What is a Solid State Relay (SSR)?

A solid state relay (SSR), as its name suggests, operates using semiconductors rather than mechanical components. Unlike electromechanical relays, which rely on physical contacts to open or close a circuit, SSRs have no moving parts. Instead, switching is achieved through semiconductors like triacs, transistors, diodes, and thyristors. The activation typically occurs via infrared LEDs or LED couplers, allowing for rapid and efficient switching compared to traditional electromechanical relays.

Thanks to the semiconductor-based design, SSRs can handle a variety of control voltages, either fixed or variable. They are particularly well-suited for resistive and high-power applications, such as heaters and heat tracing systems, which demand high load currents.

Working Principle of Solid State Relays

As shown in the figure, SSRs lack moving parts and instead incorporate semiconductors and electronic components.

The input side includes an LED, activated by the input voltage. Isolation between the input and output circuits is maintained.

The output side includes a photo-coupler, which captures the LED's emitted light and converts it into an electrical signal for the load drive circuit (using components like triacs, diodes, transistors, or thyristors). When power is applied to the input, current flows through the LED, emitting light that the photo-coupler detects and converts into electrical energy, subsequently sent to the load drive circuit to control the output voltage. When input power is removed, the load circuit turns off as well.

Thanks to opto-coupling technology, SSRs achieve fast, sensitive switching with excellent insulation levels. The output voltage can be digital or analog, depending on the design of the input and load drive circuits.