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.
Performance specifications for solid state relays include input voltage type, input voltage range, dropout voltage, input current range, output voltage type, maximum current, load voltage, and turn-on switching technique. The input voltage type and the output voltage type is either AC or DC. There are four choices for turn-on switching: instant-on, peak switching, random turn-on and zero switching. Other unlisted switching methods for solid state relays are also available.
Output device, switch configuration, and features are additional specifications for solid state relays. The output device may be a metal-oxide field effect transistor (MOSFET), bipolar junction transistor (BJT), silicon-controlled rectifier (SCR), or triac. Switch configurations include Form A, Form B, and Form C. Many different features are available for solid state relays. Some products include analog switching, an internal snubber, or built-in heat sink. Others include over-current protection, over-voltage protection, or reverse protection. Solid state relays with reverse protection, visual indicators, and transistor-transistor logic (TTL) capabilities are also available.
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