# Inductor Question

#### DC_Kid

Joined Feb 25, 2008
794
just wondering if anyone can point me to any freeware for basic (iron core cylinder, etc) multi-layer inductor design. i'm kinda working backwards for my inductor.

my knowns are:
H = ~5.42mH
DCR = ~14.4ohm
wire gauge = 28awg
wire length = ~222ft

now i need to figure out how many turns, coil mean radius, and coil length. i would also like it to be iron core. the final inductance does not have to be exact, but each coil will use 222ft of 28awg wire. any links to places for bobbins would also be great.

thanks

#### SgtWookie

Joined Jul 17, 2007
22,201
Here's the formula for a cylindrical multilayer inductor:
L(Henries) = (Da^2 x T^2) / (18 x Da + 40 x Cl)
Da = Average diameter (inches)
T = Number of turns
Cl = Length of coil (inches)
28 gauge wire has a nominal diameter of 0.01264"

In your case, for an air-core inductor with 222 feet of 28 gauge wire to come out at nominal 5.46mH:
Take a Bic pen, which has a barrel O.D. of 0.33".
Attach flanges to the outside of the barrel which are 3-1/16" apart (~3.063")
Put on 1,913 turns at 228 turns/layer for ~8.37 layers.
Your average diameter will be 0.4422", your average outside diameter will be 0.554"
The leads will be 3" long on each end.

Adding a ferrite core will radically increase the inductance, as well as changing the frequency range. What frequency the resulting inductor will work best at is then largely dependent upon the type of ferrite material used.

#### Ron H

Joined Apr 14, 2005
7,012
any links to places for bobbins would also be great.

thanks
I got 6 free samples from Cosmo Corporation. When asked for my company's name, I told them I was RMH Designs (which I will be, if I get a consulting job or develop a product ).

#### SgtWookie

Joined Jul 17, 2007
22,201
From the link that Ron_H supplied
Cosmo Part Number: 6272-0 seems to be close to what you need, but that would take 1775 turns to get 5.46mH, your resistance would be 13.65 Ohms, and your wire length would be 210.3".

P/N 6099-0 is another possibility. 0.752" diameter, 3.113" long.
1,041 turns @ 232 turns/layer, average dia. 0.8124", inductance ~5.14mH

#### Ron H

Joined Apr 14, 2005
7,012
From the link that Ron_H supplied
Cosmo Part Number: 6272-0 seems to be close to what you need, but that would take 1775 turns to get 5.46mH, your resistance would be 13.65 Ohms, and your wire length would be 210.3".

P/N 6099-0 is another possibility. 0.752" diameter, 3.113" long.
1,041 turns @ 232 turns/layer, average dia. 0.8124", inductance ~5.14mH
Is that with an air core? If you have an iron core, don't you need to know the permeability?

#### SgtWookie

Joined Jul 17, 2007
22,201
Is that with an air core? If you have an iron core, don't you need to know the permeability?
Yes, I mentioned ferrite cores in my 1st response, and that there are many materials that ferrite cores are made from which affects the frequency range the inductor is suitable for.

If the OP really is going to use a ferrite core, the coil will need to be much smaller in diameter and considerably longer.

But, they have not mentioned anything about desired operating frequency range, Q, permeability, etc.

The original suggestion I made about using the Bic pen barrel with flanges 3 1/16" apart will work as an air core inductor, and be extremely close to his stated requirements - along with having 3" leads on each end of the inductor.

#### DC_Kid

Joined Feb 25, 2008
794
air core should work fine. the inductor specs i mentioned are those of a solenoid (a fuel injector to be exact). i wish to simulate it without having to buy a bunch of injectors. the noid has a metal shaft through its center, hence why i mentioned iron core. air cores and iron cores of same inductance do behave differently so thats why i was looking for iron core.

i am a tad baffled about how the injector's metal body outside dimensions measures only 1" diameter x 3/4" long !! how is the maker getting ~5.42mH @ 14.4DCR jammed inside such a small space? perhaps it is a small iron core that is ~5.42mH but the coil wire has much less DCR and the maker just adds in a resistor to achieve 14.4DCR???

30awg is rated .86A(max) and 28awg 1.4A(max). i need to allow 1amp to flow. AWG charts are kinda confusing, "Current Carrying Max" and "Max Amp Chassis Wiring" (see below for site links). the higher #'s seem to be correct.

in a fuel injector the fuel flows around the coil to keep it cool. injectors run from 0-80% duty cycle and run most of the time 0-50% (if system is well designed, etc). this solenoid is all DC but will be pulsed up to 70Hz.

i am trying to find a inductor maker to make these coils for me, but if i cant find one that can make me a handful at acceptable price i will have to opt to make them. i could step down to 30awg and sink them in a oil bath (35mm film canister, etc).

one piece of crucial data i cant seem to find is (awg @ rated amps) how long does it take for wire to heat up and fail?

so i guess the final question is. should i just buy some off-the-shelf 5.42mH iron core inductors and add in a resistor to achieve 14.4DCR ???

http://www.powerstream.com/Wire_Size.htm
http://www.interfacebus.com/Copper_Wire_AWG_SIze.html

i found some 5.6mH (1.6DCR max) iron chokes at Newark. i got some 27ohm 10 watters from Digikey. i'll run two 27ohm'ers in parallel to the choke which should yield me roughly 14.6DCR and probably just a tad over 5.6mH because the 10 watters are wirewound. granted, i spent a few \$ but now i dont have to go through the trouble of winding my own coils, etc.