In the previous tutorial, we learned about the different properties and characteristics of inductors. These specifications are useful in determining the efficiency of an inductor in a circuit. Manufacturers provide inductors for specific application categories, including RF, power, high current, and high frequency coils. Therefore, choosing an inductor is quite simple compared to resistors or capacitors (which have many options for any application). We have already discussed some of the types of inductors such as solenoid coils, toroids, pot cores, and transmission line inductors. Some other widely used types of inductors are presented here.
Air Coils – Air coils are the simplest inductors. These are solenoid coils that use a non-magnetic core and can have a single loop of wire up to large coils depending on the frequency range they are designed for. These inductors have low inductance ratings but offer a high quality factor. Because there is no magnetic core, they do not suffer operational losses due to hysteresis, eddy currents, or distortion common in magnetic cores. Since non-magnetic cores do not heat up easily when high current flows through the coil, these inductors have a high Q factor as there is minimal energy lost by the core in the form of heat. As the signal frequency increases, the required inductance value decreases, making air coil inductors suitable for high frequency applications. Circuits that process ultra-high frequency often need air coils with a single loop of wire. Surface mount versions of the air coils are also available. Air coils are widely used in RF circuits, FM receivers, SMPS, UPS, RF power amplifiers, RF communication and radio controlled toys.
Example of Air Coil Inductors
Ceramic Core – These are robust inductors used in high frequency and high current applications, as well as in oscillators and signal generating circuits. Some typical applications of ceramic core inductors include RF filters, impedance matching, isolation circuits, audio applications, and wireless communication.
Example of a ceramic core inductor
Ferrite Core Antenna – These inductors are used in antenna applications with frequency ranges between 100 kHz and 1 MHz and may incorporate a ferrite rod, powdered iron, or non-magnetic phenolic material as the core. The coil of wire is wound around the rod used as the antenna. The operating frequency depends on the permeability of the core. The higher the required operating frequency of the signal, the lower the permeability of the core. High-frequency antennas generally use phenolic cores instead of ferrite cores.
Example of loop stick antenna with ferrite core
Toroids – As discussed in a previous tutorial, toroids can use powdered iron or a ferrite core. The toroidal shape of the core allows for high inductance in a small size and minimizes electromagnetic interference by equal and opposite currents around the core. Toroids are available in general purpose as well as surface mount variations. They are widely used as inductors in electronic circuits in almost all types of applications such as AC power coils, filter circuits, power supplies, oscillators, pulse generators, telecommunications, automotive electronics, audio circuits, etc.
Example of toroids
Pot – Potentiometer cores are useful for providing ultra-high inductance in high current applications. These inductors are known to provide stable inductance and high quality factors in small sizes. Potentiometer cores are commonly used as DC chokes, filters, and differential mode chokes in audio, telecommunications, and automotive electronics circuits. Due to their unique design, these present significant advantages of self-protection and high saturation currents.
Image of a pot core inductor
Current Sense – These inductors are available in a range of frequencies, usually center tapped. They inductors are commonly used in DC-DC converters in battery-operated consumer electronic devices.
Example of Current Sensing Inductors
High Current Coils – High current or RF coils use powdered iron or ferrite cores with a small number of coil turns to obtain high value inductance in small size. These inductors are commonly used in power supplies, communication systems, and high-current electrical appliances.
Example of a high current inductor
Balun chokes – “Balun” means the balanced and unbalanced transformation of impedance. These inductors are used in radio, television, and communications circuits specifically for impedance matching.
Example of a balun choke with air wound
Ferrite Coils – Ferrite coils are ferrite spheres without any wire coil; instead, the circuit wire is wound or passed through the inductor. These inductors are used in RF applications to remove unwanted RF frequencies. They are available in leaded and hollow versions, as well as chipped versions, where they are used as high-frequency resistors, allowing only DC to pass through. Ferrite coils are widely used with cables in different applications, whether to remove unwanted RF components from signals or to prevent unwanted external RF signals in receiver circuits.
Example of a ferrite bead used in a data cable
Common and Differential Mode Coils – Common mode and differential mode inductors are used for noise cancellation in radio and communications applications. These inductors are commonly used to cancel noise due to antenna effect on cables and prevent EMI and RF interference from power lines.
Example of common mode coils
Broadband Inductors – Broadband chokes are used to attenuate unwanted RF signals without power losses in low frequency applications. These inductors are widely used to filter EMI and radio frequency interference in communications systems, power supplies, UPS, signal generators, RF power amplifiers, I/O boards, and PC boards.
Broadband Throttling Example
Adjustable – These inductors are designed to provide variable inductance. They have a sliding core with a screw to slide in and out of the core relative to the coil (as in transmission line inductors) or they may use sliding contacts along with the coil (as in pot cores and solenoid types) . Sliding core adjustable inductors work on the concept of permeability adjustment. These inductors are used in resonant circuits operating in narrow bandwidth with high Q factor for a wide range of applications such as low frequency RF communication, RC toys, power supplies, pulse generators, signal generators and oscillators.
Example of adjustable inductors
Molded – These small inductor coils come in axial lead packages for use on printed circuit boards. The coil usually has a protective layer around the assembly. They are widely used in various applications as general purpose inductors.
Example of a molded inductor
Dipped – These inductors are similar to molded inductors. They come in an axial or radial package with protective coating, so they can be used in harsh environmental conditions. Just like molded inductors, dipped inductors are also general-purpose inductors used in a variety of applications where EMI and radio frequency interference are not major concerns.
Dipped inductor example
Shielded – These compact inductors come with a magnetic shield to prevent magnetic coupling, EMI and RF interference. They are used in high reliability applications where signal corruption is a primary concern. Shielded inductors come in a variety of packages, including axial conductor, radial conductor, and surface mount. They are widely used in high-end consumer devices, computers, communication systems, filters and DC/DC converters.
Example of shielded inductors
Multilayer Chip – These are surface mount inductors used in high density PCBs and are suitable for EMI/RFI attenuation, impedance matching and resonant oscillations in a variety of circuits such as signal generators, RF amplifiers, pulse generators, switching power supplies, bandpass filters, analog to digital converters and communication systems. These inductors are used in choke coil type applications where size, vibration, or magnetic coupling between plates are important concerns.
Example of Multilayer Chip Inductors
Obviously, different types of inductors are designed to suit specific applications. Inductors designed for high frequency applications have low value inductance, low DC current, low DC resistance and high quality factor and SFR. Inductors used in filter circuits have high inductance, high SFR and also high DC rating. Similarly, inductors used for coupling applications also have high inductance and high SFR. Inductors used in switching power supplies and DC-DC converters must have a high DC rating.
For most general purpose applications, air coils, toroids, pot cores, chip inductors, molded or immersed inductors are used. For specific applications or circuit requirements, other types of inductors may be preferred.
Activity 11
Try to discover the top inductor manufacturers on various online marketplaces. Take a look at the different types of inductors they manufacture and compare the data sheets of different types of inductors. Try to find out how different types of inductors have different technical specifications to suit their intended applications.
In the next article, we will discuss selecting an inductor for a particular circuit/application.