solid state relay

elektor electronics september 1985 J Steeman Electromagnetic relays have one or several sets of contacts which open or close when a soft-iron core is magnetized by a coil around it. Solid-state relays involve no mechanical movement whatsoever, as switching is effected by a single silicon-controlled rec- tifier (SCR) or two SCRs in a common envelope - normally called a triac. This type of relay is of great importance in digital circuits. Note that the SCR was originally called thyristor. Solid-state relays have a much better lifespan than electromagnetic types, particularly at high rates of switching. They also exhibit far less electrical noise and may be used in explosive environments since there are no contacts across which arcs can form. And, of course, they are completely free of mechanical noise. The basic concept of a solid-state relay is shown in Figure la. The switch may take the form of an SCR in a bridge circuit as in Figure lb. This configuration enables both the positiv...