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Solid state Ac relay

2017-08-16 15:13  
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Asolid-state relay (SSR)is an electronic switching device in which a small control signal controls a larger load current or voltage. It consists of a sensor which responds to an appropriate input (control signal), a solid-state electronic switching device which switches power to the load circuitry, and some coupling mechanism to enable the control signal to activate this switch without mechanical parts.When zero voltage switching is not required, methods of providing this function are illustrated. The lowest parts count version of a solid-state relay is an optoisolator, the triac driver  HlU . Unfortunately, the ability of the Hll} to drive a load on a 60-Hz line is severely limited by its power dissipation and the dynamic characteristics of the detector. These factors limit applications to 30 -50 mA resistive loads on 120 Vac, and slightly higher values at lower voltages.


These values are compatible with neon lamp drive, pilot, and indicator incandescent bulbs; low voltage control circuits, such as furnace and bell circuits, ifdVI dt are sufficient; but less than benign loads require a discrete triac. The Hll]l triac trigger optocoupler potentially allows a simple power switching circuit utilizing only the triac, a resistor, and the optocoupler. This configuration will be sensitive to high values of dV/dt and noise on normal power-line voltages, leading to the need for the configuration shown in Fig . 67-14B, where the triac snubber acts as a filter for line voltage to the optocoupler. Since the snubber is not usually used for resistive loads, the cost effectiveness of the circuit is compromised somewhat. The ac relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.