Multilayer Inductance

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Multilayer Inductance
From: Tesla List <tesla-at-stic-dot-net>
Date: Wed, 10 Sep 1997 19:13:17 -0500
Approved: tesla-at-stic-dot-net

From: 	Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: 	Wednesday, September 10, 1997 2:24 PM
To: 	Tesla List
Subject: 	Multilayer Inductance

> 
> From: 	Mark S Graalman[SMTP:wb8jkr-at-juno-dot-com]
> Sent: 	Wednesday, September 10, 1997 7:28 AM
> To: 	tesla-at-pupman-dot-com
> Subject: 	Re: New Coiler
> 
> 
> 
>           Hi Patrick,
> 
>    Here's my 2 cents worth:
> 
> On Tue, 9 Sep 1997 22:22:57 -0500 Tesla List <tesla-at-pupman-dot-com>
writes:
> >
> >From: 	Patrick J. Gustafson[SMTP:gustafpj-at-uwec.edu]
> >Sent: 	Tuesday, September 09, 1997 4:12 AM
> >To: 	Tesla List
> >Subject: 	New Coiler
> >
> >Hello all, 
> >
> >I have been patiently reading the list for quite some time now,
but I
> >could not get my questions to post on the list.   Thanks to Chip,
this
> >problem has been finally resolved. Here are the concerns that I
have 
> >as
> >of yet:
> >
> >
> >1.   I have a two layer secondary coil with 28 gauge wire close
wound
> >for 21.6" (both layers).   The first layer is at 6" in dia. and
the
> >second layer is at 6.125" in dia.   The calculated resonant freq's
are
> >108.71 kHz and 106.45 kHz respectively, and the calculated
inductances
> >are 91.84 mH and 95.49 mH also respectively.   Now since I layered
in
> >parallel, can I add together the inductance values directly? (
i.e. 
> >the
> >new value would be 187.33 mH)   
> 
>     No, when inductances are placed in parallel the total value
goes down
> and will be less than
> the least value of inductance, in this case the total inductance
would be
> about 46.8 mH
>   
> 
snip
>                            Mark Graalman
> 

Mark,
If the two coils were parallel in the usual fashion, meaning placed
side by side with their ends connected, then what you wrote would
indeed be correct. However, when a coil is wound ON TOP of an
existing coil, the inductances are NOT in parallel, but rather
co-axial. In this arrangement the effective wire SIZE is increased,
but the inductance remains the same. Think about it carefully and you
will see that such an arrangement is really the same as winding the
coil with wire that is larger, or like Litz wire, if you will. In
this arrangement the inductance is a function of number of turns and 
diameter and length. Several members on this list have built
multi-layer secondaries and verified that the inductance is
essentially the same as a single layer version. What you gain is
current carrying capability based on increased SURFACE AREA (not
cross sectional area) at high frequencies.

The fact that the inductance is not changed is not immediately
self-evident. At least, it wasn't to *ME* the first time I came
across it. Even though I am an electronics instructor and am normally
very careful in how I look at things, I initially thought that such
an arrangement would yield a lower inductance. I was wrong. When my
electronics measuring instruments showed me that the inductance
remained the same, it was only THEN that I looked at things carefully
and saw what was really going on. It is exactly like winding a coil
with Litz wire.

Someone on the List is using this method for their Magnifier's
secondary coil, and they also noted that the inductance is the same
as for a single layer coil. One of the Berts, I believe. (Bert Poole
and Wild Bill Emery?)

Hope this helps.
Fr. Tom McGahee