Re: [EE] measuring the magnetic field from a small trace carrying a < 50uA load?

Martin -

You are correct, I should have said something about moving the head
across the conductors, cassette tapes moved less than 2 inches per
second so the head wouldn't have to be moved super fast ?.

The thing I've noticed with Hall effect devices was that they are
really noisy, 50 uA probably not going to give much of a signal above
the noise.

Chuck

On Mon, Jan 6, 2025 at 2:47 PM Martin McCormick <[email protected]> wrote:
>
> Chuck Olson <[email protected]> writes:
> > Do you have an old busted cassette recorder ?  Maybe the playback head
> > could be used to sense the field ?
>
>         That's an interesting thought but you only get voltage
> from an inductor sensing a magnetic field if the inductor is
> physically moving or the magnetic field is constantly changing.
>
>         A Hall-Effect device is a solid-state component whose
> resistance changes when exposed to a magnetic field.  Think of a
> transistor's response to a current change on it's base or a
> voltage change to the gate of a FET.
>
>         The only Hall-Effect device I own is a chip that looks
> just like a flat-pack transistor.  The Hall device is in there
> along with a level detector .  There are 3 leads coming out.  One
> is Negative/Common.  Another is 5-volt VCC and the third lead is
> the sense lead and switches between 0 and 5 volts depending on
> proximity to the correct polarity of a magnetic field.  When it
> receives that field, the logic changes suddenly so there is no
> linearity to read the strength of the field.  One uses this chip
> anywhere one might use a magnetic reed relay such as in a door
> interlock or maybe a timing indicator.  I haven't played with
> that device in a while but it starts generating pulses at AC
> power line frequency if you get near a transformer or motor with
> it.  It also doesn't have the mechanical issues one sometimes
> runs in to with reed switches such as false trips or opening when
> jarred.
>
>         Hitachi once even sold a tape recorder with a Hall device in
> the playback head which would show output all the way down to no
> movement of tape at all to aid in editing.  I don't know how well
> it worked but a sensitive Hall device might do the job.
>
>         I once lost a really nice magnetic pickup tool in our
> yard and wished I had a linear Hall device that would detect a
> magnet which, I think got accidentally buried in a flower bed
> when it fell out of a tool box.  It may still be there for all I
> know.
>
> Martin  WB5AGZ
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