Sensing a magnet in motion

Discussion in 'General Electronics Chat' started by atferrari, Jun 18, 2014.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    I expected to sense a magnet in motion passing close this coil (from a discarded relay), with the scope in the lowest scale (2mV/div) but no joy.

    In case you ask, continuity is OK and magnets were moved in any position and in any sense you could imagine wrt the coil.

    I tested all the magnets in my drawers. While I know that no one is made of Neodymium and the diameter of the coil is really small, I expected a minimal output at least.

    What could I be doing wrong?
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    I just passed a magnet by a 24vac relay coil and saw full deflection at 20mv scale.

    ps.
    Mine was still on the core.
     
  3. MaxHeadRoom

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    Jul 18, 2013
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    Ouch.. that must have been painful? :p
    Max.
     
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  4. atferrari

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    I was inspired by a simple sensor in a pendulum that had no core at all.

    Gracias for replying.
     
  5. MaxHeadRoom

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    Jul 18, 2013
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    What direction WRT the coil did you move the magnet, and at what speed of motion?
    Oh I see it was in all quadrants?
    Max.
     
  6. crutschow

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    Mar 14, 2008
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    Make sure the coil has continuity.
    Add a piece of magnetic material (such as a nail) through the center of the coil to increase its sensitivity.
     
    Last edited: Jun 18, 2014
  7. inwo

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    Not at all. There are special magnets for that purpose.:eek:
    [​IMG]
     
  8. MaxHeadRoom

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    I have never had the pleasure! :)
    Max.
     
  9. MaxHeadRoom

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    Jul 18, 2013
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    To Quote Faraday.

    "Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet"

    But you probably knew that!
    Max.
     
  10. inwo

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    Repeated with no core. Still get good deflection.

    Must move magnet pretty quickly to get a high reading.;)
     
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  11. atferrari

    Thread Starter AAC Fanatic!

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    Found a more powerful magnet. This time I got an output (10 mV). With core, almost 20 mV.

    Thanks to you all replying here.
     
  12. wayneh

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    Sep 9, 2010
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    The best result should be with the magnet passing perpendicularly to the conductor (the copper windings), as close and as fast as possible. Through the center of the coil would be best, since all the windings would be contributing emf in the same direction. As it is, the windings on the far side of the coil see a portion of the moving field and generate an emf that opposes that of the nearer windings.
     
  13. alfacliff

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    Dec 13, 2013
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    it is an ac relay coil, how about the magnetic shunt that makes it an ac relay? wouldnt that make a shorted turn and reduce sensativity?
     
  14. atferrari

    Thread Starter AAC Fanatic!

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    Hola alfa,

    With no core, as in the picture, it is just a coil, right? Or do I miss something here?
     
  15. MrAl

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    Jun 17, 2014
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    Hi,

    It's mostly about speed and distance, but it's also about orientation. If the field is oriented such that it induces current in two wires that form a single loop, there may be little or no emf developed simply because the currents cancel (orientation).
    So what this means is that if the magnet is outside of the coil then it will induce less than if it was inside the coil, because each turn forms a loop and each gets a current in opposite direction, even though it's less in one side of each loop.
    When the magnet is inside the loop it will cause a current in one direction only through each turn, therefore you'll get the maximum magnitude of the voltage that way.

    So the closer your magnet is to the coil the more current gets generated (assuming at least some reasonable orientation), and the faster it moves the more current, and the better oriented it is the more current, and the stronger the field the more current.

    So to increase voltage you need to do one or more of the following:
    1. Decrease the distance (move the magnet closer to the coil)
    2. Increase the speed (move the magnet faster)
    3. Increase the field (stronger magnet)
    4. Orient the field to the coil better so that all turns produce a current in the same direction.

    You'll also note that if the magnet is too far away it produces no voltage. That's because the far field dropoff is quite severe.
     
  16. wayneh

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    Sep 9, 2010
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    The axis of magnetization also matters. Make sure your magnet's field lines are perpendicular to, and driven into, the wires. A disk magnet with its faces as opposing poles is good, as long as you pass the face, not the edge, over the coil.
     
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