To understand what an inductor is and what functions it performs in a circuit, you first have to understand what inductance is.
Inductance is the ability of a conductor or circuit to resist a change in the current flowing through it. This current flow creates a magnetic field around the conductor which can vary depending on the magnitude of the current. In effect, the current “induces” a magnetic field around the conductor. This stored energy is released when the field falls. The symbol for inductance is (L) and the magnitude is measured in units called the Henry (H). One Henry is the inductance required to induce one volt of electromotive force when the current is changing at one ampere per second.
As a component, an inductor is a coil of wire wound around a core of air, iron or magnetic material. A coil is typically used because the turns of the wire allow for increased inductance and smaller size. A magnetic field is created in the inductor when current is applied. This will cause the inductor to resist alternating current (AC) and allow direct current (DC) to flow. High-frequency signals will be blocked but lower-frequency oscillations will pass. This is the opposite function of a capacitor. In fact, when an inductor and a capacitor are used in the same circuit they can act together to cancel each other out. This is known as resonance.
This ability makes inductors very selective filters of external noise in a circuit. They can also generate oscillations making them useful in radio equipment and power supplies. In fact, inductors are also commonly known as chokes, coils, or reactors.
The basic design of an inductor can affect its performance (level of inductance) in one of four ways:
- Number of wire turns in the coil - more turns equal greater inductance
- Length of the coil - the longer the coil the more the inductance
- Area of the coil - the greater the cross section area the more inductance
- Core material - the more the core is able to accept a magnetic field (magnetic permeability) the more inductance
Manufacturers of inductors vary these factors to produce components that match specific circuit or system design needs. Circuit miniaturization in modern electronic systems, particularly in communication, has also led to different inductor designs that allow for a small footprint and surface mount ability on a pc board.
There are many different types of inductors for many different applications. They are typically differentiated based on the materials or processes used in their manufacture. Here are the basic types:
- Air Core - wire is wound in a coil, either with no support or around a ceramic core used to shape the coil (also called ceramic inductors). High quality. Low core loss. Used in high frequency applications.
- Iron Core - wire is wound around an iron core element. High power and high inductance value. Usually used in audio applications.
- Ferrite Core - magnetic material is used for the core (also known as ferro-magnetic). High level of inductance compared to air core. For higher power applications.
- Iron Powder - Uses compressed iron powder bound with epoxy for the core. Allows for higher DC currents and high temperature stability. Frequently used in switching power supplies.
- Laminated Core - Uses steel laminations aligned parallel to the field to avoid current loss. Used in low frequency detectors.
- Bobbin Based - uses wire wound on a cylindrical bobbin (like a spool of thread). Usually for PC board mounting. Specifications vary widely in terms of power, operating frequency, etc.
- Toroidal - uses wire wound on a ring of ferrite or iron. High inductance values at low frequency. Typically used in medical devices, appliances, etc.
- Multi-Layer Ceramic - uses multiple layers of wire wound on a ceramic core to increase inductance and lower operating frequency. Used to suppress noise in communication devices.
- Thin Film - uses film as the conductor to reduce size. Typically for board-mount applications in mobile devices.
- Variable - has a movable core which allows changes in inductance value. Used where tuning is required in high frequency applications.
- Coupled - uses two windings around a core that are isolated from each other. Commonly used in energy conversion devices.
Deciding on the right inductors to use in your circuit or system requires consideration of factors beyond just price or footprint. Inductors are not typically sensitive to system operating temperatures or humidity, but they can be sensitive to voltage spikes at the input, and generate spikes of their own in response.
Other factors that should be considered before specifying an inductor include: circuit requirements, RF considerations, inductor size, shielding requirements, tolerance specifications (variation from published specs), applications in power circuits, and manufacturability.
Inductors have been around and in proven use since the discovery of the induction effect in 1830, and led to the development of the first commercial transformers in the 1880’s. Today’s devices, like all components, are continuously being changed to adapt to demands for miniaturization in electronic systems.