What does transformer (inductive) isolation accomplish?

Discussion in 'General Electronics Chat' started by Ωhm, Feb 2, 2012.

  1. Ωhm

    Thread Starter New Member

    Feb 1, 2012
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    We all learn at some point that for safety reasons, a transformer should be used to provide an inductive coupling or isolation between the mains and any downstream circuitry where someone could come in contact with the circuit output.

    Besides component failure (where the failure might allow mains current to have a path to an exposed output), are there any other reasons that transformers are used for isolation?

    In the case of transformers sold as "1:1 isolation transformers", it appears to me that if you have 120v on the primary and 120v on the secondary, the secondary is just as dangerous as the primary. So what is the isolation transformer accomplishing in the name of safety? It seems these are only used in test situations where a DUT is plugged into the isolation transformer for testing.

    If the DUT already has a transformer in it, is a DUT still required? Perhaps I am missing something blatantly obvious but I am trying to understand the value of transformer isolation and when it is required in a circuit. Thanks
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    It means you are not electrically connected to any AC power line.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Not correct. Being isolated from the AC mains is a safety feature required in many cases. It basically protects you from shock in case you put a hand on a "water pipe ground" and touch something in the unit.
     
  4. EB255GTX

    Active Member

    Apr 30, 2011
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    I have seen many student struggle with this - IMHO it is not well explained.


    Remember that in order for current to flow through your body, there needs to be a complete path. For a normal mains device, if you touch the phase wire the current flows from it, through you, to earth, which is connected to neutral and thus the circuit is complete.

    If you disconnect the device from earth using an iso transformer, you can touch phase wire (please don't try it) and there is no current path to neutral since current flows from phase, to you, to earth, then has no path back to neutral.
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I guess you haven't overlooked the principal advantage of the transformer which is to transform one voltage level to one or several other levels concurrently - whether for single phase or poly-phase AC systems.

    Not to mention pulse transformers, current transformers & so forth ....
     
  6. Ωhm

    Thread Starter New Member

    Feb 1, 2012
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    Nice to see I'm not the only one who has struggled with this.

    I thought that electricity would take the shortest path to ground and going through me to earth would be a really short path, ie dangerous. This is the part that just doesn't seem intuitive.

    If I touch just phase (no neutral contact) on a normal mains device the electricity flows through me to earth and I am zapped but on the iso secondary I can touch just phase and the electricity goes through me to earth and it's OK? In both of these cases I did not touch neutral but the outcomes are so different for the same electron path. This is the part I can't reconcile.

    If I understand you correctly, touching both phase and neutral on the secondary of an iso transformer I would receive no protection (because I would be part of the circuit) from the iso but touching just phase and earth I would be OK? Whether the electricity goes from phase to me to neutral or phase to me to earth, I thought those were both dangerous paths to follow resulting in shock.
     
  7. Ωhm

    Thread Starter New Member

    Feb 1, 2012
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    I just wanted to zero in on the safety benefits :p.
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    although transformers typically isolate H and X sides, an 'isolation' transformer will have improved shielding within it's windings to prevent primary carry over. Capacitive coupling is something that isn't much considered in a 'regular' transformer, but an isolation will address the issue to some degree. Isolation transformers can be aquired in varying capacities, with some providing 'lethal' power. How you use them is up to you.

    Transformers, due to thier unique properties, are primarily used to convert voltage to current and vise versa.
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    This is where you're wrong. The current doesn't go through you, because earth is not connected to the isolated secondary. That's why it si called isolated :) (unless you connect somthing elso the the secondary, for example an oscilloscope usually is grounded, so by measuring on a normally isolated circuit you ground it through the test leads and defeat the isolation.

    If you connect one side the secondary to ground you get the plain old mains with all the dangers.
     
  10. EB255GTX

    Active Member

    Apr 30, 2011
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    Kubeek has already answered this, but sometimes i find it takes several goes of getting the same facts across in different ways and with different analogies to make it "click" so i'll try and clarify:

    I was taking liberties with my description of current flow for the purpose of illustrating what is going on....in my second case, using an iso tx the current does not flow because there is no complete path to neutral. I was trying to say that the current would *try* and flow from phase, to you, to earth, then stop because there is no path back to neutral - sorry if that was a bad way to describe it! Of course the current doesn;t flow at all and you dont get zapped, I was just trying to illustrate at what point the circuit is broken.

    Hopefully my attached diagram is clearer.

    In the diagram, what an isolation transformer does is remove the earth to neutral reference. Try and trace a current path from phase, through you, and back to neutral with that link broken :)
     
  11. Ωhm

    Thread Starter New Member

    Feb 1, 2012
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    I think I am sort of getting this now. So on the isolated secondary, I would not want to touch phase and neural because then I would become part of the circuit, but phase and earth would be fine?

    As someone said above, this is one of those topics that is not generally covered well at all.
     
  12. EB255GTX

    Active Member

    Apr 30, 2011
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    Yup - althought as I said above, please don't try it! The theory is sound, but there are real life considerations that come into it since the real world is never as simple as the textbok examples :)

    In theory though, it's as simple as looking at the current path. On my diagram, start at phase, trace the current through you, to ground, which is linekd at the switchboard to neutral, thus the circuit is complete.

    If you put an isolation transformer in the way, you remove the earth-neutral link. So current would try to go from phase, through you, to earth....and then where? There's no path back to neutral so no current can flow, so you won't get a shock.

    Another thing I have noticed a lot of my students seem to miss is this:

    "earth" is not anything magical, it's just a convenient potential to reference things to. Think of it somewhat like a large conductor that is convenient to connect to.

    You don't necessarily get a shock if you are earthed while touching something - it depends where the circuit you are touching is referenced to.

    Imagine you have a 230V battery - you will clearly get a shock if you touch both terminals, but if the battery is conencted to nothing else, and you tough one terminal at a time, you won't get a shock. Now, if you connected one terminal to earth and then stood on wet ground in bare feet and touched the other terminal, you'd get a zap.

    Earth only matters when it is used as a reference by the circuit you are dealing with, and this reference is broken by an iso transformer.
     
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  13. Ωhm

    Thread Starter New Member

    Feb 1, 2012
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    I think the battery analogy sealed the deal. Thank you EB255GTX and everyone else for taking the time to clear this up for me.
     
  14. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    most transformers provide isolation between primary and secondaries, and do not have internal taps to ground. An 'isolation' transformer in no way implies that any of it's taps have not been taken to ground somewhere in the downstream circuit.
     
  15. Ωhm

    Thread Starter New Member

    Feb 1, 2012
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    An an example of an exception to that would be a microwave oven transformer secondary right?
     
  16. Ωhm

    Thread Starter New Member

    Feb 1, 2012
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    Regardless of the status of the transformer inside the device, if you plug the device into an isolation transformer you are OK as long as you don't touch phase and neutral or use an oscilloscope that provides a path to ground correct (which is tied back to neutral)? From what you said taps that have been taken to ground only count if there is actually a path to neutral back through ground right?

    Thanks
     
  17. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    An autotransformer is a better example. A MOT is a good example of an internally grounded transformer.

    sounds good.

    my points is that 'isolation' is a method of wiring, and that an 'isolation transformer' does not imply that method has been employed. Be clear what you have in front of you.
     
  18. MrChips

    Moderator

    Oct 2, 2009
    12,437
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    Others have touched upon the question of electrical shock and safety.

    There is another practical reason for using an isolation transformer. There used to be a time when a TV set had no transformer and was connected directly to AC mains. You had a 50/50 chance of having the chassis on the live side, depending on which way you plugged in the 2-prong AC plug. It was not safe to trouble-shoot a TV circuit with an oscilloscope. Hence the DUT was first plugged into an isolation transformer before the service tech had a go at it.

    As another example, you cannot use a single-channel grounded scope to view the voltage drop across a resistor (or to measure a current on the high side of a power supply, for example). This is because the scope is grounded. An isolation transformer will overcome these limitations. (Some of the safety elephants around here will frown on this.)
     
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