Solid State Relays

Solid state relays (SSR) or semiconductor relays are semiconductor devices that can be used in place of mechanical relays to switch electricity to a load in many applications. They are purely electronic devices, normally composed of a low-current control side and a high-current load side (switching side). Many solid state relays (SSR) feature electrical isolation in the thousands of volts between the control side and the load side. Typically, this is achieved through optical isolation using an optoelectronic device (a photocoupler).
There are four main types of solid state relays: aerospace or MIL-SPEC, non-isolated, optically isolated, and transformer isolated. Aerospace of MIL-SPEC relays comply with aerospace or U.S. military standards. Non-isolated relays do not have an isolation mechanism between the control (input) and its output (load). Optically-isolated relays do not have any direct electrical connection between the input and the output. Transformer isolated devices are solid state relays (such as pulse transformers) which are used to trigger triacs.
Solid state replays carry parameters for application type, mounting style, and operating temperature. Some solid state relays are used in automotive or latching applications. Others are designed for general-purpose use. There are five choices for mounting style: bracket (flange), DIN rail, panel, printed circuit board (PCB), and socket or plug-in style. Operating temperature reflects the surrounding air-temperature limits and is a full required range.