The Effect of Iron on Inductance Value

Hi guys,

I was playing around with inductors the other day and decided to share with you an example of how much an iron core can affect the value of inductance in a coil.

Inductors air core and iron core

The red shrink-wrapped coil you see here is an air-cored inductor from a Sharp speaker. It measured in at about 0.24mH which is close to the printed value of 0.22mH. It had a paper insert to keep the coil in the correct shape. The DCR (DC Ohms Resistance, or pure resistance value) is 2.3 Ohms! This is a pretty large value for a speaker crossover inductor, usually these inductors are less than 0.5 Ohms DCR.

Now the slab of metal on the left is actually made up of iron laminations from an old “plug pack” transformer. These laminations help increase the amount of power that can go through the inductor before the iron becomes “saturated” and the inductor begins to lose its effect on the signal.

When this iron core is placed inside the air coiled inductor, the inductance value rises from 0.24mH to 2.50mH! This is 10x the amount of inductance and is a cheap way to get larger inductance values with less wire and lower DCR (moot point in this case since the DCR is already 2.3 Ohms) The new iron cored inductor is on the far right in the image above.

Furthermore, when the iron cored inductor was placed on the back of a speaker magnet, the inductance rose even further to 3.05mH! This was far too much for the intended application, so I reduced the number of turns (removed some wire). This way I was able to keep 2.5mH while glueing the inductor to the back of the speaker.

Here is a closer view of the laminated iron strips glued together:

The Iron Core

Note that you should try to keep the strips insulated from each other otherwise you will end up with a fancy piece of “solid” iron.

More on Inductors: take with a grain of salt since I am not an expert on the precise details of inductor behaviour – feel free to mention any mistakes by leaving a comment! 

• Air cored inductors are generally considered to be “better” than iron cored inductors for audio applications, i.e. speaker crossovers. The main reason for this is because iron cored inductors will eventually reach the saturation point if enough power is applied, however, usually this value is usually in the 100’s of Watts, so you won’t notice it unless you have a powerful amplifier. The cored inductors are cheaper than, and have a lower DCR than the air cored equivalent due to less wire being needed. They are also smaller, especially when it comes to large values like 2mH.

• Iron or any ferromagnetic material can help increase an inductor’s inductance value. Best results are achieved by placing the material inside the coil, however, the inductance will also change if there is metal close to the inductor, eg. screws or speaker magnets! You need to be wary of this when measuring and installing your inductor, regardless of core material.

• Saturation of the core material means that the inductor will “lose” its inductance value for power signals higher than the saturation value. If I played 300W of music through the iron cored inductor above, it would behave more like the air cored inductor for signals larger than say, 200W. This creates yukky distortion in what you hear coming out of the speaker. An easy way to hear this for yourself is to wrap 20 turns of magnet wire around a ferrite ring/donut/toroid and attach it in series with a speaker. You will get quite a large inductance value, eg. 2mH from a very small amount of turns. Even with just 2W of power the toroid core will saturate and you will be able to hear the higher frequencies leaking through, but only on the louder parts of the music, so you will probably not enjoy the effect unless you’re trying to make some kind of distortion pedal for an electric guitar.

• You can increase the power capabilities of cored inductors through various methods including using laminated iron strips (cheap) or by using more exotic materials or core shapes. A slug of iron, eg. bolt, will not give great power handling capabilities due to build up of eddy currents. The iron laminations reduce the build up of eddy currents since they are restricted to the thin piece of iron, but the magnetic field generated by current flowing through the coil is not affected significantly by this. A poorly designed toroid will saturate very easily, however a well designed toroid is usually a preferred item for transformers (and perhaps speaker crossover inductors for audio “purists”).

You can easily find well documented info on inductors by searching Google, etc. Here are some sites to look at:

All About Circuits – Inductors

Wikipedia – Inductors

Powerelectronics.com – Inductors

All About Circuit – Transformers (info on eddy currents)

 

The main point of this article was to highlight the effect that a magnetic core can have on an air cored inductor. You could delve into the theory behind inductors, or you can do many fun projects that involve wrapping wires around things, to make electromagnets, Tesla coils, induction heater, generators, and of course speaker crossover or circuit board inductors 🙂 You don’t necessarily have to know all the theory to play with these thing right? Just remember to do your research on how to keep it safe and make sure it works properly 😉

Happy coil winding,

Rennie ♪

 

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