# coil basics

Posted: 4/27/2015 8:11:07 PM

From: Moncton, New Brunswick, Canada

Joined: 4/24/2015

Hi there,

I searched the forums and read a few threads about coils, but they were all pretty specific to specific builds.
I'm looking for general info that can be used for later calculating the specifics (how to calculate wire gauge, diameter, and number of turns based on how many uH I want).

Right now, I'm looking to build a 10uH coil based on the simple schematic from this video: https://www.youtube.com/watch?v=nsxaoH9DjDo
according to the comments under the video, he did 100 turns of 29 gauge magnet wire, but there is no mention of the diameter.

I'm assuming there must be a formula to calculate these variables (wire gauge, coil diamter, number of turns).
In some of the forum threads, I saw people referring to software, used to calculate this.

Could anyone offer tips on how to calculate the variables for building a coil based on the uH?

I'm also wondering if 29 gauge magnet wire is a good place to start.  I live in a small city that doesn't have an electronics store.  No one in town has magnet wire, and so it appears I have to order online in bulk... :-/
Before ordering a lot of it, I'd like to make sure I order a useful gauge.

Thanks so much!

amanda

Posted: 4/27/2015 9:15:00 PM

From: Moncton, New Brunswick, Canada

Joined: 4/24/2015

correction... i'm working on a 100uH coil... not 10uH.
but would still like to be able to freely calculate the variables of gauge, diameter, and turns based on uH...
in case one day i want to make other coils...(which i probably will)...
(searching online for formulae)....

Posted: 4/28/2015 2:57:42 PM

From: Northern NJ, USA

Joined: 2/17/2012

Hello Amanda,

Here is a spreadsheet that I use for solenoid design:

Change any of the numbers in the colored boxes, the rest is calculated for you.  There are a couple of papers mentioned in it that make for interesting reading.  For Theremin EQ coil use (not the type of design you are looking at) you usually want to minimize the self capacitance, which means tall thin coils rather than short squat coils.

I watched the video you pointed to, from the schematic it looks like there is a ton of capacitive padding / coupling in the mixer section.  I would recommend you buffer the oscillator outputs before mixing as this will give you a larger and smoother pitch field.  Also, you don't have to use tapped inductors to make the oscillators, you can "tap" the capacitor instead.  I've found FETs to make the simplest and most reliable oscillators, though I've developed a couple of bipolar variants that work almost as well.  There are many oscillator topologies, you want one that gives you a decent voltage swing at the antenna so as to overcome transistor and environmental noise.

I haven't added to it lately, but here is my analog Theremin design thread:

http://www.thereminworld.com/Forums/T/30562/lets-design-and-build-a-simple-analog-theremin?Page=0

Posted: 4/28/2015 4:32:03 PM

From: Moncton, New Brunswick, Canada

Joined: 4/24/2015

oooo beautiful!

thanks so much!  looking into both the spreadsheeet and your theremin design thread!

(insert squeal of glee here! so much good stuff there!)

i'll be ordering my magnet wire today, and i'll start building in a few days.
i'll post photos of finished circuits and discoveries.

:-)

Posted: 4/28/2015 4:44:17 PM

From: Theremin Motherland

Joined: 11/13/2005

Here is a free calculation program "Coil32". The current version for Windows is 9.7 (13MB), for Android is v2.0 (848 KB):

Unfortunately, the download links redirect to one of the following hosting sites:

Fortunately, the application is multilanguage.

If you will have problems, I can email you the install zip.

Posted: 4/28/2015 5:05:16 PM

From: Theremin Motherland

Joined: 11/13/2005

English homepage of Coil32: http://coil32.net/

Posted: 4/28/2015 7:53:39 PM

From: Northern NJ, USA

Joined: 2/17/2012

I've bought a fair amount of stuff (components, cases, etc.) for Theremin building and I have to say 90% of it ends up never seeing actual use for one reason or another.  So my advice is to order small quantities of whatever even if the deal doesn't seem as good.

Single layer air core coils are the way to go IMO as they have high Q and minimal thermal dependence.

For coil wire, you usually want to go as fine as is manageable so as to reduce physical size of the final coil.  I've found 34 AWG to be quite cooperative on my manual winder and it doesn't break too easily.

If you get into Theremin designing / building at all I'd recommend getting an LC meter that can read tiny capacitances and small to middling inductances.  The LC200A can be found on eBay for ~\$30 and it works well.  You can even (roughly) measure things like antenna static capacitance with it.

Posted: 4/28/2015 9:26:13 PM

From: Moncton, New Brunswick, Canada

Joined: 4/24/2015

Thanks so much!

Ilya, that coil32 looks amazing... do you think I will be able to use it on a mac?
I'm working on a mac OS6.8... and I don't read Russian, so I didn't dare download it, without being certain.

Dewster, when I open the spread sheet in excel (mac version from 2004), I can't seem to change the cells... it treats the file like there are 3 images inserted into a spreadsheet...I can read everything, text, formulas etc... I just can't modify it... is there a different program I should be using to read it?

As for wire, I was about to order either 28 gauge or 30 gauge... the smallest spool is 1000 feet... so, \$100 Canadian for 1000 feet of 28 gauge magnet wire... if it's the right kind of wire, hopefully i will be making more coils later too, and it will get used.  Thanks for the tip though, I'll go for 34 instead!

Once I get the magnet wire, the only component I can't seem to find in my town is the 500pf variable capacitor... I could either try to find an old radio and extract the tuning dial... or... I am a bit intrigued by this do it yourself variable capacitor (made from tinfoil and old CDs!!!)... but I'm not really sure how reliable it would be!  What do you think of this fella'?  http://www.instructables.com/id/10-Minute-Variable-Capacitor/?ALLSTEPS

Posted: 4/28/2015 10:29:54 PM

From: Northern NJ, USA

Joined: 2/17/2012

"when I open the spread sheet in excel (mac version from 2004), I can't seem to change the cells... it treats the file like there are 3 images inserted into a spreadsheet...I can read everything, text, formulas etc... I just can't modify it... is there a different program I should be using to read it?"  - xoadc

I created it in Excel 2003 for Windows.  There are three worksheets, the first is a readme with images, the second has the calculations, the third some simple parasitic calcs.  Do you see the three worksheet tabs at the bottom?  If not I can pull out the relevant sheet.

"\$100 Canadian for 1000 feet of 28 gauge magnet wire."

That's a LOT of wire!  Why not use an eBay merchant and buy smaller spools?  Try Tech Fixx:

http://www.ebay.com/usr/tech-fixx?_trksid=p2047675.l2559

They sell single green coat solderable 34 AWG, 990 feet for \$7 or thereabouts.  Single coat makes the windings closer together, solderable means you don't have to strip the wire in order to solder to it, your iron should melt through the enamel coating when you tin it.

I strongly urge you to do your coil designing before you buy any wire.

Posted: 4/29/2015 2:13:11 PM

From: Moncton, New Brunswick, Canada

Joined: 4/24/2015

Aha!  the worksheets!  Beautiful!  How did I miss those... there they are and they work like a dream!

And thanks for the ebay link for the wire!  Hot damn!  Much better.  I'm on it!

Now to find that 500pF variable capacitor... I don't think I'm going to try out that home made variable capacitor thing... looks a bit sketchy.  http://www.instructables.com/id/10-Minute-Variable-Capacitor/?ALLSTEPS

(I just posted it again, because I just figured out how to make the link actually link... ;-)

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