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powering electronics by current flow

Discussion in 'Electrical Engineering' started by [email protected], Apr 9, 2008.

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  1. Guest

    How easy would it be to make some small solid state electronics be powered
    by current flow?

    I've been trying to imagine what it would take to make a circuit that could
    be used to communicate optically the current level from a current transformer
    with no metallic connection to the receiving point. What I imagine would be
    attached to or made part of the current transformer itself. When there is
    a sufficient amount of current, it would be able to power the electronics,
    which would measure the current level, encode it digitally, and emit that
    code over a low level LED. Nearby another electronics package powered by
    other means would receive that code and know what to do with it.

    An issue I see is that the electronics being powered by the current would
    have to be able to operate on very low levels of current since anything below
    that level would be unmeasured. But it would also have to operate on high
    levels of current, and survive faults in the system being measured (spikes
    of very high overcurrent).

    It would also have to be highly reliable to avoid having to contact that
    current sensing point for maintenance.

    Also, given the high voltages involved, the sensing package would have to be
    designed for that. I would assume it would need to have a permanent load
    resistor, but it could still see a wide range of voltage drop across that
    resistor as the current levels change.
     
  2. Palindrome

    Palindrome Guest

    I would suggest delaying the conversion from A to D until after it had
    been transmitted and received.

    For example, the current from the transformer could be applied to a coil
    whose movement in a magnetic field was controlled by, say a
    spring/torsion device plus a bit of mass damping. A mirror could be
    attached to the coil and a remote light source shone on it. An array of
    photosensors could be positioned on a cricular arc and able to detect
    the degree of deflection of the mirror and hence the coil.

    What do you think? It could be called a photo-reflective galvanometer...
    or something ;)
     
  3. Guest

    | I would suggest delaying the conversion from A to D until after it had
    | been transmitted and received.
    |
    | For example, the current from the transformer could be applied to a coil
    | whose movement in a magnetic field was controlled by, say a
    | spring/torsion device plus a bit of mass damping. A mirror could be
    | attached to the coil and a remote light source shone on it. An array of
    | photosensors could be positioned on a cricular arc and able to detect
    | the degree of deflection of the mirror and hence the coil.
    |
    | What do you think? It could be called a photo-reflective galvanometer...
    | or something ;)

    But how do you make sure the transmission path is accurate?
     
  4. Palindrome

    Palindrome Guest

    Oh, that's just mechanics. You could split the source light beam and use
    the two differentially, with a coaxial unenergised coil and mirror
    assembly and an equal length light path as the reference.

    Or use a couple of different wavelength lasers as the source and pass
    the light through a pair of gratings, instead of mirrors. Then count the
    nulls and peaks in received intensity.

    Or simply put a rare Earth alloy in the coil and use (linear) Faraday
    rotation of polarisation with magnetic field strength to change the
    polarisation of a laser beam incident on it. No moving parts at all,
    that way. Just remotely measure the degree of polarisation shift of the
    reflected laser light.
     
  5. Guest

    | Or simply put a rare Earth alloy in the coil and use (linear) Faraday
    | rotation of polarisation with magnetic field strength to change the
    | polarisation of a laser beam incident on it. No moving parts at all,
    | that way. Just remotely measure the degree of polarisation shift of the
    | reflected laser light.

    I like this one best.
     
  6. Guest

    | Piece of cake! Of course you need some numbers to see which ball park you
    | are in. As an example, lets assume that the AC current you want to measure
    | is between one amp and 100 amps, 60Hz. Lets further assume that you allow
    | 100 milliwatts to power the electronics and that the sense resistor on the
    | secondary of the current transformer produces .05volts per amp, 5 volts at
    | 100 amps.
    |
    | Let the electronics on the secondary side of the current transformer work on
    | 5 volts at up to 20mA = .1 Watts. Let the current on the primary of the
    | current transformer create that required .1 watt plus a little for the
    | sensing at one amp sensed. Therefore the voltage on the primary must be
    | .1W/1A = .1 Volt. plus a little for sensing. The current transformer must
    | step up the .1 volts to 5 volts or a ratio of 1:50.
    |
    | The sense resistor on the secondary of the current transformer is in series
    | with the electronics power supply and has a value of .05 volts at.02 amps (1
    | amp divided by the ratio) = 2.5 ohms. The electronic power supply has a
    | shunt regulator (maybe a 2 Amp zener) that absorbs the up to 2 amps on the
    | secondary of the current transformer so the electronics is not overvoltaged
    | at sensed currents up to 100 Amps.
    |
    | A micro processor with an ADC senses the voltage on the series sense
    | resistor and converts it to digital then drives an Infrared LED with a
    | serial pulse train representing the sensed current. Low baud rate RS 232 or
    | other serial transfer is used to operate the LED and is generated in the
    | micro code and internal UART.
    |
    | Another processor with a phototransistor and internal UART is used to
    | receive, detect and decode the infrared beam. You do whatever you want with
    | this digital output like drive a display, etc.
    |
    | This is just an example. Much lower power than 100 milliwatts in the
    | electronics is possible with the right processor and judicious use of LED
    | power at low rep rates.

    While it certainly would not be the same model used in both cases, consider
    the extremes:

    1. 240 volts, 100 milliamps to 400 amps working range
    2. 765000 volts, 50 amps to 5000 amps working range
     
  7. You might want to do a little more research on this. You can actually do
    this without any power at the CT. You simply do not use a conventional
    CT. There are optic materials whose properties change in the presence of
    magnetic fields. By controlling the geometry (how close to the conductor
    you install the sensor) you can determine the current by these changes.
    Of course this involves using optic fiber and lasers. At my office I
    believe I still have some materials on this. Look in T&D World
    magazine. There have been articles on optic CTs.

    As for using a normal magnetic CT and communicating down the information,
    it is doable. We have played with different designs for powering devices
    from current flow in our lab. If you need very low power levels you can
    do ok with 10s of amps flowing.

    Charles Perry P.E.
     
  8. Don Kelly

    Don Kelly Guest

    ----------------------------

    A number of years ago there was some experimentation using lasers which may
    have been something along this line. I have no idea of how it turned out.
    Possibly weather disturbances were a problem, accuracy another problem. -
     
  9. Guest

    | Are you sure the high voltage values are AC? This kind of voltage is typical
    | of DC transmission lines. Obviously you can't use a simple current
    | transformer on DC.

    I'm only considering AC. There are AC transmission lines at that voltage.
    DC would be a different challenge.
     
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