In the face of a wide variety of relay models on the datasheet, you may feel difficult to choose. Once the contact rating, coil voltage and protection function are taken into account, it will be more complicated to make decisions in many types of relay. In this article, the relay is sorted out according to the structure, contact layout and application scenarios, and the actual differences that really affect the circuit design are pointed out.
What Are the Main Types of Relay by Construction?
According to the structure, the main types of relay include electromagnetic relay (EMR), solid state relay (SSR), reed relay and hybrid relay.
Electromechanical Relay
The electromagnetic relay is composed of coil, armature and spring reset contact group. When current flows through the coil, it generates a magnetic field that attracts the armature and drives the contacts to switch, producing an audible “click” sound during the switching process. The switching speed is at a moderate level. For an universal control cabinet, a DIN-rail electromechanical relay module can meet the needs of most single-loop switches without additional cost.
Solid State Relay
Solid state relays replace coil and mechanical contacts with semiconductor components such as three-terminal bidirectional thyristors (Triac) or MOSFETs. It switches silently and does not produce arcing, and the number of switching times it can withstand is much higher than that of mechanical relays.
Reed Relay
The reed relay seals two ferromagnetic reeds in the glass tube, and the external winding coil is energized to produce a magnetic field to make the reed snap. The sealing structure makes the contact not affected by dust and moisture, so the reed relay is suitable for low current and fast switching application scenarios such as test equipment and telecommunication equipment.
Hybrid Relay
The hybrid relay combines the electromagnetic stage with a solid state stage: the solid state part handles the instantaneous switching moment to reduce arcing, then the mechanical contact carries the steady current, which limits heat buildup in the semiconductor.
How Do Relay Contact Configurations Differ?
Relay contact configurations are described by poles and throws, most commonly SPST, SPDT, DPST and DPDT. These marks determine how many circuits one relay can switch.
Pole refers to an independent circuit path inside the relay, and throw refers to the number of switch positions that the circuit can reach. A Single Pole Single Throw (SPST) relay only controls one circuit to realize the basic on/off action. A Single Pole Double Throw (SPDT) relay can switch the same circuit between two different contacts.
Double Pole Single Throw (DPST) and Double Pole Double Throw (DPDT) repeat the above logic for two isolated circuits in the same relay housing. They are suitable for occasions where the main power supply and the state indicator need to be switched at the same time with the same control signal.
Normally Open (NO) and Normally Closed (NC) states are usually marked in the datasheet to describe the position of the contact when the coil is not energized. This is easy to make a mistake when wiring a distribution cabinet: normally closed contacts remain closed when the coil is not energized, contrary to the usual perception of many people.
Compared with only referring to the specification parameter table, directly browsing TOMZN’s full relay product lineup can more intuitively compare the differences between different pole numbers and throw numbers.
What Protective Relay Types Are Commonly Used in Power Systems?
Protective relay types include overcurrent, differential, distance and earth fault relay, each of which is used to detect a specific fault state on a transmission line or transformer.
Overcurrent Relay
The overcurrent relay trips the circuit breaker when the current exceeds the set threshold, which protects the cable and equipment from thermal damage.
Differential Relay
Differential relays compare the current flowing into and out of the protected area (such as transformer windings), and only act when the two values are inconsistent, which usually means that an internal fault occurs rather than an external fault.
Distance Relay
The distance relay measures the impedance rather than the current, so it can estimate the distance of the fault point on the transmission line, so as to achieve selective tripping.
Earth Fault Relay
Earth fault relay monitors the leakage current to the earth, which is often undetectable by ordinary over-current protection.
In smaller-scale applications, similar products at the switchboard level, such as the adjustable overcurrent protection relay, apply the same tripping logic to a single loop protection in residential and light commercial switchboards, rather than to the entire substation bus.
Electromechanical vs Solid State Relay
Electromagnetic relay (EMR) is suitable for high current and low frequency switching scenarios, while solid state relay (SSR) is more suitable for fast, high frequency or quiet switching scenarios. The switching time of the solid state relay is 1 to 10 milliseconds, while the electromagnetic relay is 5 to 15 milliseconds; when the switching frequency exceeds hundreds of times per minute, the effect of this speed difference will be very obvious.
Electromagnetic relays are usually the default choice for large current loads exceeding about 50 amperes or 600 volts. Because its physical contacts can reliably withstand surge currents and the cost per pole is lower. However, the mechanical parts of the electromagnetic relay do wear out, and each switch emits an audible click sound, which is not popular in home appliances that require quiet operation.
The solid state relay does not have this wear problem because it uses a three-terminal bidirectional thyristor or MOSFET to carry the load, rather than a spring contact. This feature makes solid state relays more suitable for HVAC controllers, dimmers, and panels that need to be switched hundreds of times a day.
The disadvantage of solid-state relays is heat: once the current increases, a heatsink needs to be installed, and the leakage current in the off-state cannot always be reduced to zero.
When the load exceeds the load range of the standard relay, the heavy-duty contactor and relay range can apply the same switching control method to higher current levels.
In the mixed configuration of the distribution panel, the use of electromagnetic relays in the high current branch and the use of solid state relays in the high frequency switching branch often achieve the best balance between cost and service life.
What Are Latching and Time Delay Relay Types Used For?
The last switching state can be maintained without continuous power supply, and the time delay relay adds a delay before or after switching.
The latching relay maintains the contact in place after the coil signal ends through magnetic pulses or mechanical snaps. This feature can effectively save power in scenarios where the state can be changed by only one short pulse, such as battery power supply system and stair lighting circuit.
The delay relay covers a variety of functions: on-delay starts the load after the set waiting time is over; the off-delay allows the load to run for a period of time after the signal stops; the interval timing allows the load to repeatedly cycle the switch. The HVAC compressor often uses the power-off delay function to prevent short cycling, because the compressor restarts too fast and may cause damage.
FAQ
What is the difference between electromagnetic relay and solid state relay?
Electromagnetic relays rely on coils to drive mechanical contacts to complete switching, and solid-state relays rely on semiconductor components to complete switching, with no moving parts. The switching time of the solid-state relay is 1 to 10 milliseconds, and the electromagnetic relay is 5 to 15 milliseconds, so the solid-state relay is faster, quieter, and longer.
What do SPST, SPDT, DPST and DPDT mean for relay?
These letter combinations describe the number of poles and the number of throws: Single Pole Single Throw controls the on-off of a circuit; Single Pole Double Throw switches the same circuit between two contacts. Double Pole Single Throw and Double Pole Double Throw control the on-off or switching of two isolated circuits in the same shell.
What are the different types of protective relays?
The types of protective relays include overcurrent relays, differential relays, distance relays and ground fault relays, which are used to detect specific fault types in power systems. IEC 60255-1xx series standards have standardized such functional tests to ensure consistency of performance evaluation results worldwide.
What is a latching relay used for?
After the coil signal stops, the latching relay relies on the magnetic pulse or mechanical snap to maintain the state after switching, without continuous power supply. This is suitable for stair lighting, battery-powered devices, and circuits that pay more attention to reducing standby power consumption rather than instant reset.
What is a time delay relay used for?
The delay relay can add a settable delay in the process of circuit connection, disconnection or both, so as to protect HVAC compressors and other equipment and avoid damage caused by short-term frequent start and stop.
Choosing the Right Relay Type
By sorting out the types of relay according to their structure, contact configuration and function, you can simplify a complex specification book into a clear selection list:
- Firstly, according to the switching frequency and load current, the electromagnetic relay or solid state relay is selected.
- Then match the number of poles and the number of throws according to the number of circuits involved.
- Finally, protection, self-locking or delay functions are added only when the application is really needed.
Clarifying these steps for the first time can avoid subsequent rework and redesign. The DIN rail relay protective device and timing relay produced by manufacturers such as TOMZN are designed based on these classifications.
