Posted on Feb 17, 2013

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.

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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. Even with this disadvantage, the labor, board space, and inventory of parts savings of this circuit prove it cost-optimized for isolated logic control of power-line switching. In applications where transient voltages on the power line are prevalent, provisions should be made to protect the Hll}l from breakover triggering.

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