A relay is nothing but an electrically operated switch. To switch power on and off, a relay requires a low power electrical signal. Therefore, a relay is an automatically operated switch that uses low power electrical signal to control a high power electrical signal.
Electromechanical Relay (EMR)
As the name implies, an electromechanical relay is one that uses physically moving contact to control the flow of power in the output circuit. The movement of moving contact is generated using electromagnetic forces created by a low power input signal to the relay, which allowing the completion of the circuit that contains high power electrical signal.
Solid State Relay (SSR)
The solid state relays consist of semiconductor mechanism instead of an electromagnetic system. A low power electrical signal is used to generate optical signal within an optocoupler, which transmits and energizes the output circuit. When the relay is activated, the optocoupler acts as the switch that allows the high power signal to pass through the output circuit of the SSR. Therefore, a solid state relay does not contain any physically moving contacts for their operation.
Difference between Electromechanical Relay and Solid State Relay
| Parameter | Electromechanical Relay | Solid State Relay |
| Power Consumption | High input power is required for their operation. The power consumption depends upon the switching voltage and resistance of the contacts. | Less input power is required for the operation. Low power consumption is in the internal circuit. |
| Shock & Vibrations | Are sensitive to shock & vibrations. The electromagnetic forces acting in the mechanical system can lead to irregular and unreliable operation. | Highly resistant to shock & vibrations. Not sensitive to irregular and unreliable operation. |
| Electrical Noise | As a result of the electromagnetic system, they generate signal noise. | SSR generates less electrical disturbance. SSRs are zero voltage turn-on and zero current turn-off relays. |
| Compatibility with Control Systems | Cannot operate in the area of high electromagnetic forces. Generates arc when they interrupt current in the high power circuits. | Operation does not affected by the electromagnetic forces. Do not generate electric arcs. Do not bounce electrically or mechanically. |
| Switching Performance | Responds to control signals in around 5 to 15 milliseconds. | Responds to control signals within 100 microseconds. |
| Positional Sensitivity | The mechanical system is subjected to external forces, thus the external forces and relay action must be perpendicular to each other. | These are insensitive about position, thus can be mounted vertically or horizontally. |
