Relays are a common form of electrically operated switch capable of using single or multiple control signals to manage circuits through making and breaking contacts. While many types of relays consist of various moving parts that help facilitate operations, solid state relays, in particular, are a more specialized switching device that can switch between on and off states with the use of external voltage supplies to its control terminals. With their lack of moving components, solid state relays exhibit much longer service lives, making them a fairly reliable option for many applications. To help you decide whether or not a solid state relay is the right fit for your needs, we will discuss their functionality and design in more detail.
In general, solid state relays that are designed for switching DC loads will feature a single MOSFET or multiple MOSFETs that are arranged in a parallel array. As MOSFETS are constructed with a substrate diode that only conducts current in the reverse direction, a single one cannot block current both ways. For AC load switching, on the other hand, two MOSFETs will often be placed back-to-back so that the source pins are tied to one another. Meanwhile, the MOSFET drain pins are attached to any side of the output. When the relay is switched off, current will be blocked with the use of substrate diodes that are alternately reverse biased. Meanwhile, an on relay will rely on its photodiode to guarantee that both gates are positively biased.
When using solid state relays for various applications, it is important that the control signal is coupled to the controlled circuit so that galvanic isolation is maintained between each circuit. To achieve this, many SSRs will take advantage of optical coupling so that the control circuit is isolated from the load. With the use of a control voltage, an LED within the relay can be energized so that the photosensitive diode is switched on. This causes a current to be generated by the diode, ensuring that a TRIAC, SCR, or MOSFET is actuated for switching the load.
When compared to more typical electromechanical switching device options, solid state relays have various advantages that make them a desirable choice for many applications. Due to their design and electronic operation, solid state relays have a smaller profile as compared to their mechanical counterparts. This is beneficial for tighter packing which can be paramount to various assemblies. Furthermore, their lack of moving parts enhances their service lives, decreases their operational noise, and speeds up their switching times. As no sparks are generated during switching, solid state relays are safer to operate in areas where combustion and flammability is a concern.
Nevertheless, solid state relays are not without their shortcomings as well. As compared to mechanical relays, solid state relays have the chance of spurious switching as a result of voltage transients. Additionally, their body diodes can cause higher transient reverse recovery times, and they have a tendency to fail “shorted” on their outputs. Despite these examples of disadvantages, they can be a perfect choice for many settings where their optimal characteristics allow them to excel.
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