.00!
    There is an explanation of what and why and reason for grounding in the Appendix called The history and Mystery of Grounding.
    Max.
     
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