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Potential Transformer

Discussion in 'Electronic Design' started by Kit Talich, Jan 19, 2006.

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  1. Kit Talich

    Kit Talich Guest

    Hello all!

    I am currently working on my senior design project of a Digital
    Power Meter here at the South Dakota School of Mines and Technology
    (Shamless plug!). I plan to measure and display Voltage, Current, Power
    and Power Factor. To do this I plan to sample both waveforms and
    calculate the rest. I have found a Hall Effect type current sensing
    chip. While I have found a few 120V-5V PT's my personal requirements
    are 230V 10A. Potential Transformers that accomplish this feat seem to
    be a little more rare. Does anyone out there know of somewhere I can
    find some of these? Small size would also be desired.

    P.S.: Gimme a JOB! B.S. EE minor:CSC
  2. Ian Stirling

    Ian Stirling Guest

    If it's isolated from the user, and double insulated, a simple resistive
    divider may work well.
  3. John_H

    John_H Guest

    Why use a voltage transformer if high-value resistors work? just make sure
    you keep within the power and voltage constraints on the resistors in a
    resistor divider and you can sample within the range of your A/D converter.
  4. Is that a request for a 230V to 10V PT? Are you willing to use an
    ordinary 10 volt step down transformer with a 230 volt primary? Is 9
    volts out close enough?

    If so, you might look at some toroidal transformers with dual
    primaries (115-230). They have quite good voltage regulation under
    light load. Low Profile Miniature Transformers.pdf
  5. Joerg

    Joerg Guest

    Hello Kit,
    Buy them in Europe. Everything is 230V there. You can also buy current
    transformers (make sure to never, never leave the burden resistors off).

    Regards, Joerg
  6. Rich Grise

    Rich Grise Guest

    If you want to measure potential, how about a - may I have a drum roll
    please - voltage divider and ADC?

    You _do_ have a common ground reference, haven't you?

    Good Luck!

    P.S.: Employ me, and I'll help you reassimilate into Real Life. ;-)
  7. Phil Allison

    Phil Allison Guest

    ** Beware - gmail and Google Groupie !!!

    ** Where did the "10A" come from ??

    Or did you mean to say V ?

    A small toroidal transformer would be ideal to derive a scaled and isolated
    AC supply voltage - but do not use it for powering the device sine the
    addition of a rectifier circuit will clip the waveform.

    ......... Phil
  8. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** That last bit looks like a tautology.

    ............ Phil
  9. Pooh Bear

    Pooh Bear Guest

    How about a patent on that ?

  10. John Larkin

    John Larkin Guest

    Use a resistibe voltage divider. Or a series resistor into a small
    audio-type transformer, operated in current mode. Or any ole 240-to-6
    or whatever transformer. You'll probably have to to a phase tweak
    somewhere in the system anyhow.
    Gimmie a resume!

  11. Phil Allison

    Phil Allison Guest

    "John Larkin"

    ** Using "any ole" transformer is bad advice.

    Small E core mains transformers have high and non linear magnetising
    currents - so the output waveform is distorted and does not follow input
    voltage changes linearly. There is often significant phase shift as well.

    Small mains toroidal generally have no such issues.

    .......... Phil
  12. It is pretty close, as long as the core is no where near saturation.
    If it is near saturation, a slight change in voltage or frequency can
    change the input to output ratio. When dealing with instrumentation
    instead of power transformers, you have to be more flexible with the
    concept of regulation than just load current regulation. ;-)
  13. Agreed. That is why I offered an example of a toroidal, 50-60 Hz
  14. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** That is purest gobbledegook.

    The term "voltage regulation" when referring to transformer is related to

    The term you needed and should have used was was " voltage linearity" .

    ......... Phil
  15. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** Shame you gave a fallacious reason.

    Good thing the true one is now posted.

    .......... Phil
  16. John  Larkin

    John Larkin Guest

    The magnetizing current waveform couples weakly to the secondary
    voltage waveform. And any electric meter winds up needing some phase
    tweak somewhere anyhow, as noted.
    What makes a toroid different?

  17. Phil Allison

    Phil Allison Guest

    "John Larkin"
    "Phil Allison"

    ** Not true of most small E-cores.

    Your knowledge of them is sadly lacking too.

    " Small E core mains transformers have high and non linear magnetising
    currents - so the output waveform is distorted and does not follow input
    voltage changes linearly. "

    ** Not much help when the phase shift is varies with applied voltage.

    ** Very low Imag when used within ratings.

    ......... Phil
  18. John  Larkin

    John Larkin Guest

    How many electronic electric meters have you designed and sold? I'm
    running about 5000 so far.

    AC line voltage hardly changes. It's the CT that has the serious
    phase-versus-current problem, which can be electronically compensated
    to some extent.

    Any core can be run at low Imag. The shape doesn't matter.

  19. Phil Allison

    Phil Allison Guest

    "John Larkin"
    "Phil Allison"

    ** Big noting yourself like this does not work with me.

    In fact, it makes you look like a pathetic jerk.

    ** You are quite wrong, yet again, plus missing the point entirely and as

    The point related to using " any ole " transformer".

    Remember those words ??

    How quickly you do forget.

    ......... Phil
  20. John  Larkin

    John Larkin Guest

    Why would I care how I look to you? What matters is that the money's
    still in the bank.

    You might be a tad more specific. So how does the shape of the core
    affect the available Bmax?

    Sure. Most any AC power transformer will make a decent PT for
    metering. Even a little distortion doesn't matter much, if you do the
    math. Besides, if it's approaching magnetic saturation, and if its
    primary copper loss is so high that significant secondary distortion
    results, it will run hot without load. I suppose there may be wall
    warts this bad, but it would be pretty rate.

    And this is a school project... it doesn't have to be qualified to
    ANSI C12, as my meters were.

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