Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

AC Comparator

  1. Dec 22, 2009 #1
    There are two AC current carrying conductors.(both are of the same frequency)
    How do you compare the currents to know if they're not equal at all times....??????

    Please Answer as soon as possible .......
  2. jcsd
  3. Dec 22, 2009 #2
    If the currents are in phase or 180 degrees out of phase, just run the the conductors through a current transformer.

    If the currents are not in phase or 180 degrees out of phase, you could rectifify the currents and compare the DC value.
  4. Dec 22, 2009 #3
    Thanks carl..
    but i don't know anything about those currents.... I mean their phase relationships.
    Can we use something like 2 current transducers and a comparator??
    Do such hardware exist?
  5. Dec 22, 2009 #4
    Wind equal number M turns (or just pass the conductors through) a window-frame (or toroid) magnetic core, such that the two currents oppose each other. Wind a third coil (N turns) on the window frame, terminate it in a low resistance, and monitor the waveform on an oscilliscope. If constructed correctly, the current in the third coil, times N/M, is the current offset between the two conductors. It is necessary to conduct sufficient current in the third winding to prevent the current offset in the two main conductors from saturating the core. Very roughly, if each primary winding has M turns and the third winding is unterminated, then a current difference of ~30/M amps (very rough number) will saturate the core.
    Bob S
    Last edited: Dec 23, 2009
  6. Dec 23, 2009 #5


    User Avatar
    Science Advisor

    If the two signals are at some unknown phase to each other, you would have to rectify the signals and measure the currents as DC and then compare them.

    Rectifying them would involve something like this for each signal:
    AC current meter.PNG

    Comparing them would depend on whether the signals had a common return wire or returns that could be joined together. If there was no common return, you could just have two meters and compare them visually, I guess.

    If they did have a common return, you could use something like this:

    AC current meter 2.PNG

    The voltage across the two shunts is rectified and filtered and compared.

    Because there is a common ground, these DC voltages could be compared in a microprocessor so that allowances for small variations could be made.
  7. Dec 23, 2009 #6
    These toroids are examples of single-turn current transformers.
    http://www.toroid.com/standard_transformers/current_sensing_transformers/current_sensing.htm [Broken]
    If you run both current conductors through the hole so that the currents cancel, the output signal will be the current difference signal, even if both currents are equal amplitude and have a slight phase difference.

    [added] If you wind 10 turns of each conductor on the 1:300 transformer in the URL, the output will be 1:30 ratio for the current difference. The output is a real-time sine wave of the difference current. You could also use two toriods, and make lissajous curves on an oscilliscope.
    Bob S
    Last edited by a moderator: May 4, 2017
  8. Dec 24, 2009 #7
    Quick note -
    The Ground Fault Protector, which is installed in most bathrooms, is circuit that compares the currents in your two phases, Line and Neutral, to ensure they're the same magnitude and opposite phase. If one acts different from the other, it reacts by popping the breaker. There are similar devices on an industrial scale.
    If you're worried about shocking people, than the common GFI outlet is good. If you're worried about motor winding shorts, you may want something bigger. Contact your square D dealer regarding Ground Fault Interrupters (GFI) or Ground Fault Circuit Interrupters (GFCI)
    . Mike
  9. Dec 25, 2009 #8
    Thank you all.... (especially mike :P)

    It was helpful..
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook