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Isolation transformer

Discussion in 'Electronic Basics' started by [email protected], May 5, 2007.

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  1. Guest

    I'm trying to find out why these are used as safety precautions in
    servicing mains connected equipment. Seems to me that not being
    electrically connected to the mains is a moot point. Surely the pole
    transformer isolates me from the generating dynamo at the power
    station, but my outlet can surely still kill me. Induced currents can
    surely be as dangerous as directly conducted currents?

    Wikipedia states under this heading:

    "In electronics testing, troubleshooting and servicing, an isolation
    transformer is a 1:1 power transformer which is used as a safety
    precaution. Since the neutral wire of an outlet is directly connected
    to ground, grounded objects near the device under test (desk, lamp,
    concrete floor, oscilloscope ground lead, etc.) may be at a hazardous
    potential difference with respect to that device. By using an
    isolation transformer, the bonding is eliminated, and the shock hazard
    is entirely contained within the device."

    Why would things connected to the same ground have dangerous potential
    differences from that ground?
    Could some kind soul 'splain this to me, please?

  2. Phil Allison

    Phil Allison Guest

    ** So you nave no idea what an electrical circuit is ??

    ** Not until YOU go figure what an electrical circuit is and how come a
    broken one is not one.

    Draw yourself some diagrams.

    Then figure out the CIRCUIT by which how folk get electric shocks from
    the AC power.

    ....... Phil
  3. The classic All-American Five radio did not have a transformer. The tube
    filaments were in series, and they used tubes with differing filament
    voltages that added up to the line voltage. The ground of the radio
    was connected directly to one side of the AC line.

    The plugs were generally not polarized, so there was a good chance
    that the "ground" of the radio was connected to the hot side of the
    AC line. Touch that, and touch ground, and you've got a big shock.

    They generally fixed the problem by putting them in plastic cases,
    and not having any bits of metal on the inside touchabel on the

    But if you were servicing them, you'd risk that shock. At the
    very least, you'd risk connecting the ground lead of your VTVM (vacumm
    tube voltmeter) to the "chassis ground" of the radio, and then watch
    as that lead gets really hot as it passes the current, causing the
    insulation to melt. It happens.

    But I'd say it's also dubious about how well grounded the "neutral"
    line is. After all, three-prong outlets were brought in, and the
    third prong is most definitely ground. There may at the very least
    be some small potential between the "neutral" side of the AC line
    and actual ground.

    A unit with a transformer in it is never dangerous when you touch
    the "chassis". You know for certain it is at ground. If it's not,
    something is failing elsewhere. An isolation transformer just
    moves that transformer out of the box, for the times when
    you really need the isolation.

    Nowadays, switching supplies are common. A bridge rectifier across
    the AC line. You can't look at the primary of that supply without
    connecting a ground probe, which then shorts out the AC line to ground.

  4. Andrew Holme

    Andrew Holme Guest

    If you put your fingers between live and neutral you get zapped. But
    neutral is locally connected to ground, and your body has at least some
    capacitance to ground, so you are connected to neutral via ground. If you
    touch live, it completes the circuit. Isoltation transformer neutral is not
    connected to ground.
  5. Guest

    Thanks Michael
    That classic radio sounds like a deathtrap primed for the kill.

    AFAICR, in Australia, we have always had three-wire outlets, the green
    being usually connected to your water pipe or a copper spear, and the
    black going back to the power company's street transformer where it is
    earthed very well with a ginormous copper spear. (MEN)

    I believe any live chassis in Australia will last at that potential
    less than 30 ms or whatever time the earth leakage breaker can
    disconnect you in. Failing that, the blow time of the fuse (breaker)

    So, if I put a 1:1 transformer in the power cord of my TV set, I will
    be safer delving about in its innards with it turned on? I still don't
    quite see how. You will have the outlet black and red wires (alright,
    black and brown) connected to the primary winding and the TV set
    connected to the secondary. If I grab the black and brown wires from
    this tranny heading towards the TV I'm sure I will get deaded, or is
    there something I'm missing about induced power?
    I'm not trying to be a smart arse, just trying to understand. I've
    just invested a bunch in Horowitz and Hill's Art of Electronics, and
    finding it very mind stretching.
    Wish I'd started this 51 years ago when I was flinging rocks into
    trees for my crystal set at boarding school :)
    Thanks again, jack
  6. Guest

    Thought exercise:
    Fifty metres of copper water pipe through the wet sand.
    EVERY non-double-insulated piece of electrical equipment having it's
    metal frame/chassis firmly bonded to this water pipe.
    Earth leakage breakers on all pieces.

    Yes, I know what an electrical circuit is.
    If you can't be civil and explain to me why an isolating transformer
    might be of advantage here, perhaps you might be better to leave it to
    others and remain silent. jack

    ps Is an open circuit not a circuit? Enquiring minds want to know.
  7. Guest

    Thanks Andrew.
    Sorry, I still don't get it. How does the floating neutral of an
    isolation transformer help in one's safety? You still get a shock if
    you touch live and are in any way connected to earth.
    Surely, however you place yourself between a high potential and a low
    one is rather immaterial. If you allow electrons to be pushed through
    your body with more than about 80V, you are flirting with death?
    I understand the worst case (where an earth leakage won't protect you)
    is to hold the live/active wire in one hand and the neutral in the
    other. That is almost certain death in Australia (240V single phase).
    Even worse to do it with two phases of a three phase supply (415V).
    Does an isolating transformer perhaps only ground a live chassis with
    a non-polarised, non-earthed supply? jack

    ps could you explain that "capacitance to ground"? I would have
    expected "electrical connection to ground". But then I'm on the steep
    learning curve :)
  8. It's SEB Phil, remember? If you can't be bothered with BASIC questions,
    then don't read SEB.
  9. In circuits like TV's, manufacturers don't include transformers and often
    have circuit ground tied to one side of the AC mains. You can Google on
    "hot chassis" for information on this. This is cheaper, but it can also be
    quite dangerous. For example, if you connect an oscilloscope ground to the
    chassis of a TV, you will likely see sparks fly. It stands to reason then
    that touching the chasis while also touching an earth ground will result in
    sever shock.

    An isolation transformer breaks the DC path to ground. Don't get me wrong,
    there will still be plenty of power available in the TV to kill yourself,
    you just won't get shocked by touching one hand to the chasis. You will
    also be able to ground your scope to the chasis safeley. The output of a
    1:1 isolation transformer is just as deadly as the input, it's just not
    referenced against an earth ground.
  10. Andrew Holme

    Andrew Holme Guest

    The term "neutral" indicates which side of the mains supply is earthed.
    There is no "live" and "neutral" on the secondary of an isolation
    transformer: it is symmetrical (balanced) with respect to ground.

    Very little current can flow if you touch one end of the secondary when the
    other end is open-circuit. If you connected the open-circuit end to ground,
    that would be a different story.

    If you stand out of doors, in your bare feet, you have an electrical
    connection to ground; but you can still get a shock in an upstairs room,
    wearing shoes because there is capacitance between you and ground.
    Capacitance is a property of capacitors. A capacitor is a gap in a circuit.
    Alternating current can flow through a capacitor. The amount of current
    that flows partly depends on the capacitance. Big gaps have low capacitance
    and little current can flow.
  11. default

    default Guest

    The wikipedia explanation is excellent, what part are you having
    trouble understanding?

    An isolation transformer makes electrocution less likely since there
    are grounds all around and you are more likely to contact ground with
    one part of the body while touching a live circuit in whatever you are
    working on. You can still be electrocuted but it helps your chances
    of staying alive.

    A ground fault interrupter may be a better (and cheaper) bet for
    staying alive - but don't use it in conjunction with an isolation
    transformer since that could negate the ground error leakage signal it
    needs to interrupt power.
  12. Andrew Holme

    Andrew Holme Guest

    And if you grabbed one side of the secondary in each hand, that would be the
    end of the story.
  13. Phil Allison

    Phil Allison Guest


    ** So you nave no idea what an electrical circuit is ??

    ** Not until YOU go figure what an electrical circuit is and how come a
    broken one is not one.

    Draw yourself some diagrams.

    Then figure out the CIRCUIT by which how folk get electric shocks from
    the AC power.

    ** Go get fucked - **** head.

    ** No you do **** fucking *** not.

    YOU baby fucking pile of ASD fucked SHIT !!

    ........ Phil
  14. Phil Allison

    Phil Allison Guest


    ** So you nave no idea what an electrical circuit is ??

    ** Not until YOU go figure what an electrical circuit is and how come a
    broken one is not one.

    Draw yourself some diagrams - **** HEAD !!!!!!!!!!!

    Then figure out the CIRCUIT by which how folk get electric shocks from
    the AC power.

    ** Go get fucked - you asinine **** head !!

    ** No you do **** fucking *** not.

    YOU baby fucking pile of ASD fucked SHIT !!

    ........ Phil
  15. John Fields

    John Fields Guest

    View in Courier:

    Here's a circuit of the "safe" way to use an un-isolated mains
    HOT-----< <-----[DIODE>]--+--------+
    |+ |
    [BFC] [LOAD]
    | |
    NEUT--+-< <---------------+--------+ <-----[YOU]---+
    | | |

    GND is the earth ground to which the neutral conductor is eventually
    connected, and CH GND is the chassis to which the low side of the
    load and the supply are connected. With the circuit connected to
    the mains in this way you could be standing in a puddle of water
    barefooted, and if you touched chassis ground you wouldn't get much
    of a shock. Theoretically, that is, if all the mains wiring was
    perfect. _Don't try it at home!!!_

    Notice that the circuit will produce positive high voltage
    regardless of which way it's connected to the mains. That is,
    whether the diode is connected to HOT or NEUT, and CH GND is
    connected to the other terminal, the circuit will work.

    Now, however, if the circuit is plugged in backwards it'll look like

    NEUT--+-< <-----[DIODE>]--+--------+
    | |+ |
    GND---+ [BFC] [LOAD]
    | |
    HOT-----< <---------------+--------+ <-----[YOU]---+
    | |

    This time, if you're standing in the puddle of water, barefooted,
    and you touch CH GND it might very well be goodbye.

    That's why polarized plugs and receptacles are used when there's a
    risk of shock if the user can come in contact with metallic portions
    of the device that are carrying current. Otherwise, the device is
    designed to be so completely insulated that there's no way a user
    can accidentally be shocked. The polarized plug idea is OK, but
    there's no guarantee that an ungrounded mains receptacle _can't_ be

    The safest way around all these problems is to use a transformer to
    isolate the circuitry from the mains, like this:

    HOT-----< <--+ +---[DIODE>]--+--------+
    P||S |+ |
    R||E [BFC] [LOAD]
    I||C | |
    NEUT--+-< <--+ +-------------+--------+ <---[YOU]---+
    | | |

    In that manner, since the primary and secondary windings are
    electrically insulated from each other, current can't flow through
    you to get to GND because there's no connection between the circuit
    on the secondary side of the transformer and the mains.

    Similarly, if the mains connections to the primary were reversed:

    NEUT--+-< <--+ +---[DIODE>]--+--------+
    | P||S |+ |
    GND---+ R||E [BFC] [LOAD]
    I||C | |
    HOT---+-< <--+ +-------------+--------+ <---[YOU]---+
    | |

    there would still be no galvanic connection between the mains and
    you, so you'd be safe. Theoretically, if everything was perfect.
    But, in any case, don't try this at home.
  16. Phil Allison

    Phil Allison Guest


    ** My god - how can ANYONE be that fucking STUPID !!

    Better this brain dead **** were electrocuted right NOW.

    What a pile of vile sub human SHIT !!!!!!!!!!!!!!!!!!

    Must still vote for the long deceased Bob Menzies ......

    Maggot are feasting on his putrid brain.


    ........ Phil
  17. Guest

    Thanks Anthony. Most helpful.
    I'm just a bit curious about this DC path to ground.
    If you put _yourself_ in the way of a path to ground, surely a DC
    voltage of 80 or more will be hazardous? I honestly can't see what
    benefit there is in breaking (filtering out) the DC path via the power

    What's the characteristic of our planet that is in infinite supply? An
    infinite ability to absorb charge and not change much?
    So if you are connected to it (wet feet?) any source of high potential
    current will try to use you as conduit to earth if you give it half a

    With the isolation transformer installed in the power-in lines of a
    hot chassis TV, how is the requisite voltage achieved between the
    chassis and the other end of the daisy chain of heaters/plates/etc ?

    Is it that instead of having the chassis at mains potential, and the
    neutral at earth potential, it ensures the chassis is at earth and the
    other end is at mains? Or are these things fed by DC and it is merely
    this that you are "filtering out" with the isolation transformer?
    I think I need to sleep on this, my head hurts. I will catch up in the
    morning after a well-deserved Sunday sleep in :) jack
  18. Guest

    But you haven't actually said HOW it helps, have you?
    I'm trying to understand how an IT helps with grounded chassis
    equipment and polarised three pin plugs to the mains. jack
  19. Guest

    Aha. Now I see a glimmer of light. I understood that an IT was merely
    a 1:1 transformer. That is, mains voltage in: mains voltage out. So
    for the equipment to work, it still must have two wires entering that
    are mains potential apart, I would have thought. These would now be
    arranged to be + half mains potential and - half mains potential?
    So you switch the equipment off after the IT? I understood the aim was
    to work on the equipment while running.
    So without the IT, the live wire can damage you, even when the neutral
    is open circuit, whereas with an IT installed under the same
    circumstances, the "live wire" out of the IT is almost impotent?
    Thanks for that, I think I'm getting it.
    I understand that. The fully insulated body will allow current to flow
    into it until it is charged to it's "capacitance". On a totally dry
    wooden floor in low humidity atmosphere with dry rubber soles,
    touching 240V give one a nasty jolt -- DAMHIKT :) jack
  20. Guest

    Which is where I simplistically came in wondering why this was not the
    case with an IT. Thanks, jack
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