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Batteryless current clamps?

Discussion in 'Electronic Equipment' started by Fester Bestertester, Nov 17, 2009.

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  1. I'm curious how the Fluke i200s current clamp probe can give mV output
    without the use of batteries.

    How is this done? If one is measuring 200A I can see how the magnetic field
    could generate enough current in the probe to support some high-impedance,
    low-draw circuitry.

    But when measuring on the low scale, say, 2 or 3 amps, how could the probe
    output a few hundred mV? (The clamp is spec'd to output 100mV / amp on the
    20A low scale, 10mV on the 200A high scale.)

    Can someone explain this to me? I'm fascinated to see it's possible & curious
    to know how.

    Thanks.
     
  2. You could have added a line for the OP, saying that a passive
    current clamp is a transformer, or some such.

    Jeroen Belleman
     
  3. pimpom

    pimpom Guest

    You seem to have a preconceived notion of what constitutes large,
    small and insignificant currents levels in terms of the fields
    they generate, but such categorisations are only relative. "2 or
    3 amps" is quite huge in some contexts and generate an
    appreciable flux in the magnetic core of the clamp. The
    alternating magnetic field induces a voltage in the clamp's
    pickup coil and this voltage can certainly reach "a few hundred
    mV" if enough number of turns are used.

    You can also think of the clamp as a current transformer. The
    wire being measured for current is the primary and the pickup
    coil of the DMM is the secondary.

    If you're more familiar with voltage transformers, think of it
    this way:
    Suppose you have just 1 mV output from a microphone. Connect it
    to the primary of a 1:10 transformer and you will get 10 mV at
    the secondary terminals. Use a 1:100 transformer and you get 100
    mV and so on, theoretically up to any voltage.
     
  4. pimpom

    pimpom Guest

    I don't think that's quite what John meant. Anyway, that reminds
    me of a practice by some villagers in my area. They cannot
    afford, or don't want to pay, the power connection charge and
    monthly bills. They wire their homes for a few incandescent bulbs
    and keep a pair of solid-cored wires with the ends stripped bare
    and bent into a U shape, the other two ends feeding the house
    wiring. When it gets dark, they use a dry bamboo pole to hook the
    bare ends to the overhead power lines. Free power - until they
    get caught. The power company - the government here - usually
    does nothing more than reprimand the offenders, but the practice
    is rare now.
     
  5. You need a loop to form an air core transformer, which this has been
    done.

    Cheers
     
  6. ---
    OK.

    A passive clamp-on ammeter is essentially the secondary of a transformer
    wound on a core that can be opened or closed in order to get it around a
    conductor so the current in that conductor can be measured without
    cutting it and using a conventional ammeter.
    [...]
    JF[/QUOTE]

    FINALLY an answer on-topic. Thank you.

    After watching the 3 Stooges act that is aee / sed...

    Sheesh!

    I AM FBt
     
  7. Admitted that the S/N ratio on Usenet can be frustrating. But did
    you stop to consider the possibility that a) you failed to grasp
    other attempts to explain it to you; b) your question was so
    elementary for *this* group that few people bothered; c) your
    last post might be taken as a slap in the face by those who tried
    to help.
     
  8. Admitted that the S/N ratio on Usenet can be frustrating. But did
    Right you are.

    A big thank you to those responders who gave answers to my question. Much
    appreciated.

    My comment was addressed to the "noise". :)

    FBt
     
  9. John Nagle

    John Nagle Guest

    Yes. Classic AC clamp-on ammeters are simply transformers. One
    "turn" through the clamp, many turns in the fixed coil for output.
    The output feeds into a voltmeter.

    Those are AC-only devices. There are also Hall-effect clamp-on
    ammeters, and those work for both AC and DC. These have been
    available for a decade or so, and pricing is now down as low as
    $60. I used to have one that could read down to about 500mA DC,
    and it only cost $129. Very useful in robotics and controls work.

    John Nagle
     
  10. DaveC

    DaveC Guest

    [...]
    Which make & model would that $129 model be? It's always useful to know
    someone else's favorite tools...

    Dave
     
  11. There's nothing fancy about that, the electricity meters of a medium-voltage
    consumer (real and reactive energy) are powered from the two potential and
    the two current transformers, without any other power supply (medium
    voltage=15 kV in Crete).
     
  12. So, for a millivolt output probe, this might be as simple as 2 windings (or a
    tapped single winding) with a range switch to select the winding?
     
  13. daestrom

    daestrom Guest

    If you make a 'tap' upstream of the revenue meter, even with just
    transformer action, you're stealing. Revenue meters (kilowatt-hour
    meters) have always had terminal voltage as one of their inputs. An
    illegal tap upstream may affect the voltage at the service entrance some
    small amount, but the metering will reduce the billed kWh accordingly.
    So regardless of the exact voltage supplied by the utility (it often
    varies slightly throughout the day), the amount of energy delivered at
    the service entrance is what is billed for. Power drawn off before the
    meter isn't measured and is 'stolen'.

    Of course if you just 'wrap some turns around the power line' without
    orienting the coil properly in relation to the line, you're not going to
    get any power because transformer action won't work when your turns of
    wire are parallel to the power line's magnetic field (i.e. 'wrapped
    around' the power line). And I think that was John Field's point.

    daestrom
     
  14. Anyway, current transformers must always be operated with the secondary
    shorted. In the generating facilities in Kozani, West Macedonia, where 400
    kV current transformers were involved, the operators of the plant had a
    special indicator whether the secondary was shorted.
     
  15. daestrom

    daestrom Guest

    Some old switchboard CT's I worked on in the Navy had very thin
    insulator between two spring clips. Whenever we wanted to remove a
    meter for cal, we slip the insulator out so the two clips would short
    together, shorting the CT. Then we could open circuit the meter and
    remove it from the panel. I don't remember exactly what the blade was
    made of, but it's surface wasn't perfectly smooth like polished
    material, more porous like unglazed ceramic (of course it wasn't any metal).

    The reason they built the insulator so thin was that if one accidentally
    open-circuited the CT without removing the wafer first, the high voltage
    developed by the CT would just 'punch thru' the wafer and safely short
    the CT. Then all you had to do to repair things was make sure you
    closed the circuit and replace the wafer-thin insulator blade.

    Was kind of surprised when I moved to commercial power systems that they
    didn't use something similar. Just has to have a breakdown voltage that
    is low enough to avoid damaging the CT.

    daestrom
     
  16. JosephKK

    JosephKK Guest

    Actually about equally ferrite and plastic with copper coming at about
    1/3 either of those. No battery whatsoever and AC only.
    Only active probes do that.
    I can make any output ratio i want, i know how they work.
     
  17. JosephKK

    JosephKK Guest

    There is no CT in residential metering, the energy meter is connected
    directly. Even commercial / light industrial you do not see CTs in
    the meter circuit until 600 A, and before that you are typically at
    480 V 3-phase (in the US).
     
  18. James Sweet

    James Sweet Guest


    My uncle's house (in the US) has a 400A service with current
    transformers. They're not common but they do exist. IIRC 200A is the
    largest residential meter.
     
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