# Toroid inductance

## Main Question or Discussion Point

Hello, I need help with calculating inductance of a toroid core. I tried googling and came up with 3 results, but they don't really help. Links are:

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indtor.html
http://www.nessengr.com/techdata/toroid/toroid.html
http://www.phys.uri.edu/~gerhard/PHY204/tsl267.pdf

The information I have:

Toroid inner diameter: 8 mm
Toroid outside diameter: 15 mm
Toroid height: 6 mm
Wire radius: 0.63 mm
Number of turns: 60

I need a formula that takes into consideration inside and outside diameter (or radius), height, wire radius, number of turns and permeability of the core. None of the formulas in above links seem to do that.

## Answers and Replies

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marcusl
Gold Member
The first link above tells you how to come up with the formula. This link carries it out
http://www.phys.ufl.edu/~acosta/phy2061/lectures/Inductance.pdf" [Broken]

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The first link above tells you how to come up with the formula. This link carries it out
http://www.phys.ufl.edu/~acosta/phy2061/lectures/Inductance.pdf" [Broken]
Thanks for link, I'll give it a read tomorrow (going to bed now). After that I'll ask further questions (if there are any). :)

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vk6kro
You have some very good references already, but I have attached another one which gives some information about the uses of different types of toroid.
I found it on HamRadioIndia.org. Very well presented and hopefully accurate.
View attachment toroid_datasheet.pdf

Wire diameter doesn't have much effect on the inductance of a toroid as long as you can fit the required number of turns into the air gap.

Frequency does have a huge effect on some toroid materials. Permeability can drop to 10% of the low frequency value, or less.
So, it really needs to be measured or specified at the frequency you wish to use it at.

Also, temperature has a big effect and if stability is an issue (such as when making coils for oscillators) this should be considered.

Another useful document is this one:
which deals with broadband transformers using ferrite materials.

Try to get the makers data sheet for the actual core you are using. Usually, their data is very thorough and helpful.

It is a bit of a black art, though, and eventually you have to put some wire on a ferrite and start measuring.

Many thanks guys, I'll give them a thorough read when I get back from college.

The thing is, I'm trying to make a DC-DC 12V/24V converter and the author of circuit I'm using hasn't specified inductance, but has given above information instead. There is no talk about permeability (i.e. specific material used for toroid) so I don't even know where to start to calculate needed inductance. :/ Any ideas?

Here's a direct quote from the author:

"L1 is made by winding 60 turns of 0.63MM magnet wire on a toroidial core measuring 15MM (OD) by 8MM (ID) by 6MM (H)."

what is the magnetic field outside the torroidal coil - ie in the centre and on the outside