Making a Simple Air Core Inductor (Induction Coil)

Picture of 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.
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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.

Step 2: Turning the wire

Picture of Turning the wire

Using the drill bit as a template, wrap the wire, counting up to the number of desired turns. Allow 3cm of wire clear before starting the turns and keep the wire taught while turning. Kogawa has a video for this step on his site here [.wmv].

For Niftymitter, make 4 complete turns, as close together as possible.

When complete, snip off from the reel at 3cm distance from the last turn.

Step 3: Forming the legs

Picture of Forming the legs
Use some needle nosed pliers to grip the coil on the bit as shown. Bend the legs as shown so that they are parallel. Remove from the drill bit.

Step 4: Tinning the legs

Picture of Tinning the legs

The legs of the coil require tinning to remove the enamel and prime the surface for soldering onto a board.

Use some needle nosed pliers to grip the coil while tinning - it can get very hot. Heat one leg with the soldering iron for a few seconds.

Introduce some solder to the heated leg and continue applying the iron, moving back and forth on the leg. The enamel will begin to separate from the copper and bobble.

Continue adding solder until the leg is silvered all over. You will have to turn the coil over as you do this to ensure coverage. Coax any surplus solder and enamel to the end of the leg.

Repeat the procedure for the other leg. When complete, snip of the ends of the legs, with the surplus solder attached, leaving at least 1cm of straight leg before the turns.

hello, what type of wire can you suggest in making 200uH inductor for AM transmitter?


Arghus10 months ago

i was thinking of induction coil to heat the soldering iron point, but i am afraid it would short, is all i need a 220v diode or 2 perhaps?

gussmith1 year ago

I'm just starting in electronics learning as a hobby.

When you have made an air core inductor like this with a certain value in millihenries. Can you then replace this with the same value but with a fixed value inductor (the ones that have the shape of a resistor)?

If not, what is the difference between those two inductors?

royshearer (author)  gussmith1 year ago

Sorry to say I'm not familiar with the types of inductor you are describing (axial lead inductors?). I suspect there is a reason for choosing one over the other, as the transmitter circuit I have worked with did specify an air core conductor.

techno guy3 years ago
How can I make a 1 milihenry air core induction coil? It's for an EMF detector that I might make into an instructable.
check this out
infanati1 year ago
Hi, im working on a project at the moment and im wondering how would you go about wiring 2 coils together. One being the inductor and the other the collector. so that the electricity one flows one way. Into the collector and straight out the inductor.
royshearer (author)  infanati1 year ago
I'm afraid I can't help you there, not sure about that!
mrmerino2 years ago
How much inductance do these little guys have?
royshearer (author)  mrmerino2 years ago
we're not sure - see comments below!
JakeTobak5 years ago
I don't know if it would be more or less work, but you could probably also sand paper the enamel off.
royshearer (author)  JakeTobak5 years ago
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?
Scrape it with a knife blade is more effective than sandpaper.
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.
burdockwing3 years ago
what does an induction coil do?
royshearer (author)  burdockwing3 years ago
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!

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'.
Blofish5 years ago

inductance calculator

agis685 years ago
 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 (author)  agis685 years ago
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.
1.598E-002 microhenries, is what this site gives.
royshearer (author)  revelae5 years ago

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 try this you can find it also assembled 
hivoltage5 years ago
 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 (author) 5 years ago

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.

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.