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Solid State Relays | Working principles, differences & application

October 26, 2019

Solid State Relays | Working principles, differences & application


Solid-state relays are the modified form of the relays. A solid-state relay is composed of a solid-state material whose features are better than the typical relays. Relays are used for automatic switching of power. The relays are used to get rid of the typical switching techniques which are manual and difficult to handle. A solid-state relay is a type of switch which is a non-contact switch. The manufacturing material for the solid-state relays is a solid-state element. The benefit of using the solid-state element is that the solid-state element can control high load current with a smaller control signal. In the solid-state relay, there is no possibility of sparking due to the characteristics of the solid-state element used in the relay. The switching OFF and ON state in the solid-state relay can be achieved easily.

Solid-state relays have different features which are:

  • There are no mechanical elements in the solid-state relays
  • Voltage rise in very low
  • Zero-crossing technology is used in solid-state relays

Solid-state relays have other advantages like:

  • Corrosion resistance
  • Vibration resistance
  • High reliability and
  • Long life.

The main feature of the solid-state relays is the higher load capacity.

Solid State Relay Working

The working of the solid-state relays can be understood by categorizing the solid-state relays into two categories. One is the AC solid state relay and the other is the DC solid state relay. It consists of one input and output terminals. The inputs and the output of the relay are attached to the specified terminals. When a specific control signal is provided to the input terminals of the solid-state relay, the ON and OFF function on the output terminal is performed accordingly and after attaining that function the switching function of the relay is turned ON. With the help of the coupling circuit in the solid-state relay, a channel is formed between the input and output of the relay.

The cutoff function is also performed by this coupling circuit in case of the unwanted or uncertain situations which are set initially before installing the relay. Optical couplers are used in the coupling circuit of the solid-state relay. The optical couplers have good sensitivity, higher response speed, higher input, and output insulation levels. The load used in the solid-state relay is a LED which is used to match the level of the input signal. The output is either “0” or “1” in the solid-state relay. The output of the solid-state relay is connected with a computer for interfacing.

In the case of the DC solid state relays, there is no snubber circuit and zero-crossing control circuit. A transistor with a larger value is used for the switching of the solid-state relay. Other working principle for this relay is the same as the above working principle.

Difference between Solid State Relays and Normal Relays

There are a number of differences between a solid-state relay and a normal relay.

Few important differences are:

  • Normal relays have mechanical elements in them whereas solid-state relays have no mechanical elements.
  • The load capacity for the solid-state relay is higher than that of the normal relay.
  • Moving part crossing is used in normal relays whereas zero crossing is used in the solid-state relays for its operation.
  • The elements used in solid-state relays are long-lasting and efficient as compared to normal relay.

Applications of Solid State Relays

The most common application of the solid state relay is the load switching of AC load. A number of household circuits have solid state relays for AC load switching. Solid state relays have different applications with the power control, moreover, different solid state relay are used for heater controls.


  • Furnaces (e.g., Baking furnaces)
  • Robots
  • Machine tools
  • Molding machines

Solid State Relay Diagram

SSR structure (image)

Structural parts of SSR (Example)