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Measuring 120V/240VAC current

Discussion in 'Electronic Design' started by Angus, Oct 16, 2008.

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  1. Angus

    Angus Guest

    Hi all,

    I am looking to put together a circuit that measures power usage of
    mains devices (both 120V and/or 240V for here in the UK) but am
    struggling to find a current transducer that will give enough
    sensitivity at low currents (e.g. a few tens of mA for a phone
    charger) as well as full scale measurement of 13A. Has anyone here
    done anything similar, and if so could suggest a suitable device?
    Ideally I don't want to simply put a series resistor in line as I'm
    trying to keep this as efficient as possible!

    Many thanks

  2. Phil Allison

    Phil Allison Guest

    "Angus the Ass "

    ** That will require some form of range changing.

    ** What you are tediously crapping on about is yet another Greenie " Holy
    Grail " wank off machine that costs nothing to make and does nothing worth

    ** Then you are an utter WANKER !!

    **** off.

    ...... Phil
  3. spamme9

    spamme9 Guest

    Use a standard clamp-on ac current meter.
    For more sensitivity, wrap more turns thru the sensor.
    you can buy gizmos with a plug on one end, socket on the other
    and two or more openings to stick your clamp-on. Different
    numbers of turns == sensitivity multiplier.

    Or spend $20 on a kill-a-watt...don't know if they like 240V.
  4. Angus

    Angus Guest

    Many thanks for all the advice (apart from the 1st reply, obviously!).
    Some good ideas to look into...

  5. Mark

    Mark Guest

    note that in an AC circuit POWER is not simply AMPS x VOLTS.

    Look up "power factor"

    Did you want to measure current or power?

  6. Angus

    Angus Guest

    I actually want to measure power, so yes, I will be taking PF into
    account. Thanks!
  7. DJ Delorie

    DJ Delorie Guest

    Analog Devices ADE7753 ?
  8. Phil Allison

    Phil Allison Guest


    ** Go drop dead you IDIOTIC Greenie wanker.

    ..... Phil
  9. Phil Allison

    Phil Allison Guest


    I actually want to measure power, so yes, I will be taking PF into
    account. Thanks!

    ** Compute it from the phase angle - right ??

    You moronic wanker.

    ...... Phil
  10. whit3rd

    whit3rd Guest

    If you want a wattmeter that handles 13A and 13 mA both with
    a percent or so accuracy, your ammeter part will have to be
    dual-scale. It's going to be a small series resistor (a copper
    wire, for instance) at 13A, and a much larger series resistor,
    in a current-transformer secondary, for the 13 mA.

    The small sense resistor and current transformer primary are
    both in series with the load.

    The trick is, your current-transformer will saturate (like, at 50 mA)
    so the series resistance on its secondary is no longer in-circuit
    when the power is high. Use transconductance multipliers
    to multiply the sense resistor voltage drop by the mains voltage,
    converting the product to frequency (volt/frequency converter), then
    use a
    microprocessor to count the pulses. There may have to be
    four volt/frequency converters, one for the (+) power/low current and
    one for (-) power/low current, third for the (+) power/high current
    and fourth for the (-) power/high current sections; that's because
    voltage/frequency conversion doesn't behave well if the input
    ever goes negative.

    The microprocessor, each power cycle, must determine if the
    low-current section saturated, and use the high-current data for
    that time period, suitably scaled, if it did. If the low-current
    section didn't reach saturation current, its associated count is
    used instead, because it will be more accurate.

    The 240V doesn't need similarly wide-range treatment, I trust.
    If there were significant DC current drawn, that would render
    the current-transformer circuit inaccurate, of course.

    For extra credit, use multiple floating power supplies and op amp
    current mirrors, and generalize the scheme to five separate
    current ranges instead of just two...
  11. JosephKK

    JosephKK Guest

    You do not seem know or understand squat about current transformers.
    They are normally linear over 3 to 4 orders of magnitude, and can be
    really fast (sub-nanosecond pulses, wound low nanosecond 300 A ones
    myself). Specialized ones are useful to 6 orders of magnitude. You
    can buy ones that go up to 10,000 A Commercial-Off-The-Shelf (COTS).
    Try Ion Physics stuff or clamp on ammeters for examples. Electric
    service providers use them all the time for metering purposes.
    The rest of the measurement system is up to the designer.
  12. whit3rd

    whit3rd Guest

    Note, however, that the simple copper sense resistor also
    is linear over 4 orders of magnitude, and is much less expensive.
    My intent was to put a low-current sensor (the current transformer,
    of an inexpensive size and no great capacity) in series with a
    high current sensor so as to make a low-Z current sense
    array with two gain ranges.

    Two resistors won't do it. A high-current transformer (my 800A
    example here weighs about 2 kg) is expensive. A low-current
    transformer, which drops out of the circuit when it saturates,
    seemed suitable.
  13. JosephKK

    JosephKK Guest

    It might be made to work if you can also provide the HV isolation that
    is typical with a current transformer.
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