This article will give an in-depth discussion about Inductors and Inductor Coils.
The article will bring more understanding about topics such as:
Inductors are passive two-terminal components of an electric or electronic circuit that are capable of storing energy in magnetic form. They oppose sudden changes in current and are also called coils or chokes. They are known by their electrical symbol L.
An inductor coil is an electrical conductor that passes electricity and generates a magnetic field. It is wound in the form of a coil or spiral.
When electric current begins to flow through a conductor, a magnetic field is created around it, following the right-hand rule. If current flows through an inductor with conductors wound around it in the same direction, the magnetic field around the wire is consolidated, turning the inductor into an electromagnet. Conversely, a magnetic field can also induce an electric current.
Once an inductor has been magnetized, the magnetic field or flux around it can be altered by moving a magnet closer or farther away. This change induces an electric current to create a "force against change" that aims to regulate the direction and strength of the magnetic field. This process is known as electromagnetic induction. As illustrated in the circuit diagram below, when a DC current starts to flow through an inductor, it immediately generates an electromotive force that opposes the direction of the current flow.
This property is known as the self-inductive effect. However, as the DC current continues to flow, it eventually reaches a steady state where the magnetic flux stops changing, and the current flow is no longer impeded since the electromotive force ceases to be generated. The electromotive force in an inductor is directly proportional to the rate of change of the current (ΔI/Δt). Conversely, when an AC current is applied (as shown in the figure below), the voltage increases significantly when the electric current rises from zero because the rate at which the current is changing is at its maximum.
As the rate of current increase starts to slow down, the voltage decreases and eventually becomes zero when the current reaches its maximum value. When the current begins to decrease from its peak, a negative voltage is generated, and as the current drops to zero, the voltage reaches its lowest point. The electromotive force is generated with a phase shift of ¼ cycle relative to the voltage and current waveforms, as illustrated in the figure above. Consequently, AC current is more challenging to pass through an inductor compared to DC current. Additionally, if the frequency of the AC current exceeds a certain threshold, the electromotive force will continuously oppose the current, eventually stopping its flow. Thus, higher-frequency AC voltage makes it increasingly difficult for the current to pass through.
Inductors can be distinguished in three main ways. One method is based on the type of core around which the inductor is wound. There are various types of inductor cores, including air cores or cores made from magnetic materials that enhance the inductor’s ability to store energy.
Another way to distinguish inductors is by their characteristics, such as the shape of the coil or the construction. Some inductors have coils wound in circular shapes, while others are cylindrical.
The final distinguishing feature is whether the inductor is adjustable or variable. Adjustable inductors have a movable inner core that alters their inductance. If the core is made of a magnetic material, moving the core towards the center of the windings increases the inductance. Conversely, if the core is made of brass, moving it towards the center decreases the inductance.
The types of inductor coils based on their cores are detailed below:
Below are the details on the construction, description, and applications.
As the name implies, an "Air core" inductor does not need a coil form for support and is self-sustaining. It uses air as the medium for storing magnetic energy instead of relying on magnetic materials like ferrite. In certain instances, the coil of an air core inductor can be wound in a way that allows it to support itself. However, in other cases, a ceramic or insulated material may be used to provide structural support. Additionally, to stabilize the inductance of this type of inductor, it may be coated with varnish or secured with wax.
Air core inductors offer several advantages due to the absence of a ferromagnetic core. These benefits include high linearity, no core saturation, and no iron losses at high frequencies. Constructing an air core inductor is straightforward, and it is unaffected by the value of the electric current it carries. However, the lack of a ferromagnetic core limits the L-I product, making air core inductors more suitable for low-power applications. They are commonly used in commodity electronic products, computer devices, communication equipment, and other consumer goods. Due to their large size and low factors, high inductance values are not achievable with air core inductors.
Below are details on the construction, description, and applications.
These inductors are made from non-magnetic ceramic material, which functions similarly to air. The ceramic material serves as the core, providing shape to the coil and structure to its terminals.
Because it is a non-magnetic material, its magnetic permeability is low, resulting in low inductance. Core losses are minimized in this type of inductor, but high inductance values are not achievable.
Ceramic core inductors are used in applications that require low inductance levels, a high Q factor, and minimal core losses.
Below are details on the construction, description, and applications.
These inductors are made by wrapping a length of wire around a ferrite core. The ferrite material is created by mixing iron oxide (Fe2O3) with a small percentage of other metal oxides like nickel, barium, zinc, or magnesium at temperatures between 1000 and 1300 degrees Celsius.
These inductors exhibit high permeability, low eddy current losses at high frequencies, and high electrical resistivity. Ferrite core inductors can be used without additional laminating material as long as they remain below the Curie temperature, where they maintain good magnetic properties. Due to these characteristics, ferrite core inductors are well-suited for high-frequency applications and offer the advantage of being cost-effective. However, they have several disadvantages, including the issue of core saturation. Saturation losses can occur when the magnetic flux density reaches 400 mT. Additionally, there is a limitation on the upper operating frequency due to other core losses, and temperature drift can affect inductance, potentially altering the performance of a tuned filter.
The construction, description, and applications are detailed below.
As the name suggests, "iron core inductors" are made by winding a conductor around an iron material.
They provide higher inductance values due to the properties of the material used. The iron material amplifies the inductor’s magnetic field, making the iron core inductor more effective at storing energy in the magnetic form compared to air core inductors. This means that an iron core inductor can store more magnetic energy than an air core inductor with the same number of wraps or turns. Although an iron core increases the magnitude of the inductance, it also exhibits high core loss at high frequencies. Therefore, iron core inductors are used in devices that require high power levels and low frequencies, such as audio equipment, power conditioning systems, and inverters.
These inductors are used for low-frequency applications such as
Below are details on the construction, description, and applications.
These inductors are made of laminated core materials, which are typically thin steel sheets arranged in stacks.
These laminated core inductors minimize loop action by blocking eddy currents using thin steel sheets arranged in stacks. This approach reduces the loop area for current travel, thereby decreasing energy losses. The primary advantage of these inductors is their reduced weight compared to other solid core materials.
Laminated steel core inductors are primarily used in the manufacture of transformers.
Below are details on the construction, description, and applications.
As the name suggests, these inductors have cores with magnetic materials containing air gaps. This type of construction provides the advantage of storing larger amounts of energy compared to other types.
Iron powder core inductors exhibit low eddy current losses as well as low hysteresis losses. They are also very inexpensive and offer excellent inductance stability. However, a disadvantage of this type of inductor is that hysteresis loss and eddy current loss, although low, are still present. Additionally, there is air gap loss, which results in excess losses in both the core and the winding.
Below are the types of inductor coils categorized by their core design:
The construction, description, and applications are detailed below.
These inductors are constructed by wrapping a length of wire around a cylindrical bobbin and enclosing it with a shrink tube. The core material used is ferrite, which imparts the same properties as those of a ferrite inductor.
These inductors are found in small sizes, making them suitable for use in power adapters.
The construction, description, and applications are detailed below.
By winding a length of wire around a doughnut-shaped core, a toroid core inductor is made. The material of the core is ferrite, so this inductor’s properties resemble those of a ferrite core inductor.
Due to its closed-loop design, this type of core generates a stronger magnetic field, which increases both the size and inductance. It also offers a higher Q factor compared to an inductor of the same value with solenoid coils and a straight core. Toroidal core inductors improve efficiency with less impedance due to a high magnetic field and high inductance magnitude, achieved with only a few windings. Additionally, they benefit from low flux leakage because of their magnetic circuit's symmetry. Toroidal core inductors are made with fewer materials, resulting in a lighter and more compact design.
Below are the types of inductor coils categorized by their core usage:
Below are the construction, description, and applications:
As indicated by the name, this inductor consists of multilayers. It is constructed by layering thin plates of ferrite material. The sheets are properly placed one layer after another, forming a coil and thus creating inductance. A special metallic paste is used to print the coil pattern on the layers.
They have increased inductance and capacitance. Higher inductance results can be achieved at lower operating frequencies.
The construction, description, and applications are detailed below.
This type of inductor is made by using a substrate of very thin ferrite or any magnetic material. It is constructed by placing a spiral-shaped trace of conductive copper on top of the substrate.
The design of a thin film inductor provides resistance to vibrations and ensures stability.
The construction, description, and applications are detailed below.
Just like resistors, this type of inductor is built by coating it with insulation such as molded plastic or ceramic material. The core material is either ferrite or phenolic. The winding is available in various designs and shapes, such as cylindrical, bar shapes, and axial forms.
They can achieve greater inductance levels and handle higher currents while maintaining a small volume, making them suitable for unobtrusive mounting in small, compact devices. These inductors enhance power optimization and reliability due to the stability of inductance across a wide current range, with only a gradual drop beyond rated currents.
The construction, description, and applications are detailed below.
It is built by winding two lengths of wire around a common core. The windings can be connected in series, parallel, or configured as a transformer, depending on the application. These inductors operate based on the principle of mutual inductance, transferring energy from one winding to the other.
They reduce inductor current ripple, maintain transient performance, and provide higher converter efficiency. Additionally, they experience relatively insignificant power loss due to current ripple.
The construction, description, and applications are detailed below.
These inductors are designed to handle high currents without reaching magnetic saturation. To increase the saturation current rating, the inductor’s magnetic field is amplified. This increase in magnetic field can lead to EMI (electromagnetic interference). Most power inductors use proper shielding to mitigate EMI.
Power inductors feature low resistance values, high current capability, low magnetic flux leakage, and high inductance values. They are lightweight and space-saving, with an optimized temperature range of up to +150°C.
They are used to convert a certain voltage in a step-up or step-down circuit to the required voltage.
Below are the details on the construction, description, and applications.
This type of inductor is very simple but is specifically designed to block (choke) high-frequency signals. As the frequency increases, the impedance of the choke rises significantly. Consequently, it allows low-frequency AC and DC to pass through while blocking high-frequency AC. Choke inductors are constructed without employing techniques for impedance reduction that are used to increase their Q-factor. Instead, chokes are intentionally designed to have a low Q-factor so that their impedance increases as the frequency rises.
Chokes allow low-frequency AC and DC to flow while blocking high-frequency AC. They have a low Q-factor, meaning their impedance increases as the frequency rises.
Detailed information on the construction, description, and applications is provided below.
By wrapping a very thin copper wire around a dumbbell-shaped ferrite core and attaching two lids at the top and bottom of the core, a color ring inductor is created. The inductor is then molded with a green material coating. Values are indicated by colored bands printed on the coating. These colors can be read and compared with a color code chart, similar to how resistor values are determined.
These inductors feature a compact structure, being both small and lightweight. They are resistant to humidity due to their epoxy resin coating, which enhances their durability. They offer a high resonant frequency along with a high Q factor and are RoHS compliant.
Below are the details regarding the construction, description, and applications.
It is constructed by wrapping a length of wire in a cylindrical bobbin and enclosing it in a special housing form of ferrite, shielded surface mount inductor.
The shielding minimizes EMI and noise from the inductor, enabling its use in high-density designs. These inductors are particularly suited for PCB-mounted applications.
The construction, description, and applications are detailed below.
These inductors are made by coiling a length of multi-stranded wire and then placing it in a ferrite material. The use of multi-stranded wire helps reduce the skin effect, enabling the generation of a high-frequency magnetic field that penetrates to a certain depth. If a solid wire were used, most of the current would flow through the outer part of the conductor, increasing resistance. The ferrite plate beneath the coil enhances the inductance, concentrates the magnetic field, and reduces emissions.
They are efficient in charging, reliable, and cost-effective, offering reduced thickness for various applications. They feature low thermal loading and low DC resistance, ensuring high efficiency.
The construction, description, and applications are detailed below.
This type of inductor is constructed by winding a length of wire around a hollow cylindrical bobbin. The inductance value can be adjusted by positioning and moving a ferromagnetic or brass core. When using a ferrite core, the inductance increases as the core is moved towards the center of the winding. Conversely, with a brass core, the inductance decreases as the core is moved to the center of the winding.
The inductance can be adjusted by altering the position of the core, making it suitable for highly sensitive applications where a fixed inductor might not offer precise alignment.
This chapter will cover the inductance of an inductor coil and the factors that influence it.
The concept of inductance in an inductor coil will be explained in detail below.
The concept of inductance in an inductor coil will be discussed below.
Inductance is a property of an electrical circuit that resists changes in current. This resistance is caused by the creation or destruction of a magnetic field. When current starts to flow, it generates magnetic field lines of force. These lines induce a counter electromotive force (emf) that opposes the current by interacting with the conductor. In other words, inductance is the phenomenon in which a changing magnetic field affects the flow of electricity within the circuit.
Self-inductance refers to the process where a circuit uses its own changing magnetic field to induce an emf within itself. This property is inherent in all electrical circuits. Self-inductance only manifests when there is a change in the electric current, opposing only changes in current rather than the current itself.
Two coils exhibit mutual inductance when the magnetic flux from one coil intersects the turns of the other coil. The extent of mutual inductance is influenced by several factors, including:
The degree of coupling between the coils is defined by the coefficient of coupling \( K \). The coefficient of coupling \( K \) is equal to 1 (or unity) if all the magnetic flux from one coil intersects all the turns of the other coil. Conversely, if none of the flux from one coil intersects the turns of the other coil, the coefficient of coupling is zero.
Several factors can influence coil inductance, including:
Increasing the number of turns in a coil enhances its inductance, assuming all other factors remain constant. Conversely, fewer turns result in lower inductance. This is because additional turns generate a stronger magnetic field (measured in amp-turns) for the same amount of current, thereby increasing the coil's inductance.
Assuming all other factors are equal, a larger coil area results in higher inductance. Conversely, a smaller coil area leads to lower inductance. This is because a larger coil area offers less resistance to the formation of magnetic flux for a given field force (amp-turns), thereby increasing the inductance.
All other factors being equal, a longer coil length results in lower inductance, while a shorter coil length provides higher inductance. This is because a longer coil length increases the path that the magnetic field flux must travel, leading to greater opposition to the formation of the flux for any given field force.
Assuming all other factors are equal, a core material with higher magnetic permeability will result in greater inductance. Conversely, a core with lower permeability will produce less inductance. This is because materials with higher magnetic permeability generate a stronger magnetic field flux for a given magnitude of field force (amp-turns).
When selecting inductor coils, several factors need to be considered:
When reviewing application requirements, an engineer must select the appropriate type of inductor. The chosen inductor should meet circuit specifications and enhance performance. Many inductors are crucial for power circuits or for blocking radio frequency interference.
In power circuit applications, both incremental and maximum currents must be considered. Incremental current refers to the current level at which inductance starts to decrease, while maximum current pertains to the current level that exceeds the temperature rating of the application device.
When choosing an inductor for an RF application, two factors must be kept in mind:
The size of the inductor is determined by the application. For instance, power circuits necessitate large inductors, whereas RF applications typically use small ferrite core inductors. Additionally, compatibility with filter capacitors is crucial for larger inductors. RF devices usually have lower power requirements. To minimize magnetic coupling between components, all inductors should be shielded.
The tolerance percentage should be compared with the device's inductive value by reviewing the manufacturer's datasheet. Before purchasing an inductor, it is essential to check the datasheet to ensure that the specifications align with the application requirements.
There are many different types of inductor coils with different characteristics as a result of their core material, shape, or use. All these inductors have different properties and functions; therefore, one must be aware of these properties and functions in order to choose the right inductor for a certain application. The factors affecting inductance must also be taken into consideration.
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