50_μA_SOLID_STATE_RELAY

Published:2009/7/17 4:06:00 Author:Jessie | From:SeekIC

The relay circuit described here reduces the control current to 50 μA, but can switch up to 600 W of 120-Vac power. The circuit uses a conventional 10-A, 400-V triac. The triac gate current is routed through a small bridge rectifier, a 15-V zener diode, a sensitive SCR (Q2), and a 180-Ω resistor (R2). The circuit allows a small SCR to control ac current through the triac's gate terminal. The voltage needed to control the SCR's gate terminal is developed by rectifying and filtering the ac voltage across the triac. Capacitors C2, C4, and C5, resistor R3, bridge rectifier BR2, and zener diode D2 form the rectifier circuit. C5 and D2 filter and limit the supply voltage to about 8 V, while the 470-Ω resistor R3 limits the charging current. To develop a slightly higher voltage (up to 30 V p-p), a 15-V zener diode (D1) is inserted in series with the SCR. D1 delays the triac's conduction trigger point each half cycle and produces only a slight reduction in rms power to the load. The dc control voltage, produced by the rectifier circuit, is switched to the SCR's gate using a sensitive Darlington-type optoisolator (A1)i Only about 50 μA of LED current in the isolator is needed to fully turn on the SCR.



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