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    Transformer & Grounding

    Hi All,

    I have a system which interacts with transformer and the same system again takes the output of the transformer to analyze. I want to isolate both the sections of transformer from shorting each other. What shall I do?

    Regards
    Kshatriya

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    Re: Transformer & Grounding

    say again.. ? you have a step up/down or isolation or current transfomer and you want to what?
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
    ... Tony Stewart EE since 1975



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    Re: Transformer & Grounding

    I have a system which inputs sine wave to the primary side of a transformer and the same system takes output at the secondary (center tapped output) Now if my system ground is shorted at primary and the same system ground is shorted to the output at the secondary, then that means I have put a path between primary and secondary . is it not . How can i avoid it?
    Last edited by KSHATRIYA; 17th July 2012 at 07:13.



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    Re: Transformer & Grounding

    You do not need to connect directly to the secondary. An op amp can be used to provide a ground reference for a floating differential signal applied to the inputs.



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    Re: Transformer & Grounding

    Quote Originally Posted by BradtheRad View Post
    You do not need to connect directly to the secondary. An op amp can be used to provide a ground reference for a floating differential signal applied to the inputs.
    Please elaborate, could not get you.



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    Re: Transformer & Grounding

    Quote Originally Posted by KSHATRIYA View Post
    Now if my system ground is shorted at primary and the same system ground is shorted to the output at the secondary, then that means I have put a path between primary and secondary . is it not
    Yes, but that is OK. There is no connection between primary and secondary inside the transformer.

    You can connect ground to one side of the primary and also to one side of the secondary, or to the center tap of the secondary.



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    Re: Transformer & Grounding

    Quote Originally Posted by godfreyl View Post
    Yes, but that is OK. There is no connection between primary and secondary inside the transformer.

    You can connect ground to one side of the primary and also to one side of the secondary, or to the center tap of the secondary.
    So you mean to say no problem If I connect like that? My transformer would still work fine.



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    Re: Transformer & Grounding

    Quote Originally Posted by KSHATRIYA View Post
    So you mean to say no problem If I connect like that? My transformer would still work fine.
    It should be fine. Can you show your circuit?



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    Re: Transformer & Grounding

    Please elaborate, could not get you.
    If you decide you need to avoid a connection between the primary and secondary coils...

    You can use an op amp to obtain the differential voltage across the secondary...

    Then take its output level as referenced to your zero ground, which you said comes off the primary coil.

    Link to article (particularly the bottom 1/3 of the webpage):

    http://www.allaboutcircuits.com/vol_3/chpt_8/2.html

    You may need to attenuate the signal coming from the secondary.



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    Re: Transformer & Grounding

    This is essentially how power is transferred around a city, country and the world. For safety the center tap which is Neutral is connected to safety ground. The isolation between primary and secondary is provided by the dielectric properties of the transformer prevent arcing across windings a shunt stray lightning to ground. You will not get a floating output of course as it is tied to the same ground as the primary neutral is connected. All current on secondary neutral is shared by the complementary outputs on the secondary side. Any ground currents are only from LC filters called line filters which use tiny caps to ground such that the leakage at residential line voltage does not exceed 0.5mA demanded by UL for certification. GFI or ground fault interruptors are sensitive breakers that can react to a mA of current on the ground line.

    This same characteristic would apply to any signal, be it 50/60Hz or microwave for isolation. The critical factors are attenuation of lightning stray noise and surge protection at the meter to 6kV. Other factors that are common mode are leakage inductance for power and return loss for microwave.

    Since there is no current generally passing from primary center tap to secondary center tap ( which may be ground or not as in the case of telephony) serves to reduce common-mode nose and does not alter the transformer current coupled between primary and secondary.
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
    ... Tony Stewart EE since 1975



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