An Introduction to Relays (2024)

Relays play a vital role in countless consumer, commercial, and industrial applications and systems. They are often employed in everyday systems without notice. For example, relays are used when adjusting a household furnace, clicking a TV remote, or operating an elevator. Originally conceived in 1835 for the establishment of extended-distance telegraph connections, relays were subsequently adapted for use in telephone exchanges and continue to operate dependably, inconspicuously, and with high efficiency.

From a pragmatic perspective, relays function as electrical switches. They utilize a low-power signal to govern a high-power circuit whether in proximity or situated at a distance. Their inherent design facilitates effective isolation between the low-power signal and the high-power circuit, often referred to as galvanic isolation. This isolation ensures the safe and robust operation of electrical systems for users. Relays exhibit versatility, as they can be employed to regulate a single circuit or multiple circuits, and they can serve as amplifiers or circuit breakers.

The ability to control device power remotely also translates into enhanced safety measures, especially in industrial processes that might present physical hazards to operators. These versatile devices are available in a wide array of packages, offering diverse current capacities, mounting options, and physical footprints, making them as prevalent as the everyday, ubiquitous switch.

Relays – how they function

For a better understanding of the subject, relays can be categorized into two fundamental types: electromechanical (EMR) and solid-state (SSR). The key distinction lies in the presence or absence of moving components.

The oldest of the two types is electromechanical, which comprises contacts, an armature, a spring, and an electromagnet. In its most basic operation, a spring maintains the armature's position. Upon the application of current, the electromagnet exerts an attractive force on the armature, causing it to move and close a set of contacts, thus enabling the flow of current through the circuit.

Figure 1: The basic internal structure of an electromechanical relay. (Image source: CUI Devices)

Conversely, solid-state relays, which emerged in the late 1950s, serve the same fundamental purpose as electromechanical relays but are entirely electronic without moving parts. They encompass an input circuit, a control circuit, and an output circuit to regulate current. When a voltage surpassing the specified pickup voltage is applied, the control circuit triggers the relay. Once the voltage falls below the dropout voltage, the relay deactivates.

Electromechanical vs solid-state relays continued

As with any electronic component technology, electromechanical and solid-state relays each come with distinct advantages and disadvantages that need to be considered.

Due to their two-century-old design, electromechanical relays are robust devices with straightforward operation, demonstrating reliability in a range of applications, including those involving high current and hazardous environments. They offer complete electrical isolation, can withstand high current and voltage surges, and are impervious to electrical noise, such as electromagnetic interference and radio-frequency interference (EMI/RFI).

Nevertheless, because electromechanical relays have moving parts, physical degradation does occur over time, and contacts can deteriorate due to corrosion and oxidation. They may be susceptible to contact arcing, resulting in pitting and short circuits. Their mechanical nature makes them vulnerable to contact bounce from shock and vibration, and they can generate their own EMI/RFI noise. Additionally, external magnetic fields may affect their operation.

In contrast, solid-state relays offer a longer operational life and function with significantly lower control power thanks to their design without moving parts. They provide faster on/off cycling, eliminate arcing and contact bounce, and remain unaffected by external mechanical shock, vibration, or magnetic fields. Solid-state relays operate within a lower voltage range than electromechanical relays, making them suitable for electronic devices but less suited for high-power applications.

However, due to their unique design, solid-state relays may be susceptible to voltage or current transients and EMI/RFI noise. They tend to generate more heat than their mechanical counterparts and may be sensitive to ambient temperatures. Notably, achieving complete electrical isolation between the control signal and the load is not inherently possible with standard semiconductor switches, but it can be accomplished using optoelectronic components in photo-coupled devices, which isolate the input and output signals.

Figure 2: The basic internal schematic of solid-state relays. (Image source: CUI Devices)

Common relay types

A diverse array of relay types is available, each tailored to address specific application requirements. It is worth noting that manufacturers may employ slightly different terminology for their product offerings, but here is a generalized summary of the major relay types:

• General Purpose: These are common electromechanical relays that operate with AC or DC current in the range of 12 to 230 volts and can control currents from 2 to 30 amps.
• Signal: Signal relays are used to control low-power loads, typically less than 2 amperes. For more information, you can refer to CUI Devices’ "Signal Relays – Understanding the Basics" article.
• Power: Power relays are specifically designed to manage high-power loads, minimizing heat generation and reducing arcing. For more information, you can refer to CUI Devices’ "An Introduction to Power Relays" article.
• Machine Control: These are heavy-duty and rugged relays intended for use in large industrial applications.
• Latching: Latching relays maintain their set or reset condition (either ON or OFF) until they receive an inverting voltage signal.
• Reed: Reed relays are compact and operate swiftly. They use an electromagnet to control one or more hermetically sealed reed switches, allowing for immunity to external contaminants or moisture.
• Zero Switching: These relays activate the load when the control voltage is applied, and the load voltage is close to zero. They deactivate the load when the control voltage is removed.

Figure 3: Basic principle of zero switching with relays. (Image source: CUI Devices)

• Peak Switching: Peak switching relays activate the load when a control voltage is applied, and the load voltage is at its peak. They deactivate the load when the control voltage is removed and the current load approaches zero.
• Instant ON: Instant ON relays promptly activate the load when the pickup voltage is applied.
• Time Delay: Time delay relays incorporate a built-in timer to control events based on time.
• Analog Switching: Analog switching relays manage the output voltage as a function of the input voltage, allowing for infinite output voltages within the relay's rating.
• Optically Coupled: These are solid-state relays that provide isolation between the control and power circuits by switching in response to an internal light source.
• Military/Hi-Rel: These relays are specially engineered to operate in demanding and harsh environments.

Relays can further be categorized as normally open (NO), where the contacts are open and the circuit is not energized, or normally closed (NC), where the contacts are closed when the circuit is not energized. In summary, relays are typically specified as either NO or NC when no power is applied.

Ratings and configurations

Relays are rated based on their capacity to safely switch electrical power through the device. These ratings are categorized as either AC or DC and are typically expressed in amperes. It is crucial that the relay's rating is equal to or greater than the device it will be controlling.

Relays have the capability to control multiple circuits simultaneously and are designated to indicate their specific characteristics. If familiar with poles and throws, these designations include SPST, DPDT, 3PDT, and SP3T.

In addition to the number of poles and throws, relays can describe important characteristics using the term "forms." Phrases like "1 Form A" or "2 Form C" highlight two key pieces of information. The form type indicates whether the switch is normally open or normally closed, and in the case of SPDT switches, whether they are "break-before-make" or "make-before-break." The number (1 or 2) before the form signifies how many contacts of that style are available within the relay. Some common styles include:

• Form A - Normally Open
• Form B - Normally Closed
• Form C - Break-Before-Make SPDT switches
• Form D - Make-Before-Break SPDT switches

Conclusion

Given their simplicity of design and reliable operation, relays are used in devices and systems across a wide range of industries and markets. Originating as basic components in telegraph systems, and even contributing to the early development of computers, relays maintain their essential role in the present day, ensuring the safe and efficient control of electrically powered devices from a remote location.

To fulfill an engineer’s requirements for low-level or high-level current switching, CUI Devices offers a comprehensive selection of power and signal relays with a variety of ratings and configurations.