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 supply inductors for application-specific categories including RF chokes, power, high current, and high-frequency. So, choosing an inductor is quite simple compared to resistors or capacitors (which have a lot of options for any given application). We have already discussed some of the inductor types, such as solenoidal coils, toroids, pot cores, and transmission line inductors. Some other widely used inductor types are presented here.
Air Coils – Air coils are the simplest inductors. These are solenoidal coils that use a non-magnetic core and can have a single loop of wire to large coils depending upon the range of frequency they are designed for. These inductors have low nominal values of inductance but offer a high-quality factor. As there is no magnetic core, they do not suffer from operating losses due to the hysteresis, eddy current, or distortion common in magnetic cores. Because non-magnetic cores do not easily heat up when the high current flows through the coil, these inductors have a high Q-factor as there is minimal loss of energy by the core in the form of heat. As the frequency of the signal increases, the value of required inductance decreases, making air coil inductors suitable for high-frequency applications. The circuits processing ultra-high frequency usually needs air coils with a single loop of wire. Surface mount versions of the air coils are also available. The air coils are widely used in RF circuits, FM receivers, SMPS, UPS, RF power amplifiers, RF communication, and radio-controlled toys.
Ceramic Core – These are robust inductors used in high-frequency high current applications, as well as in oscillators and signal generator circuits. Some typical applications of ceramic core inductors include RF filters, impedance matching, isolation circuits, audio applications, and wireless communication.
Ferrite Core Antenna – These inductors are used in antenna applications with frequency ranges between of 100 kHz to 1 MHz and may incorporate a rod of ferrite, 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. The high-frequency antennas generally use phenolic cores rather than ferrite cores.
Toroids – As discussed in a previous tutorial, toroids may use powdered iron or ferrite core. The toroid shape of the core enables high inductance in small size and minimizes electromagnetic interference by equal and opposite currents around the core. Toroids come 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 like AC power chokes, filter circuits, power supplies, oscillators, pulse generators, telecommunication, auto electronics, audio circuits, etc.
Pot – Pot cores are useful in providing ultra-high inductance in high current applications. These inductors are known to provide stable inductance and high-quality factors in small sizes. Pot cores are commonly used as DC chokes, filters, and differential mode chokes in audio circuits, telecommunication, and automotive electronics. Due to their unique design, these have significant self-shielding advantages and high saturation currents.
Current Sense – These inductors come available by a range of frequencies, usually with a center tap. They inductors are commonly used in DC-to-DC converters in battery-operated consumer electronic devices.
High Current Chokes – High current or RF chokes use powdered iron or ferrite cores with a small number of coil turns to achieve high-value inductance in small size. These inductors are commonly used in power supplies, communication systems, and high current electrical appliances.
Balun Chokes – “Balun” stands for the balanced and unbalanced transformation of impedance. These inductors are used in radio, television, and communication circuits specifically for impedance matching.
Ferrite Chokes – Ferrite chokes are ferrite beads without any coil of wire; instead, the wire from the circuit is wound around or passed through the choke. These inductors are used in RF applications to remove unwanted RF frequencies. They come available in leaded and hollow versions, as well as chip bead versions where they are used as high-frequency resistors allowing only DC to pass through. Ferrite chokes are widely used with cables in different applications either to remove unwanted RF components of signals or prevent unwanted external RF signals in receiver circuits.
Common and Differential Mode Chokes – The common mode and differential mode chokes are used for noise cancelation in radio and communication applications. These inductors are commonly used to cancel noise due to antenna effect in cables and avoid EMI and RF interference from power supply lines.
Wideband Chokes – Wideband chokes are used to attenuate unwanted RF signals without power losses at low-frequency applications. These inductors are widely used to filter out EMI and radio frequency interference in communication systems, power supplies, UPS, signal generators, RF power amplifiers, I/O boards, and PC boards.
Adjustable – These inductors are designed to provide variable inductance. They either have a sliding core with a screw to slide in and out the core with respect to the coil (like in transmission line inductors), or may use slider contacts along with coil (like in pot cores and solenoid types). The adjustable inductors using a sliding core work on the concept of permeability tuning. These inductors are used in resonant circuits operating over 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.
Molded – These small inductor coils come in axial lead packages for use on PC boards. The coil usually has a protective coating around the assembly. They are widely used in various applications as general-purpose inductors.
Dipped – These inductors are similar to molded inductors. They come in an axial or radial package with a protective coating, so they can be used in harsh environmental conditions. 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.
Shielded – These compact inductors come with a magnetic shield to avoid magnetic coupling, EMI and RF interference. They are used in high-reliability applications where signal corruption is a prime concern. Shieldedinductors come in a variety of packages including axial lead, radial lead, and surface mount. They are widely used in high-end consumer devices, computers, communication systems, filters, and DC/DC converters.
Multilayer Chip – These are surface mount inductors are used in high-density PCBs and are suitable for EMI/RFI attenuation, impedance matching and resonant oscillations in a variety of circuits like 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 inter-board magnetic coupling are important concerns.
Obviously, different types of inductors are designed to suit specific applications. The inductors designed for high-frequency applications have low-value inductance, low DC current rating, low DC resistance, and high-quality factor, and SFR. The inductors used in filter circuits have high inductance, high SFR as well as high DC rating. Similarly, inductors used for coupling applications also have high inductance and high SFR. The inductors used in switching power supplies and DC-DC converters should have a high DC rating.
For most of the general-purpose applications, air coils, toroids, pot cores, chip inductors, molded or dipped inductors are used. For specific applications or circuit requirements, other types of inductors can be preferred.
Try to find out major manufacturers of inductors from various online market places. Take a look at different types of inductors manufactured by them and compare datasheets of different inductor types. Try to figure out how different types of inductors have different technical specs to suit their intended applications.
In the next article, we will discuss selecting an inductor for a given circuit/application.