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Re: Flat Coils



Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Jim, John,

First, I must clear up a mistake. In the first part of my reply I said "The
inductance is
increasing because...", I should have said "The inductance is decreasing
because...". I was
talking about the helix case (sorry about that). When I went to bed last
night, I knew I didn't
say that correctly.

Yes, neither case may be proper comparisons for the two extremes.  I knew
while replying to the
post that there would be a few post referencing the difference in radius,
as well as turn to
turn inductance incrementing out. There may not be a real means of comparison.

If one takes a flat coil, winds it into a helix at a radius less than 2 x
the flat coils inner
radius, the turns will be increased to fulfill the same length of wire and
the inductance will
be less. If turns are decreased and radius increased, the inductance will
be the same when the
turns are the same. This will occur when the radius reaches 2 x the inner
radius of the flat
coil and is the maximum inductance achievable in the helix (keeping all
else the same). Any
radius above or below this value will decrease inductance in the helix. But
I agree, this may
still not be a true comparison.

Take care,
Bart

Tesla list wrote:

> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jimlux-at-earthlink-dot-net>
>
> Tesla list wrote:
> >
> > Original poster: "Barton B. Anderson by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>
> >
> > Hi John T.-
> >
> > The inductance is increasing because you are dramatically increasing the
> number
> > of turns in the helical compared to the flat coil to keep the wire
length the
> > same. Inductance is a function of the turns.
> >
> > A helical or flat coil with the same length of wire must come out to
the same
> > inductance as long as the length, wire size, spacing between turns, and
> number
> > of turns remains constant (if your going to do a comparison of inductance).
> > When accomodating these constants, you will need to wind a helical coil
at a
> > diameter twice the inner diameter of the flat coil. You should find that
> R and
> > L remain the same for the same length of wire.
>
> I don't know that this is true.  The interwinding mutual inductance has a
> large effect on the overall inductance, and the mutual inductance will be
> very different in a flat spiral compared with a helix.  I, off hand, can't
> think how you could keep the interturn distances the same for helix vs
> flat.  The coupling between turns is why L varies as Nturns^2 (roughly) for
> a closewound coil.  If the distance between turns is different, then the
> coupling is different, etc.
>
> Now, it is possible that for some restricted range of sizes, number of
> turns, etc, the two might generally work out the same.  It is such
> restrictions that lets us use approximations like Medhurst's and Wheeler's,
> and the fact that TC's in general, are 5-10% tolerance kinds of devices.
>
> Circular 74 does give (exceedingly long and complex) equations for all
> these cases, and, as expected, they all are strongly dependent on N^2 plus
> some "geometry factors".
>
> Wheeler's equations are:
> Single layer solenoid
> L (uH) = r^2 * n^2 / (9 * r + 10 * l)
>
> where
>
> r = coil radius in inches
> l = coil length in inches
> n = number of turns
>
> Flat "pancake" coil
> L (uH) = r^2 * n^2 / (8 * r + 11 * w)
>
> where
>
> r = radius to center of windings in inches
> w = width of windings (in inches)
> n = number of turns
>
> >
> > I like your method of winding the flat coil. That's the route I would go.
> >
> > Take care,
> > Bart
> >
> >