Making a Simple Air Core Inductor (Induction Coil)

This instructable will tell you how to make a simple small air core inductor, specifically for Niftymitter. Niftymitter is an open source FM transmitter based on Tetsuo Kogawa's Simplest FM Transmitter,  a free running oscillator circuit, hence the need for a coil.

Step 1 What you need

A small reel of enamelled copper wire.
A drill bit of the desired internal diameter of the coil.
Soldering kit and solder.
Wire snips and needle nosed pliers.

For Niftymitter, use 0.75mm (or 22SWG) diameter wire, such as this from Rapid and a 5mm drill bit.

SWG/metric conversions can be found on this page.

Comments

[JakeTobak]

I don't know if it would be more or less work, but you could probably also sand paper the enamel off.

[royshearer]

I had a shot at burning off the enamel and then sanding clean, but still had to work the coil with the soldering iron to get it to tin. Any tips?

[steveastrouk]

Scrape it with a knife blade is more effective than sandpaper.

[t.rohner]

At 100 MHz the current flows only in the outer 10 micrometers of your wire. This is due to the skin effect. So normally i would take a silver plated copper wire for a air coil of this dimension. The windings have to be apart anyway, so i makes no sense to take enamelled wire. By using silver plated wire, you have a lower resistance in the skin region and you don't have the hassle with tinning.

[techno guy]

How can I make a 1 milihenry air core induction coil? It's for an EMF detector that I might make into an instructable.

[burdockwing]

what does an induction coil do?

[royshearer]

That is a good question. I didn't know, but typed your question into google and read this forum page and now have a slightly better idea!
http://www.physicsforums.com/archive/index.php/t-101306.html

I think inductors are kind of like flywheels, in that, they resist being activated, and resist being deactivated. So when current is applied, they take some energy and put it into their magnetic field, slowing the increase of the current. And when the current to them is decreased, the magnetic field acts to keep current going, slowing that decrease. Hence their use in AC circuits almost as dampers.

Capacitors behave in a similar way, but resist changes in voltage rather than current.

I've also realised that what I've described making hear is not technically an induction coil, but an 'air core inductor'.

[Blofish]

http://www.66pacific.com/calculators/coil_calc.aspx

inductance calculator

[agis68]

 how do you know the appropriate length and spiral step of any of the coils you ve made??? Do you have any LC meter? be explained 

[royshearer]

I am simply using the specification from Kogawa's transmitter page here. He does not give an inductance for the coil, I would love to know how to measure it, or a refresher on how one calculates the spec for a coil for a given inductance.

[revelae]

1.598E-002 microhenries, is what this site gives.

[royshearer]

cheers!

In practice, I find I need to separate the windings of the coil to get the frequency of the transmitter up, so, assuming the coil is 0.8 cm long, I get an inductance of 3.841E-002 microhenries

[agis68]

electronics-diy.com/lc_meter.php try this you can find it also assembled 

[hivoltage]

 I like to use my mini torch to burn off the enamel, followed by light sandpaper to get rid of the soot. Makes for a nice clean copper surface that solder tins very easily, especially if you use a little flux.

[royshearer]

I have tried sanding/ filing but its awfully fiddly and one has to spend as much time tinning anyway. Shall try the lighter method though, cheers. This is a collaborative instructable so just message me if you would like to augment / correct anything in it.

[mothflavour2]

I find burning it off with a lighter is the best way.  This works really well on headphone wire.  It also gets rid of the reinforcing fibres.