An Electromechanical Relay (EMR) is based on a coil, a magnetic field, springs and mechanical connectors. An electromagnet is designed with a coil winded around an iron core. The electromagnet is driven by a current that controls the position of one or several metal springs so that they either get in contact or loses contact. Switching time is typically 10 to 15 milliseconds.
Solid State Relays
In a Solid State Relay (SSR), the electrical and optical characteristics of semiconductor materials are used for isolation and switching functions. Without moving parts, switching can be done faster and more reliable. Switching time is lower than one millisecond. Electrical interference is also reduced. In a solid state relay, either AC or DC currents can be switched depending on if a thyristor, TRIAC or switch-transistor is used on the output. A small DC voltage, usually between 3 and 32 V, can be used to control a higher voltage and current, for example 240 V and 10 A. An optocoupler isolates the input from the output. The optocoupler consist of a light emitting diode and a photo sensitive transistor that detects the light. Since the connection between input and output is light, it is possible to achieve a high voltage isolation of several thousand volts.