What is the working principle of solid-state relays?

Solid State Relays (SSRs) have revolutionized automation and control systems since their introduction in 1974. These integrated electronic switches, composed of integrated circuits and discrete components, perform the functions of traditional electromechanical relays without any moving parts. SSRs achieve electrical isolation between the control system (input circuit) and the controlled system (output circuit) through optocouplers or other coupling methods. Due to this design, SSR can operate at high frequencies, with little control power, great reliability, noiseless operation, quick switching speeds, and high working frequencies.

Typically, SSRs are modular four-terminal active devices. Two terminals serve as the input control side, while the other two are for the output side. Optocouplers are commonly used within SSRs to ensure electrical isolation between input and output. To achieve contactless and spark-free switching operations for external control circuits, the output terminal makes use of semiconductor devices (such as switching transistors or three terminal bidirectional thyristor switches). Without moving parts or contacts, SSRs are capable of carrying out tasks that conventional electromechanical relays might handle.

The internal equivalent circuit of a typical SSR is straightforward. When an appropriate control signal is applied to the input terminal (IN), the output terminal (OUT) transitions from an off state to an on state. Removing the control signal returns the output to the off state. This mechanism enables contactless automatic control of the power supply to the load connected to the output terminal circuit.

SSRs are classified into DC and AC types based on the polarity of the output terminals. The circuit principle of a DC SSR involves the control voltage being input through the IN terminal. This signal is coupled via an optocoupler to the receiving circuit, amplified, and used to drive the switching transistor (VT) into conduction. Hence, the OUT terminal in a DC SSR has distinct positive and negative polarities. Conversely, an AC SSR uses a triac (VS) or other AC switches for its switching element, meaning the OUT terminal has no polarity and can control AC circuit switching.

Despite their many advantages, SSRs also have some limitations. One major drawback is the voltage drop at the output terminal when conducting, leading to power dissipation that necessitates appropriate heat dissipation measures. Furthermore, SSRs exhibit a certain amount of leakage current when off. Other limitations include a relatively simple control state, poor overload capacity, and the lack of interchangeability between DC and AC SSRs.

In certain ways,this relays are prime examples of the continuous improvements in relay technology. Compared to conventional electromechanical relays, they provide numerous additional features and advantages. SSRs are anticipated to gradually replace electromechanical relays in a variety of applications as technology advances, despite several drawbacks.As time goes on, our XRS also works nonstop to find solutions to supply issues for clients in need of this kind of relay. We have a large number and a wide variety of relay model products. If you are still struggling to find the corresponding product, why not consult us? Maybe you can find an unexpected shortcut to purchase.