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inductive sensing relay ?

Discussion in 'Electrical Engineering' started by GH, Mar 9, 2011.

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  1. GH

    GH Guest

    Looking to have a relay close when
    current passes through a wire that
    I'm NOT allowed to cut into, or probe.

    Basically leaves me with needing an inductive
    pickup which ultimately can pull in a N/O relay

    Is there an off the shelf solution, or any other ideas ?

    Thank in advance!

  2. GH

    GH Guest

    AC and its to a variable speed exhaust fan, so I I would imaging from 20
    to 120V.

    Again, we are willing to spend a few bucks for off the shelf solutions...
  3. John Rye

    John Rye Guest

    Hello Glenn

    How about a minature split core current transformer providing the current is
    ac. then rectify the output current and use it to drive your relay. It would
    only give you a low energy output so you might need to use a reed switch or
    something like that to drive the final relay. If you have not come across
    split core transformers "Google".

  4. Beware that a current transformer must never be allowed to have an
    open circuit secondary - that can result in some nasty unsafe voltages.

    I can't see any load resistor on the current transformer secondary
    in your setup. If you are really measuring the *voltage* output of
    a *current* transformer into a high impedance scope, it isn't
    giving you a useful representation of the actual current waveform,
    and indeed the current waveform for the 100W lamp looks wrong.
    You need to have a relatively low load resistance across the CT
    secondary, and then measure the current through it by measuring the
    voltage across it. The value of the resistor depends on the CT turns
    ratio, CT power rating, and the max current on the primary side.
  5. Don Kelly

    Don Kelly Guest

    I have to agree with Andrew about the waveform which looks a lot like
    transformer magnetizing current.
    The thing is that you want the load on the secondary of the CT to be low as
    it will reflect to the primary.
    Ct's should , ideally be at the minimum load resistance for which you have
    a reasonable voltage for your sensor. 1K seems high
    Do you have any information with regard to the turns ratio?
    What is the voltage across the 100W lamp with the CT and scope vs without
    the CT and scope?
    In other words, does the addition of the CT with its load have any
    appreciable effect on the lamp load/

    Don Kelly
    cross out to reply
    wrote in message

    My bad, I didn't talk about the resistor. The packaged unit I came up
    (device picture) with has a 1k resistor potted in there. That will
    screw into a 3/4" knockout in a regular US electric box. That was big
    enough to provide a load without actually crushing the output. It
    dropped about half with 100 ohms.
    If you are sensing with an OP amp it really would not matter that much
    since they are pretty easy to drive into saturation.

    I am using LM324s
  6. Don Kelly

    Don Kelly Guest

    wrote in message
    I was just playing with the CTs. I don't really know much about them I
    got them for a buck apiece from a surplus outfit.

    The 1k did not seem to affect the output much so I went with it
    I don't think the CT had any appreciable loading effect on a 120v 100w
    light bulb.

    OK, trying to remember the scope trace information, the resistance reflected
    to the primary is quite negligible.

    Just don't open circuit the CT. as it is a step up voltage transformer.

    Don Kelly
    cross out to reply
  7. GH

    GH Guest


    What im trying to do is to open a powered damper when a commercial range
    hood is turned on.

    No silly stuff here...

    Im not sure what the actual voltage will be, or the current the motor
    draws. All I know is that as soon as the hood is turned on, I need to
    open the damper. I thought that I could do it easily electrically but
    it looks like using air pressure switches will be much simpler.

    Thanks any way guys

  8. Rich.

    Rich. Guest

    So just come off the switch for the range hood and run it to the damper
    motor. If the voltages are different then run it to a relay or a transformer
    which will in turn control the damper. It's not that complicated.
  9. GH

    GH Guest

    Owner does not want 1 wire touched in the hood.
    They dont want any warranty issues.

    All I have is the house wiring going to the hood.
    And yes, because of this, it is a little more complicated.

    If I could, I could grab power from the power light, pull
    a relay.

    If I could I would make the on button open the damper, and when fully
    open and only then pull a relay that powers the fan...

    There are a ton of ways, I know.

    But do it NON destructively ?

    Still looks like pressure
    switch is the only way, but the problem with that is grease..
  10. Rich.

    Rich. Guest

    How are they turning the hood on? Since it's a commercial hood, there should
    be a contactor powering the hood so that the fire suppression system has a
    means for turning the hood off. Right?
  11. GH

    GH Guest

    Commercial hood retrofitted for residential being place in
    elite home(s) No fire suppression. I know, its better by the min...

    Im thinking of getting one of the circuit testers that light
    up when placed near a live wire and seeing if it can power a

    Again, I cant believe that something isn't already available.

    Maybe I should try looking into remote house management or alarm
    system control's...
  12. GH

    GH Guest

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