What is Solid State Relay (SSR)? How do Solid State Relays Work?

What is Solid State Relay?

The Solid State Relay (SSR) is a variety of relay that is running using the electrical and optical properties of the switching process and isolation between the input and exit of the moving mechanical part of the mechanical part of the semiconductors. In industrial environments, the solid-state relay or briefly is also called SSR. As in normal relays, high currents can be switched using very low currents with SSR. An external (AC or DC) voltage is applied to the control terminals, has a structure that transmits or cuts energy.

How do Solid State Relays Work?

The Solid State Relay has no moving contacts. Physically has no mechanical contacts working with a thrust power. It works similar to the mechanical relay, but SSR uses semiconductor switching elements, such as tristor, triac, diode, and transistor. It also uses the optical semiconductor called Photocell (LED) to isolate SSR, input, and output signals. The photocell transforms electrical signals into optical signals and transmits the signals from the cavity. This completely isolates the input and output sections when transmitting the signals at high speed. (Galvanic Isolation)

ALSO SEE: What is a Superconductor? Definition, Types, and Uses

What are the advantages of the Solid State Relay?

The advantages of Solid-State relay use and the differences between an electromechanical control relay are:

  • There are no moving mechanical parts. (The greatest difference from the normal relay)
  • They provide high-speed, high-frequency switching operations.
  • Low power is consumption.
  • They are long-lived because there are no contacts to be physically frayed.
  • Contact errors are not observed.
  • They do not form noise when they work.
  • They can switch higher current loads.
  • Does not constitute sparks during his work. They can be used in explosive and challenging environments.
  • They do not form electromagnetic parasites.
  • Sizes are smaller.

What are the Disadvantages of Solid State Relay?

  • The costs are higher.
  • Depends on certain current/voltage characteristics.
  • Due to the internal impedance, the voltage drop in the fixed value may be observed.
  • Even if the relay is in closed mode, leakage current occurs.
  • Produces waste heat in open mode. This situation is proportional to the voltage drop. Some configurations of solid-state relays may require external cooling.
  • Unlike the electromagnetic relay, it can be triggered with short voltage rises on the input side.
  • Unlike the electromechanical relay, it is more sensitive to the fluctuations and sudden elevations in the current side on the output side.

What are Solid State Relay Mounting Types?

In addition to the fact that there are many different types of solid-state relay switches sold in the world markets, there is also a wide range of installation solutions to physically assemble their circuits or to maintain as part of a wider electrical system.

SSR with PCB connection

Designed to be mounted directly on a printed circuit board.
Provides quick and easy installation with the main cards and other PCB types through the push pin fittings or the requirement of soldering directly to the PCB surface.

Panel Assembled SSR

The most common and flexible key types are between and are manufactured to be fitted with various industrial and equipment panels, covers, or coolants. Mounted versions of the casing offer functionality and comfort. The SSR base is attached through the screws that allow direct fixing through the body or purpose-designed rings.

Where is the Solid State relay used?

Solid-State relays are widely used in sectors and industries in a wide range of applications, including:

  • Industrial controls
  • Engine control
  • Robotic
  • Medical equipment and patient/equipment isolation
  • Instrumentation
  • Multiplexers
  • Data collecting
  • I / O Systems
  • Counters (electricity, water, gas)
  • IC equipment
  • Household appliances


  1. https://www.electronics-tutorials.ws/power/solid-state-relay.html
  2. https://www.linkedin.com/pulse/what-working-principle-solid-state-relays-jacob-chen/

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