# inductance question

Discussion in 'Electronic Design' started by Yzordderrex, Jun 14, 2007.

1. ### YzordderrexGuest

I have 50' of wire and I want to build an air core inductor and
maximize the inductance. Is 1 big turn going to give me the most
inductance?

regards,
Bob
N9NEO

Just say NEO!

2. ### Fred BartoliGuest

Yzordderrex a écrit :
NEO! It isn't just one big turn

What you're after is called a Brook's coil.
The very nearly optimal coil has a square section (a x a), internal
diameter 2a (and consequently external diameter 4a ).
You'll have to solve this for your wire length and diameter.

Inductance value is L = 2.55E-9.a.N^2 (in H) with a given in mm.

3. ### Harry DellamanoGuest

"Fred Bartoli"
Sweet!

4. ### Tim WescottGuest

Expect lots of interwinding capacitance.

A one-turn coil, by the way, makes a nice antenna when it gets to be one
wavelength in circumference -- think "loop antenna".

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

5. ### D from BCGuest

Square section?? Is that why those wallwart transformers have square
bobbins?

D from BC

6. ### kellGuest

square in cross section. wound round.

7. ### John PopelishGuest

He'll get more inductance by tightly bundling those turns
than by winding them is a single layer solenoid of the same
diameter. The bundled form provides tighter coupling
between the turns, that most closely approaches the turns
squared effect on inductance.

The inter winding capacitance will be higher, also. I don't
know exactly what the optimum relationship between bundle
diameter and average turn diameter for optimum inductance
per wire length, but I am guessing that the bundle cross
section diameter needs to be quite a bit less than the
average turn diameter.

8. ### The PhantomGuest

Just one of many web pages on the Brooks coil:

http://home.san.rr.com/nessengr/techdata/brooks/brooks.html

Making the cross section circular or hexagonal rather than square increases
the inductance by about 1%; hardly worth doing.

9. ### LVMarcGuest

If you want to get the highest inductance and the highest "q" and at the
highest possible frequecny:

then you want a shape that incloses the maxium volume, with a minimium
shaped wire envelop I geomtery tis wold yield a sphere. not so goo for
coils.

the next shapoe woulb be apporx 2:1 right hand clynder shape,with
several turns, and the angle of wires about 42 degrees....

Marc

10. ### John PopelishGuest

1% more is more. By the way, do you have a reference that
confirms this magnitude of improvement for a round copper
cross section? I would have guessed a couple more percent,
because it reduces the number of longest turns (that use the
most of the total wire length per turn) and the shortest
turns (that enclose the least area) and increases the number
of turns near the average length, Reducing the shortest flux
line that encircles all the turns, as well as reducing the
distance between each turn and its average neighbor,
improving their flux linkages. I would be a bit surprised
if all those slight but compounding factors added up to only

11. ### D from BCGuest

Right hand cylinder?

Is it like this

| ----/////////-
1 ---/////////-- 42deg angled windings
| --/////////---
<-----2---->
D from BC

12. ### The PhantomGuest

Courtesy of Google, I see that Brooks inductors were discussed on SED back
on August 25, 2004, where you asked Winfield about circular cross sections.

Winfield mentioned a "definitive" study of Brooks inductors, but apparently
nobody has ever posted it.