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If you want to convince yourself that eddy currents really matter ...

Discussion in 'Just Pickups' started by TeleTucson, Aug 4, 2018.

  1. TeleTucson

    TeleTucson Tele-Afflicted

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    Here's a really simple and fun non-destructive demo you can do for yourself. :)

    Last month I posted on an eddy-current equivalent electrical circuit which I think is based on some simple, basic physical principles.

    http://www.tdpri.com/threads/physically-based-eddy-current-equivalent-circuit.846745/#post-8468750

    One of the good things about that model was that it allows some quantitative estimates of real things you can do. Here's one you can try yourself at home, and if you ever held any skepticism about how important eddy currents can be, this will convince you in a simple, non-destructive experiment on your own guitar:

    Eddy Loop.jpg

    Take some 8-gauge copper wire, cut it and form it into a loop that can go around your tele neck pickup as shown above. Solder the loop closed (that's critical), fit it on, and try strumming your guitar with, and without, this loop in place (you'll probably need to lay the guitar flat on your lap so it doesn't fall off).

    Wow - the difference is remarkable! Strumming closer to the bridge where you'd normally get a lot of treble will make it more noticeable. The heavy 8 gauge of the copper wire matters, as the change is much more subtle with lighter gauge.

    Note: You should make it a reasonably good fit to get a good mutual inductance coupling, but if your tele neck has screw height adjustments in the pickguard, just make the loop a bit longer so it lays flat on the pickguard.

    I found this a fun little demo, but I'm probably just a geek :D
     
    Last edited: Aug 4, 2018
  2. rigatele

    rigatele Tele-Afflicted

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    As a kind of bonus, try the same thing placed around a humbucker and see how it makes no difference. :)
     
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  3. TeleTucson

    TeleTucson Tele-Afflicted

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    Yes, with little to no net changing magnetic flux through a copper "eddy current" surround loop in the case of the humbucker, that would not surprise me, or Prof. Faraday :) But if you laid a single-coil-sized copper loop on top of just one of the coils of a standard side-by-side PAF-style humbucker, or put two single-coil-sized copper loops side-by-side with one on top of each of the coils, you'd see it for sure.
     
    Last edited: Aug 5, 2018
  4. rigatele

    rigatele Tele-Afflicted

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    BorderRadio and TeleTucson like this.
  5. Inglese

    Inglese Tele-Meister

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    Not sure if the point is Eddy current or rather a shorted winding coupled to the same magnetic circuit the pu coil is sensing... Eddy current usually are considered where you don't have a defined electric circuit but a plane or a mass so that "simple" current induction formula are hard to use.
     
  6. rigatele

    rigatele Tele-Afflicted

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    The paths of eddy currents are defined both by the fields that produce them and the geometry of the conductive material. The time varying magnetic field of the string induces currents in specific directions in a material. The resulting current in a bulk conductor like a metal plate would still be singular and circular. Since it is so hard to measure currents in a plate directly, you can estimate it by substituting and comparing partitions of it in the same environment. That will give you a good idea of where the current will flow in a bulk conductor.

    In fact, a pickup eddy current is effectively, "a shorted winding coupled to the same magnetic circuit the pu coil is sensing".

    It is true that it is not possible to obtain exact measurements by substitution, that would require multiple test coils. However, with pre-knowledge of the field orientation it is easy to obtain reliable approximations. Furthermore, with a good theoretical knowledge of how the field produces current, it is entirely possible to predict where the eddy current paths will be, and also the approximate degree of coupling. That is the whole point of my paper.
     
    Last edited: Aug 5, 2018
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  7. TeleTucson

    TeleTucson Tele-Afflicted

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    I had not seen this & didn't know you had evaluated all those "copper loop" configurations. Very nice data, even if I may take issue with some of the explanatory field/current dynamics that precede it. All that data motivates me even more to get to the bottom of the "pre-resonance droop" you are seeing when using the exciter coil. :)

    Update - I did make an updated simple Spice model with the exciter that seems to get the droop, and posted it in the other thread on "Physically Based Eddy Current Equivalent Circuit".
     
    Last edited: Aug 5, 2018
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  8. David Barnett

    David Barnett Doctor of Teleocity

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