What are toroids? Sometimes these are incorrectly spelt torroids. These come in two types, powdered iron toroids or ferrite toroids. Both introduce permeability. Toroids look exactly like doughnuts and come in various diameters, thicknesses, permeabilities and types depending upon the frequency range of interest. Includes links to Micrometals inductor design software and Micrometals RF design software."




What are toroids?

These come in two types. Powdered Iron or Ferrites. Both introduce permeability, that means an increased inductance for a given number of turns. Toroids look exactly like doughnuts and come in various diameters, thicknesses, permeabilities and types depending upon the frequency range of interest. Note the correct spelling, not torroids

Some of the advantages of toroids are:

1. High inductance for the physical space occupied.

2. No interaction or coupling with adjacent components (unlike air wound and other inductors).

3. Various permeabilities are available.

4. Exceptional Q values when wound correctly and optimum core and windings selected.

5. Wide range of diameters and thicknesses.

6. Relatively low cost

7. Often simple to mount or secure mechanically.

Some of the disadvantages of toroids are:

1. Nearly impossible to introduce variable tuning of the inductance.

2. Subject to some thermal drift.

This image is copyright © by Ian C. Purdie VK2TIP - toroid core

Fig 1. - Toroid Core

A typically popular type is made available by Micrometals and a representative example is the T50-2. This core is lacquered red (so you know the type) and has the following main properties.

Being T50 it's outside diameter is 0.5", the ID is 0.3" and the thickness is 0.19"

The permeabilities or in this case AL factors i.e. ( inductance per 100 turns2 ) are:

TYPE            COLOR        AL                      Freq. Range

T50-26        Yel-Wh        320uH                  power freq.
T50-3          Gray           175                      50 Khz to 500 Khz
T50-1          Blue            100                      500 Khz to 5 Mhz
T50-2          Red              57                      2 Mhz to 30 Mhz
T50-6          Yellow          47                       10 to 50 Mhz
T50-10        Black            32                        30 to 100 Mhz

This is only a small sample to give you an idea. Your turns required to give a certain inductance based on the above AL  is as follows:

N = 100 * Ö[ L / AL]

Therefore to obtain an inductance of 3.685 uH using a T50-6 toroid would require 28 turns (of course I cheated again) but check it out on your calculator as I may have left in a deliberate mistake to see if you're awake.

By the way don't get too paranoid about the exact number of turns because cores do vary in value anyway and particularly with temperature changes.

I finally got around to photographing some coils and toroids.

This image is copyright © by Ian C. Purdie VK2TIP - photograph of some coils and toroids

Figure 2 - photograph of some coils and toroids

Sorry for the poor lighting. The top two toroids on the left are T68-2 types (red). Below that is the yellow T50-6 type. To the right of the yellow toroid are two RFC (radio frequency chokes) one is 1 mH and the other is 2.5 mH. Above the RFC, lying diagonally, is a 4.7 uH (nominal) variable inductor from J.W. Miller. At the extreme left of the photo is the tip of a similar variable inductor where the tuning screw is a little more clearer.

Finally to the middle right is a wound toroid I salvaged from somewhere.

Mike Czuhajewski WA8MCQ has produced a Toroid Chart

Mike Czuhajewski has this excellent chart for toroids, print it out.

See: Mike's Toroid Chart

Mounting Toroids

A considerable amount of work has been done by Roy Lewallen, W7EL on the important topic of mounting toroids.

It is quite often important in applications such as LC filters to preserve "Q" as high as possible. Roy has done quite a lot of research in this area.

In part, Roy says...

"Of the materials tested, both types of RTV stand out as having a negligible effect on inductor Q. Hot melt glue and paraffin have a small enough effect that they should be tolerable for many applications. Duco cement seriously degrades Q, even in a much thinner layer than the other coatings. The "sealing tape", tested out of curiosity, shows just how great a degradation can be caused by a poor coating........."

See: http://www.qrp.pops.net/w7el.asp

Micrometals RF Design Software

FREE Micrometals RF design software, RF-Toroid was written as a supplement to the Micrometals RF Application catalog. Much emphasis was put into calculating the location of the Self resonate frequencies for series and shunt inductors. RF-Toroid is for use DC to 300 MHz. Much research went in to the calculation and corrections from the standard formulas for low perm & phenolic cores to better predict the RF style of winding and application.

Some features include SRF for series and shunt cores, peak Q curve ranges and 200 & 360 degree coverage to achieve the best product possible for the microwave designer who needs accurate and abundant information at the finger tips to compare one core to another simultaneously. The custom core feature allows for custom shaped cores, stacking cores side by side , ferrite cores and machining custom phenolic cores from plastic, wood, ect... Micrometals hope this tool will serve you well.

Download Micrometals RF Design Software - 397Kb

Micrometals Inductor Design Software

Go to Micrometals Inductor Design Software for other inductor software.

PLEASE NOTE:The toroid part numbers mentioned herein and elsewhere on my sites are copyright © Micrometals, Inc.

Other Toroid Software

mini Ring Core Calculator - English Version

The easy way to calculate toroid cores. Mini Ringkern-Rechner V. 1.0 was first published in German Amateur Radio Magazine "FUNKAMATEUR" 11/2000.

The program is homemade and free for use by radio amateurs. Version 1.0a running on Windows 3.1 and up, but will not developed any longer. Version 1.1 is 32-bit, including English.

Download http://www.qsl.net/dl5swb/minirk_e.htm - 624Kb

Inexpensive source of Toroids in USA

My very good friend Dieter (DIZ) Gentzow - W8DIZ - of Loveland, Ohio, tells me, "as far as I know, my prices for Toroids are the best in town/county/world :)"

Diz goes on to say "the web page with the info is at... http://www.kitsandparts.com/

I'm sure Diz will look after you kindly.


1. audio transformers

2. chokes

3. coil formers and cores

4. inductive reactance

5. mobius winding techniques

6. toroid cores

7. wide band rf transformers

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Updated 6th April, 2003