Technical Article

Unraveling Passive Components: A Deep Dive Into Resistors, Inductors, and Capacitors

December 05, 2023 by Amna Ahmad

Passive components, including resistors, inductors, and capacitors, play essential roles in circuits, influencing current flow, storing energy, and affecting electrical system behavior.

Three types of passive components are used in electrical circuits: resistors, inductors, and capacitors. Passive means the component's behavior changes little with voltage or current fluctuations.


Figure 1. PCB mount trimmer potentiometers. Image used courtesy of Wikimedia Commons


Other components, known as actives, have a non-linear reaction to voltage and current. Diodes, transistors, and thermionic valves are examples of active components.

Resistors, inductors, and capacitors come in various styles and types, depending on use.



Resistors resist the flow of electricity or, more specifically, electric current. In doing so, resistors cause a drop in voltage and radiate heat. If enough heat is generated, a resistor glows with incandescent light. Resistors are used to:

  • restrict current flow
  • develop a voltage drop (PD)
  • generate heat
  • generate light

Resistors consist of a conductor length, sometimes wound into a coil or laid into a grid so heat can escape. In electronics, resistors can be as small as 1/8 watt and just 2 mm by 1.5 mm. Even smaller resistors exist in microelectronics, while larger resistors can be as large as a manufacturer requires.

Resistors are the most common method of generating heat from electricity, and almost every electrical heat source you can think of is a resistor. Electric toasters, ovens, cooktops, space heaters, hot water systems, and even bathroom heat lamps are based on resistors.

High-power resistors find applications in diverse fields, such as power generation, distribution, high-voltage systems, and control systems. Grounding resistors facilitate resistance grounding in industrial power systems, allowing controlled fault currents to protect equipment. Dynamic braking resistors, designed for high-heat and high-power scenarios, are crucial in material handling equipment, elevators, escalators, cranes, power inverters, and industrial drives. Load banks are instrumental in testing uninterruptible power supplies, generators, and backup generation systems, applying a dummy load to assess power sources.

Electric lighting is undergoing a revolution, replacing incandescent lamps with arc lamps and light-emitting diodes. Electric lighting based on Thomas Edison's incandescent lamp loses most of its energy, heat, and not light, making it too inefficient for today's energy-conscious requirements.

Resistors regulate the flow of current in electronics and certain electrical applications. The operator can sometimes change some settings, such as volume and tone in an amplifier. In such cases, an adjustable 'rheostat' or 'potentiometer' is generally mounted on the front panel and controlled by a knob that is rotated to adjust the resistance setting.



An inductor is a component that demonstrates inductance, which means it induces an electromagnetic field in the space around a conductor. The electromagnetic field is stored energy, which the inductor can later return as a current. Every conductor is also an inductor, although usually with a weak magnetic effect. An inductor is, therefore, also an electromagnet.

Normally, an inductor is made as a wire coil, sometimes wound around a core of magnetic material, usually iron.


Figure 2. Inductors. Image used courtesy of Wikimedia Commons


Inductors store current as an electromagnetic field when the current increases and give it back when it decreases. Therefore, an inductor is often used to smooth the current value. That is called 'filtering,' and in that case, the inductor is called a 'choke.'

An inductive coil by itself is called a 'solenoid coil.' If a solenoid has a movable magnetic core, called a 'slug,' energizing the solenoid will pull the slug into the center of the magnetic field. This is the best-known electromagnetic effect, commonly used for providing a mechanical force.

A solenoid can magnetically attract an iron armature to operate a set of contacts. The contacts can then be used to operate another circuit and thus control a larger current. A solenoid switch can also operate a series of circuits, each with its own solenoid switch. Such a circuit was used for telegraphic communications to relay messages, so solenoid switches became relays or relay switches.


Figure 3. Electromagnetic relay. Imaged used courtesy of Wikimedia Commons


As current flows through an inductor, it generates a magnetic field that passes through other coil sections, causing voltage within the coil. This induced voltage opposes the original current flow. This only occurs when there is a relative movement between the field and the turns, so it only occurs briefly when the current is first applied or removed from the coil.

When the current changes constantly, as in AC electric current, there is a continually changing magnetic field, and electricity is generated in any nearby coil. This is known as the transformer effect or mutual coupling.


Figure 4. Step-down transformer. Image used courtesy of Wikimedia Commons


Passing a conductor through a magnetic field, or a magnetic field past a conductor, also generates a voltage in the conductor. This effect is used in generators and alternators.

A transformer transforms voltages from one level to another level. It does this by converting electrical energy to magnetic energy, then converting the magnetic energy back into electrical energy in a coil with a different number of turns, generating a voltage appropriate to needs, e.g., a battery charger can convert 230 volts from the mains supply into approximately 12 volts for a battery.

Inductors designed for high currents typically feature a single winding composed of insulated wire, effectively storing energy, and regulating current flow. This design ensures a consistent and reliable current delivery to equipment even during shifts in current.

These high-current inductors are crucial in various applications, such as solar inverters, HVAC inverters, and server power supplies. Additionally, bidirectional, string, and PV inverters commonly rely on high-power inductors. Their significance in these applications lies in their ability to enhance overall performance by:

  • Filtering low current ripples
  • Dissipating heat
  • Storing energy
  • Increasing efficiency



Capacitors are devices that store an electrical charge. While inductors store a current as a magnetic field, capacitors store voltage as an electrostatic field.

Capacitors come in many sizes and shapes depending on the manufacturer and their intended use. A capacitor is constructed of two conductive surfaces separated by an insulator to store an electrostatic field between those surfaces.

Thus, a capacitor is a device for storing electric energy when the voltage is high and returning it when it is low. Capacitors are commonly used in power supplies to remove high-voltage surges and to smooth out the voltage after it has been rectified. Capacitors are also used in electric single-phase motors to help them start and develop full torque. While inductors are common in electric circuits, capacitors are more common in electronic circuits.


Figure 5. Capacitors. Image used courtesy of Wikimedia Commons


Capacitors are typically rated by their capacitance value and maximum voltage rating. The standard capacitance unit value is the farad (F). Common automotive capacitors are generally rated in microfarads (µF), which are one-millionth of a farad. This value may be indicated on the capacitor. Common capacitors are made of ceramic, plastic, or electrolytic material.

For practical purposes, capacitors employ thin metal sheets, typically aluminum, as plates. The high-resistance material between the plates can be wax paper, oil-saturated paper, or a chemical electrolyte. In air-conditioning and refrigeration services, two common capacitor types are oil-filled and electrolytic capacitors.

Oil-filled capacitors enhance motor torque by being used in series with the start winding of AC motors. They are also employed when the application requires the capacitor to be in the circuit continuously. On the other hand, electrolytic capacitors find application in the start circuits of high-horsepower AC motors, offering high capacitance in a compact space. These capacitors are typically connected for intermittent use.

Capacitors, alone or in conjunction with resistors, can form RC (resistor-capacitor) networks. These networks find applications in filtering, DC blocking, decoupling, and coupling phase-shift circuits.


Passive Component Takeaways

A solid understanding of passive components is crucial for engineers delving into electronics. With their resistance values, resistors control the current flow, limiting it to specific levels as required by the circuit. Upon the passage of current, inductors generate magnetic fields, leading to self-inductance that opposes changes in current flow and enables them to function in energy storage and filtering applications. On the other hand, capacitors store electrical energy as an electric field across their plates, facilitating functions like energy storage, filtering, and coupling in electronic systems. Understanding the technical nuances of these passive components enables engineers to tailor their design choices for optimal performance, efficiency, and reliability in numerous electronic applications.