How Do Solid-State Relays Work?

Solid-state relays (SSRs) are a type of electronic device with a binary switching function. Compared to similar devices, such as the electromechanical relay, SSRs provide a longer operational lifetime and can be used in a wider range of operating conditions. With many device manufacturers implementing SSRs in their architecture, it is beneficial to understand the operating principles of these switches. In this blog, we will discuss how solid-state relays work and how they are being implemented in various devices.

Unlike electromechanical switching devices, there are no moving components in an SSR. Instead, they use LED couplers to create a circuit through galvanic separation. This strategy employs an optical isolator or optocoupler, which involves an emitter that turns an electrical signal into a near-infrared LED and a receiver that detects the light and triggers an electric current on its side. SSRs also employ some other associated components to aid in the switching mechanism, some of which will be defined in the next section.

Transistors: Many SSRs use metal-oxide-semiconductor field-effect transistors (MOSFET) to perform the switching function.

TRIAC: As the name suggests, TRIACS are three-terminal components that can facilitate switching by controlling current in either direction. They are generally implemented in low to medium AC power requirement applications.

While all SSRs are configured with similar components and functions, there are several different classes to choose from.

Zero Switching: One unique class effect attribute of the zero switching SSRs is their ability to handle resistive and capacitive loads. This design allows for fast switching but has the side effect of electromagnetic interference (EMI).

Instant-On Switching: These devices are preferentially used for inductive loads and have the fastest switching time, usually close to or under 1ms.

Analog Switching: A great choice for applications that demand strict temperature control, analog switches produce a linear and reproducible output.

DC Switching: Paired with DC motors and valves with resistive and inductive loads, DC switches can deliver a response time of less than 100ms.

Zero Switching with System Monitoring: These zero-switch relays offer an added layer of safety, with an alarm that will sound in case of circuit failure.

Compared to traditional mechanical switches, SSRs pose several notable benefits. First, they deliver switching speeds far beyond what can be produced by conventional methods. Also, since they rely on semiconductors, no contact erosion occurs due to repeated switching. Finally, they generally emit little to no noise in normal operation. However, they are not without any faults. Particularly, they create a considerable amount of heat that must be safely dissipated. They also tend to fail in the "fail-closed" state, which is considered more dangerous but not of particular concern to their electromechanical counterparts. Additionally, they are more sensitive to the negative effects of transient overload occurrences.

When choosing an SSR, there are several considerations to evaluate. First, it is necessary to establish whether the relay will be switching AC or DC power since most switches are only compatible with one such function. It is also important to recognize if the application requires dimming capacity, as only certain designs can modulate the output to an exact level. Finally, one must ensure that the relay has optimal surge protection capabilities if being paired with AC power.

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