A relay is a type of electrically operated switch, and they are employed in electrical systems for the means of controlling a circuit with a low power signal or for managing several circuits with a single signal. Relays have been extensively used in technology since their inception, and many advanced types have come about over the years to provide unique advantages and characteristics. Electromechanical and solid-state relays are the two common classifications for relay components, and they both feature various differences and similarities that warrant their comparison.
An electromechanical relay is capable of transferring signals between contacts through the means of mechanical motion. A typical electromechanical relay is composed of two main sections, those of which are the electromagnet and the armature and contact section. The electromagnet of the electromechanical relay is a magnetic core that is surrounded by a wound wire coil. As electricity is induced to the relay, current will flow through the coil, causing it to become energized. This results in the generation of a magnetic field, that of which is powerful enough to actuate the armature assembly for the means of establishing or breaking contact connections due to their attachment. When the electricity is released from the relay, the magnetic field drops so that the armature may return to its resting state with the aid of a spring return arrangement.
Solid-state relays, or SSRs, are most notably different due to their complete lack of any mechanically moving parts. Instead, such relay types are fitted with a number of semiconductor and electronic components that serve to replace the mechanical components of the electromechanical relay. Rather than rely on an electromagnet for the means of operating an armature, a typical solid-state relay features an optocoupler and driver circuits. The output contact section is also replaced for such relays, and they instead use either a TRIAC or a transistor, snubber, and driver circuits. Similar to the electromechanical relay, electrical current is induced in the solid-state relay and it will flow through the optocoupler. Based on the output of the optocoupler, a switching circuit is used to provide the TRIAC with a gate pulse to allow for current flow. When the current is removed, the optocoupler ceases to operate the TRIAC switching circuit and thus the TRIAC will no longer conduct.
Due to the difference in parts and operations, there are various advantages and disadvantages to each relay type that should be considered when making a purchasing decision. Due to the lack of any moving mechanical parts, a solid-state relay will often exhibit a much longer service life due to less wear and is quieter in operation for similar reasons. Despite this, solid-state relays can generate large amounts of heat due to their semiconductor components and thus require a heat sink for proper thermal management, unlike electromechanical types where heat is rarely a concern. Generally speaking, electromechanical relays are most often found in motor controls, automotive applications, and industrial applications where there needs to be control over high voltages and currents. Solid-state relays, on the other hand, are primarily found in applications where they are switching AC loads for ON/OFF switching, motor speed control, light dimming, and more.
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