Relays play a vital role in countless consumer, commercial, and industrial applications and systems. They are often inadvertently used in everyday systems. For example, we use relays when adjusting a home stove, tapping a TV remote control, or operating an elevator. Relays were originally conceived in 1835 to establish long-distance telegraph connections, and were later adapted for use in telephone exchanges. Although it is inconspicuous in itself, it still operates efficiently and reliably as always.
From a practical point of view, the function of a relay is to switch electrically. Whether at short range or at long distance, all use low-power signals to control high-power circuits. Relays are inherently designed to help effectively isolate low-power signals from high-power circuits, often referred to as galvanic isolation. This isolation ensures the safe and robust operation of the user's electrical system. Relays are versatile as they can be used to regulate a single circuit or multiple circuits, and can also be used as amplifiers or circuit breakers.
In addition, relays enhance safety measures by enabling remote control of equipment power, especially in industrial processes where there may be a physical hazard to operators. Available in a variety of packages, with a variety of current capacities, mounting options, and physical footprints, these versatile devices are as ubiquitous as commonly used switches.
Relays - How they work
To better understand this topic, we can divide relays into two basic types: electromechanical relays (EMRs) and solid-state relays (SSRs). The key difference is the presence or absence of moving parts.
Of the two types, electromechanical relays have the longest history and include contacts, armatures, springs, and electromagnets. In the most basic operation, the spring maintains the position of the armature. When energized, the electromagnet creates an attraction on the armature, causing it to move and close a set of contacts, allowing current to flow through the circuit.
Figure 1: The basic internal structure of an electromechanical relay. (Image source: CUI Devices)
In contrast, solid-state relays serve the same basic purpose as electromechanical relays, but are entirely electronic components with no moving parts. Such relays appeared in the late 50s of the last century. They include input, control, and output circuits that can be used to regulate current. When the applied voltage exceeds the specified starting voltage, the control circuit triggers the relay. As soon as the voltage drops below the dropout, the relay is deactivated.
Comparison of electromechanical relays and solid-state relays
As with any electronic component technology, electromechanical relays and solid-state relays have their own distinct advantages and disadvantages that need to be considered.
Because electromechanical relays have been designed for two centuries, they are easy-to-use, stable devices that demonstrate good reliability in a wide range of applications, including high currents and hazardous environments. They provide complete galvanic isolation, are resistant to high currents and voltage surges, and are immune to electrical noises such as electromagnetic interference and radio frequency interference (EMI/RFI).
However, because electromechanical relays have moving parts, they can physically degrade over time and the contacts can be damaged due to corrosion and oxidation. They can be susceptible to contact arcing, leading to pitting and short circuits. Due to their mechanical properties, they are susceptible to contact chatter caused by shock and vibration, and generate their own EMI/RFI noise. In addition, an external magnetic field may affect the operation of the relay.
In contrast, solid state relays have a longer operating life and have significantly reduced control power due to the absence of moving parts in the design. These devices provide faster switching cycles, eliminate arcing and contact chatter, and are unaffected by external mechanical shock, vibration, or magnetic fields. Solid state relays operate in a smaller voltage range than electromechanical relays, making them suitable for electronic devices, but less suitable for high-power applications.
However, due to their unique design, solid state relays can be susceptible to voltage or current transients as well as EMI/RFI noise. The heat generated is often higher than that of mechanical relays and can be very sensitive to ambient temperatures. It is important to note that with standard semiconductor switches, complete electrical isolation between the control signal and the load is not inherently possible, but it can be achieved using optoelectronic components in optocoupler devices to isolate the input and output signals.
Common relay types
There are a variety of relay types available on the market, each tailored to meet specific application requirements. It's important to note that while manufacturers may have slightly different names for their products, the main types of relays can be broadly summarized as follows:
General Purpose: These are common electromechanical relays that use AC or DC power, operate in a voltage range of 12 to 230 V, and are capable of controlling currents from 2 to 30 A.
Signal: Signal relays can be used to control low-power loads, typically less than 2 A. For more information,
Power: Power relays are specifically designed to manage high-power loads, minimizing heat generation and arcing.
Machine Control: These are durable, heavy-duty relays for large industrial applications.
Self-latching: The self-latching relay remains set or reset (on or off) until an inverting voltage signal is received.
Magnetic reeds: Reed relays are compact in structure and easy to operate. These relays use solenoids to control one or more enclosed reed switches, making them unaffected by external contaminants or moisture.
Zero switches: These relays activate the load when a control voltage is applied and the load voltage is close to zero. When the control voltages disappear, they disconnect the load.
Peak Switching: When a control voltage is applied and the load voltage is at its peak, the peak switching relay activates the load. They disconnect the load when the control voltage disappears and the current load approaches zero.
Momentary Opening: The Momentary Switching Relay activates the load as soon as the starting voltage is applied.
Delay: The delay relay has a built-in timer that controls events based on time.
Analog Switching: An analog switching relay manages the output voltage as a function of the input voltage, providing an infinite output voltage over the relay's rated range.
Optocoupled: These are solid-state relays that can switch in response to an internal light source, providing isolation between the control circuit and the power circuit.
Military/High Reliability: These relays are specifically designed to operate in harsh environments.
Relays can be further classified as normally open (NO) and normally closed (NC), with the former having the contacts open when the circuit is not energized; The latter closes the contacts when the circuit is not energized. In summary, relays are usually designated as NO or NC when not energized.
Ratings and configurations
Relays are rated based on their ability to safely switch power through the device. These ratings can be classified as AC and DC and are typically measured in amps. It is important that the rating of the relay is equal to or greater than the device to be controlled.
Relays have the ability to control multiple circuits at the same time, and their names indicate specific characteristics. These relay names include SPST, DPDT, 3PDT, and SP3T, where P and T stand for the familiar "knife" and "throw", respectively.
In addition to the number of knives and throws, the word "form" can also be used to describe the important characteristics of relays. Something like "1 Form A" or "2 Form C" highlights two key pieces of information. The Form type indicates whether the switches are normally open or normally closed, and in the case of SPDT switches, whether they are "open before closing" or "close before closing". The number (1 or 2) before the form indicates the number of contacts of this type in the relay. Some common ones include:
Form A - Normally Open
Form B - Normally closed
Form C - Make-and-close SPDT switch
Form D - Close-Then-Open SPDT Switch
Summary
Due to their simple design and reliable operation, relays are widely used in equipment and systems in various industries and markets. Relays were originally a fundamental component of telegraph systems and even contributed to the early development of computers, and still play an important role today in ensuring the safe and efficient control of electrically driven devices from remote locations.
Signal relays
Since its appearance in 1835, relays have become a widely used and very important electronic device. Despite their age, relays still play an important role in various fields. With signal relays, circuits can be controlled remotely, making them useful in a wide range of applications. Even early computers used a large number of relays to implement Boolean logic functions. Signal relays are a major subclass of relays and are purpose-specific, often playing an important role in the field of communication. This article will introduce signal relays, including the concept of signal relays, their differences from other relays, and key selection criteria.
The basics of signal relays
Signal relays are essentially electrically operated electromechanical switches that are used to control the current in a circuit. Relays use the magnetic force generated by the control current through a coil near the contact to move the internal moving parts or contacts between the pick-up and open positions. In this way, small signals can be controlled by large signals. Signal relays are similar to power relays, but are used to handle low voltages and small currents typically less than 2 A, and to switch low power signals, typically rated between 5 VDC and 30 VDC. For this reason, these relays are also known as "low-signal relays".
As mentioned above, a signal relay is an electromechanical relay that is best suited for low voltage and low current applications, and its contacts are specifically designed for low power. While power relays capable of handling higher currents and voltages may be more economical for some applications, such relays can disrupt low-power signals in audio or video circuits, thus making signal relays a more suitable choice. The signal relays are available in a small package, which is ideal for board mounting and has faster switching times. Signal relays are typically much less expensive than solid-state relays and are not affected by voltage or current transients and are not susceptible to EMI/RFI. Due to the low power handling capability of signal relays and the fact that they generate less heat than solid state relays, thermal management solutions in circuits are often not required.
Advantages of signal relays
Signal relays, like other electromechanical relays, offer a variety of advantages in a project:
The design is simple
Galvanic isolation
Stable operation
Save on the cost of long-distance cabling
Multiple package and feature options
Immunity to EMI/RFI interference
When properly matched to the power requirements of the circuit, signal relays have other advantages, such as:
Easy to operate
Small size
Affordable
Resistant to mechanical shock
The internal coil and contacts are highly insulated
The main specifications and selection of signal relays
There are several factors to consider when selecting a signal relay model for a specific design, including:
Voltage Rating: The highest voltage that the relay can switch, usually in VDC or VAC.
Rated current: The maximum current that the relay can switch in A.
Contact Resistance: The resistance added to the load circuit, in Ω.
Coil Voltage: The rated control voltage of the relay coil.
Coil Current: The rated current that the coil is subjected to at the rated voltage.
Contact type: Switching configuration of the relay (number of poles and normally open or normally closed configuration). For example, SPDT (type 1C) and DPDT (type 2C).
Contact Rating: The current and voltage values that guarantee the performance of the relay. For example, a relay rating is typically expressed as 1 A 30 VDC.
Switching time: The working speed of the relay from the application of the control current to the closing of the contacts, and vice versa.
Installation type: How the app is installed. This is usually a through-hole or surface-mount mount on a PC board.
Dielectric strength: The maximum voltage that a relay can withstand for a specified period of time without causing damage.
Operating Temperature: A specified temperature range within which the relay can operate safely and normally without degradation in performance.
Using the above checklist, engineers can select signal relays for their project. First, the required voltage and current ratings are determined based on the maximum switching load of the circuit. It is important to keep in mind that the switching capability of signal relays is typically 2 A or less.
Next, determine the required control voltage and type (whether AC or DC) and specify the number of poles/circuits to switch. Also, whether the circuit/switch layout requires normally open (NO) or normally closed (NC)
Finally, consider the method of mounting the relay in the circuit, such as panel mount, DIN rail mount, surface mount, or through-hole mount. By determining the specific parameters required, it is possible to identify a relay that meets the needs of the system without the selected specifications being too high.
Applications of signal relays
Due to their low-power switching capabilities, signal relays have been widely used in both consumer and commercial applications. Signal relays provide an effective solution for network equipment that requires signals over long distances that are higher in voltage and current than most electronic products can handle. Signal relays are also useful in applications with fast response capabilities, without the need for power relays like power relays. In addition, in everyday environments such as homes and offices, thermostats operate at low voltages and currents, but need to send signals to the furnace or air conditioning (AC) unit. Signal relays can take small, low-power control signals and use this signal to relay information to heating or air conditioning equipment so that these devices work as required.
Conclusion
As electronic systems become more advanced, it is often necessary to achieve complete galvanic isolation between the switching signal and the signal that needs to be switched. Signal relays provide a solution for the safe remote control of current and voltage. Signal relays are widely used in a variety of consumer and industrial products and systems to switch power in different types of circuits.
Power relays
The main function of the relay is to act as a switch, controlling the operation of other switches in the circuit. They use low-power input signals to control high-power circuits. The low-power signal that activates the relay triggers the electromagnet to energize, which initiates armature movement. This movement, in turn, causes the electrical contacts to close, which in turn facilitates the transfer of power to the controlled circuit.
One of the biggest advantages of this design is the ability to isolate low-power control signals from high-power circuitry. This isolation not only protects the operator from potential hazards, but also protects the equipment from potential damage. In addition, this arrangement facilitates remote control of equipment or systems for remote operation.
The origins of electromechanical relays date back to 1835, and although their components and diversity have improved greatly over the years, their basic functions have remained the same. One of the most well-known relays in history is the power relay. While all electrical relays inherently control power, it is not appropriate to refer to all relays as "power relays". This article will take a closer look at power relays, including their benefits, configurations, and main selection criteria.
Power relay basics
Power relays are known for their expertise in managing high-level current switches, ranging from a few amperes to higher orders. Due to the stronger construction and larger size of the power relay, their contacts are able to withstand large currents, which is ideal for applications that typically require currents in excess of 10 amps.
They are used in a wide range of industries, including automotive systems, elevator controllers, valve actuators, and a variety of devices characterized by high initial current surges, such as motors, solenoid valves, power supplies, and electronic ballasts.
Like other electrical components, relays have their inherent limitations when it comes to managing power safely. Each model has a maximum power rating, ensuring an effective match for a wide range of loads, from low-power entities such as light bulbs to heavy machinery such as large motors. However, exceeding the specified power rating can cause permanent damage to the relay.
In addition, contact misalignment causes contact arcing, which is characterized by the discharge of current through the air gap between the contacts when the relay contacts are open but close to each other. The risk of this phenomenon is not only the generation of sparks and heat, but also the erosion of contacts and unwanted electrical interference, which can damage nearby equipment.
Power relays are designed to address electrical loads on high-current equipment such as heaters, motors, lighting arrays, and industrial equipment. The higher current and voltage ratings of a power relay are mainly due to the fact that it uses a different switching contact material than a normal relay. These materials were selected because of their ability to withstand the rigors of high-power applications, ensuring reliable operation and durability in demanding industrial environments.
Power relay contact material
Resistance is encountered as current flows through the relay contacts, depending on the size and material composition of the contacts. The increased resistance not only amplifies the power dissipation inside the relay, but also increases the heat generation. One way to reduce contact resistance is to carefully select the contact material.
The contacts of traditional relays are usually made of silver-nickel, a metal known for its ubiquitous use in relay construction. Silver-nickel contacts perform well when switching resistive loads, where current and voltage are in phase.
In contrast, relays designed for higher loads, such as power relays, choose contacts made of materials such as silver cadmium oxide, silver tin oxide, or gold alloys. These materials are ideal for handling inductive loads, which are characterized by current and voltage being out of sync and can generate huge current or voltage spikes. Both silver cadmium oxide and silver tin oxide contacts reduce resistance and reduce the risk of contact soldering due to high inrush currents. Notably, the adoption of silver tin oxide avoids the environmental issues associated with cadmium-based alloys, thus meeting the regulatory standards adhered to by certain countries.
Comparison of power relays vs. signal relays
Power relays and signal relays are two common variants in the field of relays. While power relays prioritize handling higher voltages and currents, they typically have shorter life cycles. Conversely, signal relays are designed for longer life cycles, but operate at lower voltages and with minimal current.
The contact materials used in power relays, while good at managing high-power situations, are not well-suited for low-power switching. This is because at lower voltages, the physical connection between the contacts is critical, which is determined by factors such as contact pressure and cleanliness, rather than by the contact material.
In addition, there are inherent risks associated with the use of signal relays in power applications, which can lead to catastrophic failures due to overvoltage or overcurrent. Even if such a relay survives, it will lack critical features such as arc protection and contact self-cleaning, impacting long-term reliability.
Therefore, when deciding whether to use a power relay or a signal relay, there is an important basic guideline: always match the power level of the switch to the power rating of the relay. This ensures optimal performance, reduces the risk of failure, and maintains the integrity of the relay and associated systems.
Power relay type
Power relays, like ordinary relays, are of two main types: electromechanical and solid-state.
Electromechanical power relays rely on a combination of coils, magnetic fields, springs, movable armatures, and contacts to regulate the power output of the device.
Solid state relays, on the other hand, do not use moving parts. Instead, they utilize semiconductor devices such as silicon-controlled rectifiers (SCRs), TRIACs (AC triodes), or switching transistors to switch AC and DC currents. Compared with electromechanical relays, solid state relays have the advantages of faster switching speed and stronger reliability. However, as power demand increases, so does the cost-effectiveness due to the higher costs associated with rugged power semiconductors and the need to incorporate additional thermal management components.
Common configurations and ratings
Power relays, like non-power relays, are classified according to the contact configuration, which indicates the number of devices that the relay can control at the same time. Common classifications include:
SPST (Single Pole Single Throw)
DPDT (Double Pole Double Throw)
3PDT (Triple Pole Double Throw)
SP3T (SPDT)
Relay contacts are either normally open (NO) or normally closed (NC), depending on their state when the relay is not powered.
The relay rating indicates the maximum power at which the relay can be switched safely and efficiently. These ratings are typically expressed in amperage for AC and DC currents. The rating of the relay must exceed the rating of the device being switched, taking into account the safety factor.
Similar to non-power relays, power relays can also be described by the word "form". Phrases such as "1 Form A" or "2 Form C" can give an idea of the characteristics of the relay. The number in front of "Form" indicates the number of contacts available in the relay. "Form A" indicates a normally open relay and "Form B" indicates a normally closed relay. "Form C" and "Form D" apply to SPDT relays and indicate which position is considered normally closed, and whether the relay is open and closed or closed and closed and opened respectively. While there are many other forms, these four are the most commonly used.
Form A - Normally Open
Form B - Normally closed
Form C - Make-and-close SPDT switch
Form D - Close-Then-Open SPDT Switch
Other considerations
Other factors to consider when selecting a device include:
Input Power Surge: Some devices may generate significant power surges when they are turned on. These surges must be identified before relays are specified to prevent damage to the equipment.
Coil Suppression: Relay cycling creates high-voltage transients. Coil suppression involves the use of additional components in the circuit to protect the device from these transients. However, this may shorten the life of the relay. Determine if it is necessary to use a specific coil suppression strategy for a specific application.
Latching: The latching relay retains the last contact position after disconnecting the starting power supply. Some apps may require this feature.
Noise: Relays generate electromagnetic interference (EMI) or radio frequency interference (RFI) noise, which is more pronounced in high-power equipment. The sensitivity of the device or system to this noise should be determined beforehand.
Contact flutter: When a relay is operating, its contacts may experience a short open/close cycle, i.e., contact chatter, resulting in electrical impulses. Depending on the sensitivity of the application, this rebound can have undesirable effects, so it is important to determine whether contact flutter will affect the application before specifying the relay.
Epilogue
Relays are trusted, efficient, and reliable devices that provide safe electrical control of systems and equipment while maintaining safe isolation of the operator from the operating current. Both electromechanical and solid-state power relays are specifically designed with enhanced features to manage higher voltages and currents.
