PSN-L Email List Message

Subject: Re: Mu-metal/Permalloy to build your own homemade fluxgate
From: "Charles R. Patton" charles.r.patton@........
Date: Fri, 16 Jun 2000 23:18:06 -0700

Some answers and thoughts:

Bent anti-theft tags -- Generally the one=92s on CD=92s (and the ones mos=
plentiful) are the least likely to have been bent because they=92re
generally on the cellophane wrapping which is ripped off in haste after
they get out of the store.  In any case, use an Exacto knife to slit the
package as if you were filleting fish.  The two metal pieces and the
plastic interlayer should come out easily.  Look at the bright, glassy
looking piece, the Metglas.  It=92s either OK or it has a permanent kink
in it.  Bending, per se, does not hurt it until it kinks.  It acts
similar to highly tempered spring steel.  It generally will not scratch
with the Exacto knife until you crease it. Toss the remains.  I do not
suggest trying to cut it at all.  This is extremely difficult to do
right and has a high potential to instigate small cracks from the edge
being cut.  Use it as is.

As far as coil suggestions, it is kind of cut-n-try.  Hopefully you have
an oscilloscope and an audio generator, preferably square wave with
microsecond rise and fall times.  Find a coil form to use such as a
plastic drinking straw from McDonalds or some such where the strip will
slip inside snuggly.  If the straw is a bit small, after it is wound you
can squish it in a vise with smooth jaws and make an oval that the strip
will slide into.  I then take a wood dowel that fits snuggly inside the
straw and chuck the combination into a variable speed electric drill and
wind a single layer of #40 just as long as the Metglas strip.   To lower
the frequency it runs at, just wind more coil to your hearts content
until the diameter starts to approach the length, at which point the law
of diminishing returns takes over.   Slip out the wood dowel and slip in
the Metglas.  Drive the coil in series with 50 to 100 ohms to ground.
Monitor across the resistor with the scope.  When the frequency is high
enough and the voltage low enough, you will just see a triangular
waveform.  As you lower the frequency and/or raise the voltage, at some
point the ramp starts to develop peaks =96 this is saturation of the
ribbon.    If you rotate the coil/ribbon combination in the earth=92s
field, the peak should diminish in one polarity and increase in the
other.  Measuring across the resistor with a DC meter should show a DC
change as you rotate the combination.  You have just made a sensing
circuit of the incremental inductance style used in the Precision
Navigation modules.  To make a flux gate version out of it, lower the
frequency and/or increase the drive voltage so that the peaks occur all
the time strongly.  Wind another coil whose diameter is equal to the
length of the coil/ribbon combination.  Insert the ribbon/coil across
the diameter of this new coil and monitor the output of the big coil.
As you move everything, you should see small spike in the big coil when
it=92s axis is North/South.  However the effect will be small since the
ribbon=92s small dimension in the N/S direction works against it use as a=

flux gate in this fashion.  The frequency of the spikes should be twice
the drive frequency if it working properly and you=92re not just picking
up the drive waveform through mutual inductance.

As a reference point, a 10 V P-P square drives a library book permalloy
strip inside a coffee stirrer straw single layer wound with #40 to
saturation in about 15 to 20 microseconds.

Charles R. Patton


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Larry Cochrane <cochrane@..............>