5 Applications of Resonant Circuits

To many, global communication is something that is often taken for granted; and like many of the technologies that help to shape our world, there is a considerable gap between passing knowledge of the technology and how it actually works.

This article aims to shed some light on how resonant circuits work and how they have shaped the ways in which information is shared and consumed all over the globe.

Resonant Circuits: Back to Basics

Understanding the many applications of resonant circuits is one thing, but it is quite another to understand what makes up a resonant circuit and how it works. Let’s begin by reviewing how a resonant circuit is created.

In order for a resonant circuit to work, the circuit’s inductive reactance and capacitive reactance must be in a state of equilibrium. This state of equilibrium allows these circuit elements to be used in a number of electrical resonance applications; as the frequency of the current increases, the crucial equilibrium collapses. When the frequency increases, so too does the inductive reactance. The capacitive resonance, however, heads in the opposite direction. The point at which both the capacitive resonance and inductive reactance are in harmony is known as the resonant frequency.  

Once energized by an electrical current, the energy contained within a resonant circuit will be simultaneously stored as an electrostatic charge within the capacitor and as a magnetic field around the inductor. Courtesy of resistance, the charge will slowly diminish until it ceases to exist. However, if an electrical current is continuously driven into the circuit, a state known as “oscillation” will occur. 

There are also different kinds of resonant circuits you may already be familiar with: series resonant circuits and parallel resonant circuits. When resonance occurs in circuits where capacitors and inductors are connected in series, it’s a series circuit. Series circuits are also sometimes delightfully referred to as “daisy chains,” as they share a resemblance to a daisy garland. Parallel resonance is when the resonance occurs in circuits where the capacitors and inductors are connected in parallel. While connected in parallel, equal voltage is applied to all components, as is seen in Kirchhoff's current law. The law states that “for any node  in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node.”

to further differentiate between these two circuits, we can use light bulbs as an example. If each light bulb is connected by a wire, and the wire traces back to one single light switch, that’s considered a series. If the light bulbs are each individually wired to a light switch but don’t directly connect to each other, they’re considered parallel. If one bulb in the series circuit breaks, the entire circuit breaks. But in a parallel circuit, if one bulb breaks, at least one other will still function.

Applications of Resonant Circuits

Let’s take a look at five applications of resonant circuits and how they appear in our everyday lives

1. Radio Transmissions and Television Broadcasts

Most of us will experience using a resonant circuit as we attempt to tune into a radio frequency or pick up an analog television station. In doing so, we’re adjusting the frequency so that the elements of the resonant circuit are in equilibrium. In the radio receiver, the tuning circuit is connected to the vehicle’s antenna. The antenna receives electromagnetic waves of varying frequencies transmitted by different stations within a specific range. As you turn the dial, the frequencies generated in the antenna will have the same frequency as a local radio station. This means the tuning circuit in the radio receiver will only allow the current that has a matching frequency to that of the circuit to pass which in turn allows you to listen to a crystal-clear signal of your selected station. 

2. Signal Processing and Communication Systems

Television and radio signals are an example of an application of resonance effect that many of us are most familiar with, but there are many other applications in which resonance circuits play an important role. Case in point: fiber optic data transmission. Fiber optics communication is used to transmit video, telemetry, and voice data all over the globe. Infrared light is sent through an optical cable, with the light carrying information—otherwise known as a carrier wave. The reason fiber is used instead of electrical cables is because it’s immune to electromagnetic interference. It’s also better at accommodating high bandwidth and longer distances. 

3. Voltage Magnification

When the total circuit impedance is less than either the capacitive or inductive reactance at a resonant frequency, the supply voltage (developed across the circuit resistance) will be less than either of the opposing reactive voltages VC or VL. This effect, where the internal component reactive voltages are greater than the supply voltage is known as voltage magnification and there are several benefits associated with it. For example, when a circuit is configured in series, it can be used to magnify the voltage amplitude of a signal When it comes to AC signal voltages, voltage magnification can be done passively, meaning the amplitude of the signal can be increased without a commensurate increase in power consumption. 

Voltage magnification is very useful and is most notably used in radio antennas. Radio receivers magnify the voltage amplitude of the signal voltage that is being received before it’s fed to the transistor amplifiers in the circuit. Also, voltage magnification doesn’t require an external power supply, making it even more beneficial. 

4. Induction Heating

Typically, an induction heating system relies on access to a power supply, a circuit that contains matching impedance, a resonant circuit, and an applicator. In this instance, the resonant circuit is generally configured in parallel; within this circuit, the capacitors and inductors will store electrostatic and electromagnetic energy respectively. Once the circuit is at its resonant frequency, the elements will transfer power back and forth. Since the circuit is configured in parallel, this conversion will occur at a very high current. Energy is lost in the form of heat in the capacitor which results in the creation of induction heating. 

Induction heating is used within many industrial processes and is a quick way to heat metals. Technicians can use induction heating devices to create an electrical current in the metal object, generating heat. This is useful because then the metal becomes pliable enough to be used for machining.

5. Oscillator Circuits

Unlike the other applications, a crystal oscillator is an electronic oscillator circuit that is used to create an electrical signal with a given frequency. This frequency is commonly used to keep track of time and is used in a number of different timepieces. The crystal oscillator works in conjunction with a principle known as the inverse piezoelectric effect. Simply put, an electrical field produces a mechanical deformation within the crystal material. This deformation produces a vibration of the material’s mechanical resonance to produce a very specific frequency. 

Common applications of oscillator circuits include alarms or buzzers within clocks or other devices. They can even be found in recreational or decorative products such as dance lights. 

Resonance Shapes Our World

Understanding the underlying technologies that affect our lives can be a daunting task. Though many may take them for granted, resonant circuits are essential to the technologies that we all use on a daily basis. To expand on what you already know about resonant circuits, consider applying for our Electronics Technician Training program. You’ll gain hands-on experience in an online setting and grow within this fascinating field.