Introduction to Industrial Automation

2.1.9   Solid State Relays

Solid state relays (SSRs), like the one presented in Figure 2.14, are electronic devices with no 

mechanical contacts capable of switching various AC or DC loads, such as heating elements, 

Power

supply 

t

t

ON

Open

V

in

NO contact 

Closed

OFF

Figure 2.13  Time behavior of an electric contact of a pulse bistable relay due to a series of four 

successive pulses.

Figure 2.14  Typical external view of a solid-state relay by Siemens.

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Hardware Components for Automation and Process Control 

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motors, and transformers. SSRs perform the same switching function as the electromechanical 

relays, however their structure is quite different, consisting of semiconductor switching elements, 

such as thyristors, triacs, transistors, and diodes. Also, SSRs include a semiconductor-type opto-

coupler to separate the input circuit from the output circuit, offering complete isolation of the 

input and output signals. As shown in Figure 2.15, the control signal (which corresponds to the 

coil voltage of the electromechanical relays) is applied to a light-emitting diode (LED). The light, 

or infrared radiation, is detected from a phototransistor that triggers the triac, which switches 

on the load current supplied from an external source. In order to avoid temperature rise in the 

triac junction, SSRs are usually equipped with a heat sink made by a heat-conductive metal and 

integrated in their body in order to dissipate the heat to the surrounding air. Because of their elec-

tronic structure, SSRs provide high-speed or high-frequency ON-OFF switching operations and 

generate low noise. Furthermore, due to the absence of movable contacts, they do not produce 

electric arcs and hence are suitable for use in hazardous areas. In chemical, petrochemical, min-

ing, and many other industries where combustible materials are transported, stored, or processed, 

potentially explosive atmospheres are inevitable. In such cases, the use of SSRs as load switches is 

mandatory, according to national laws and regulations, since they ensure reliable operation and 

safety for personnel and machinery, even in extremely explosive environments.