In this guide, we’ll examine various aspects of solid state relays, how they function, and what sorts of environments they’re best suited to. Firstly, in order to understand what a solid state relay (SSR) is, it’s important to know what a basic or standard electromechanical relay (EMR) is, and precisely how the two types differ.
In electrical terms, a relay is a relatively simple switching device that’s used to automatically close or open a set of contacts between two circuits. This process is triggered by an electrical input or control signal of some kind, in response to which the relay switch usually moves from an ‘off’ to an ‘on’ position.
In so doing, the relay closes (or, less commonly, opens) a specific set of contacts, thus completing (or breaking) the circuit(s). In most cases, when the electrical relay device is receiving no signal, the contacts are open and the circuit is effectively incomplete. When an electrical control signal is sent to the relay terminals, it triggers a physical response in the relay switch, typically causing it to close those contacts and complete the circuit as required.
In a standard relay, this process is electromechanical (hence these relays are referred to by the acronym EMRs). In other words, an electrical control signal triggers a mechanical response in the relay. The standard mechanical relay switch is therefore composed of key moving parts, and it’s these parts that physically change position in order to open/close the contacts and complete or break the circuit as required.
By contrast, a solid state relay has no mechanical or moving parts. As electrical components, SSRs are widely used to perform much the same on/off, open/close functions as standard relays, but a solid state switch does so entirely without any physical movement within the relay itself. (We’ll look more closely at how this works in the next section of this guide.)
Solid state relays can be designed to operate either based on AC or DC input currents, depending on the specific model and applications. Common voltages for DC input include 5V, 12V and 24V DC solid state relays, while widely available examples of AC solid state relays are often based around 120V or 240V AC input.
The term ‘solid state relay’ is actually a fairly generic one, and can, in fact, refer to all manner of different relay components and configurations used to achieve the basic on/off switching function.
Many of these configurations will be designed around specific industries or applications. Solid state relays made for automotive use, for example, tend to be optimised for plug-and-play installation methods and resilience against harsher environmental conditions (anti-vibration, over-voltage protection, polarity reversal and short circuit protection) than you’d typically encounter in, say, industrial or production applications.