Discussion in 'Electronic Design' started by George Herold, Oct 31, 2012.

1. ### George HeroldGuest

Yeah no torroids.... unless you're making a flux gate thing.
So you're saying there will be very little voltage gain, with a
ferrite plus coil, vs just the coil? (I must admit I find magnetic
materials a confusing lot.) That's OK I hopefully won't make the same
mistake twice.

George H.

2. ### Tim WilliamsGuest

Oh, there's gain, just not mu times -- should be roughly the ratio of
lengths, core to coil. So a 6" loopstick in an AM radio works a lot
better than the teeny winding on it, etc.

Tim

3. ### George HeroldGuest

Hmm OK thanks, If I want to shake it around it's not worth the extra
mass... which I guess leads back to James's original idea.

George H.

4. ### josephkkGuest

The cheap answer is like a Pringles.

http://en.wikipedia.org/wiki/Paraboloid

5. ### George HeroldGuest

'scratch scratch'... You're riffin' on James's idea? Isn't this like
the fluxgate that Jan made (and posted here) a while back? (I've half
forgotten how that works.)

George H.

6. ### George HeroldGuest

Hmm, maybe we are talking about different geometries.
Googling "toroid fluxgate" I get this,

www.geotech1.com/pages/mag/projects/fmx1/fmx1.pdf

The first figure is what I was picturing. I remember going over this
in my mind when Jan posted it, and I thought I got it.... but I'm
going to have to draw pictures again.
(The pickup coil sees a signal at twice the drive frequency, IIRC)
Say Jan, if you're lurking do you have a good reference for the
toroidal fluxgate?

George H.

7. ### Tim WilliamsGuest

Hmm, nonuniformity in the toroid is on the order of N turns; you'd need
over 1000 turns, in a single even layer, to get to that level I'd think.

Hmm, nah, it wouldn't quite be that bad, nor that easily defined. Each
turn acts like a dipole at a slightly different angle. After ten turns,
you have a dodecapole in a circle. A suitable distance from that and your
far field drops off ridiculously fast (that is, ignoring the solenoidal
component of a standard progressive winding), isn't it ~1/R^N for an
N-pole?

So it comes down to the compromise between relative size (how much space
you can get between the toroid and solenoid), near field nonuniformity
(number of turns, and their distribution), and the amount of with-core vs.
without-core gain you experience from using the toroid as a bar magnet.

I'm carrying out a thought experiment which seems to suggest there's no
free lunch here, i.e., it reduces to a linear/folded form of the "no free
lunch" once-you-saturate-it-the-control-winding-couples-in hypothesis.
It's just an hypothesis, but there may be a theorem in there.

Tim

8. ### Jasen BettsGuest

This is soething I've just thought up.

someone said a rotating coil coupled to the leads by a coaxial
transfromer (like found under VHS drums)

someone else made a remark (that I may have misread) that started me
thinking that you could stir the magnetic field and make it pass over
turns of a stationary coil by combining the two rotating coils into
a single stirrer

given that the inner radius of the coils is 1 unit

For the field baffle with the axle on the Z axis though the origin
the surface shhould approximate the plane z=y (where xx+yy < 1)

on further thought most of the current will flow in the edges of the
baffle, so, a simple inclined elliptical ring would probably work as well
as he disc does.
try "z=xy"