An inductor is a two-terminal passive electrical component, comprised usually of windings of copper wire, and which exhibits the behavior of resisting changes in current flow through the inductor, i.e. changing inductor current. The strength of an inductor coil in opposing changes in current is termed inductance (L) and expressed in the unit of Henry's (H). The AC inductor voltage measured across the terminals of an inductor is equal to the inductance (L) times the rate of change of the current passing through the inductor (v=L(di/dt)), with the polarity opposing the change in the current.

The higher the frequency, the faster current changes. Thus, another way of expressing an inductor's opposition to current change is in terms of it's reactance (X) in ohms, which is proportional to both the frequency and the inductance, thus (X=2[pi]fL). Reactance is added vectorally to resistance to get impedance (Z), giving v=iZ where all three variables are vector quantities. When choosing inductors for your business, you should:

1. Buy a ferromagnetic core choke inductor for power or audio applications.

2. Buy ferrite core inductors for high frequency applications.

3. Buy ferrite core devices for very high frequency inductor circuits.

Action Steps
The best contacts and resources to help you get it done

Find inductors made for power supply use or for audio use

Because reactance is very low at low frequencies, the inductor core for the power inductor and for audio inductors/transformers is made of a magnetic material like iron, which greatly increases the inductance.

I recommend: Lenco Electronics manufactures inductors, transformers and chokes to your industry’s specifications. Teco Corporation builds inductors and chokes to handle currents from 1 amp to 200 amps.

Locate distributors of ferrite core inductors

Middle to high frequency inductor circuits often use ferrite cores, which boosts the inductance, though not as much as ferromagnetic core materials.

I recommend: Communication Coil designs and manufactures baluns, inductors, chokes and coils, including ones with a ferrite core. It serves industrial and commercial clients in an array of areas including military, avionics, medical and telecommunications. National Magnetics Group allows you to select from a table of ferrite cores when ordering inductors you need made for your industry.

Acquire air core inductors for a VHF inductor circuit

In very high frequency applications, the inductive reactance of an inductor is already high due to the frequency, thus a magnetic core isn't not necessary and could in fact lead to the inductor burning up due to the fast changing current. Therefore, engineers use air inductors for very high frequency applications.

I recommend: Tranex manufactures air core and foil wound coils to your specifications. Perfect for many industries, such as medical, food preparation, telecommunications and aerospace. Regional Mfg. Specialists manufactures air core inductors for industrial, commercial and military applications and are willing to work with short runs.

Tips & Tactics

Helpful advice for making the most of this Guide

• •  Recently developed inductors called shaped-foil inductors reduce losses by approximately 50%. The design also works with integrated gate bipolar technology, used to control high power electronic devices designed to operate at current levels at 100 amps or higher and at frequencies above 10 KHz.

Inductors store energy in coil-generated magnetic fields. A capacitor performs a similar task in that it stores energy, but it stores electrostatic energy between two plates. Inductors and capacitors often combine to either amplify or dampen signal frequencies.

Inductors and capacitors have many uses, from reducing the hum of DC outputs, to preventing radio waves from interfering with sensitive electronic devices. Alone, a power inductor controls voltage levels, and the tiny chip inductor has a role on many electronics boards. Two inductors can also combine as elements of transformers, which step voltage or current up and down and also affect frequencies.

Besides the radius of the coil, the two factors that most affect inductor properties are the composition of the inductor core that the coil winds around and the shape of the inductor. Here are basic tips on these inductor properties:

1. The center of an air core inductor does not increase inductive properties. Neither do glass, plastic or wood cores.

2. Ferrous cores increase the inductance of an inductor coil.

3. A toroidal inductor looks like a donut and has greater inductance than a solenoid (cylindrical) inductor.

Action Steps
The best contacts and resources to help you get it done

Determine the electrical specifications you need in inductors and buy the right parts

Inductors have many variables. You should mainly concern yourself with inductor current, as well as inductance range and tolerance. Maximum DC resistance is also a factor.

I recommend: Vishay does a good job of publishing their inductor properties. On their website, you can look up inductors by application, inductance range, mounting, shape and other basic variables. Wilco Corporation has an outstanding selection and clearly publishes part specifications to help you find the right inductor.

Find inductors that have the right physical characteristics to suit your applications

Radius and core material are primary options, but also look for surface (including PCB) or through-hole mount inductors. Also, some inductors have variable inductance.

I recommend: Coilcraft carries surface mount inductors to filter both radio frequency interference (RFI) and electromagnetic interference (EMI). Inductors, Inc. covers an array of inductor types, including a full line of each mount type and a number of core choices, including air and ferrous materials. They also carry chips.

Shop for inductors at sources that also have a wide selection of capacitors

If you are shopping for inductors, maybe you're shopping for capacitors as well, since the two often combine in an inductor circuit. You'll want to look for the best one-stop electronics sources.

I recommend: KOA Speer Electronics, Inc. carries electronic components, including resistors, inductors, capacitors and other products. Their stock includes a hybrid, three-terminal inductor/capacitor. Also, Marsh Electronics, Inc. carries capacitors and frequency filters, as well as many brands of basic coils, chokes, and inductors.

Tips & Tactics

Helpful advice for making the most of this Guide

• •  Even the best inductor coils give off some heat due to resistance. Take this into account when mounting an inductor.