Difference between solid state relay (SSR) and mechanical relays

By April 9, 2020Article
Omron G3RV

Difference between Solid State Relay (SSRs) and Mechanical Relays

Solid state relay, commonly known as SSR, are a type of relay found everywhere in the world. The main difference between solid state relays and general relays is that there is no movable contacts in solid state relay (SSR). In general, solid state relays are quite similar to the mechanical relays that have movable contacts. One thing to highlight is that solid state relay (SSR) employ semiconductor switching elements, such as diodes, thyristors, triacs and transistors. Moreover, solid state relays (SSR) employ optical semiconductors which are called photocouplers to isolate input and output signals. Photocouplers change electric signals into optical signals and relay the signals through space, therefore completely isolating the input and output sections while relaying the signals at high speed.

Solid state relay (SSR) do not contain any mechanical contacts and are mostly of electronic parts. Thus, SSR have a wide range of features that mechanical relays do not have. The most significant feature of SSR is that they do not employ switching contacts that will wear out physically.

Solid state relay (SSR) are widely suitable in many ranges of applications due to below special characteristics:

  • SSR provide high-speed, high-frequency switching operations.
  • SSR have no contact failures
  • SSR generate minimum noise
  • SSR have no operation noise

solid state relays

Control of Solid State Relay

ON/OFF Control

ON/OFF control is a form of control where a heater is turned ON or OFF by turning an SSR ON or OFF in response to voltage output signals from a Temperature Controller. The same kind of control is also possible with an electromagnetic relay but if control where the heater is turned ON and OFF at intervals of a few seconds over a period of several years, then a SSR must be used.

Cycle Conrol

With cycle control (SSR modelG32A-EA), output voltage is turned ON/OFF at a fixed interval of 0.2 s. Control is performed in response to current output from a Temperature Controller in the range 4 to 20 mA.

Optimum Cycle Control

The basic principle used for optimum cycle control is zero cross control, which determines the ON/OFF status each half cycle. A waveform that accurately matches the average output time is output. The accuracy of the zero cross function is the same as for conventionally zero cross control.

Phase Control

With conventional zero cross control, however, the output remains ON continuously for a specific period of time, whereas with optimum cycle control, the ON/OFF status is determined each cycle to improve output accuracy.