A solid state relay is often used when a line powered device needs to be turned on and off by a control circuit, isolated from the power line. Most off-the-shelf relays require 3ma to 15ma control currents. That much power can put a strain on a current conscience battery powered control system. The relay circuit shown (Adobe PDF file) reduces the needed control current to a much more tolerable 50ua, but can still switch up to 600 watts of 120vac power. The relay circuit uses a conventional 10 amp 400 volt triac, equipped with a dv/dt snubber circuit, to switch line power to a load. However, to turn the triac on, an unconventional gate control circuit is used. The triac gate current is routed through a small bridge rectifier, a 15 volt zener diode, a sensitive SCR (Q2) and a 180 ohm 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. The capacitor C5 and the zener diode D2 filter and limit the supply voltage to about 8v while the 470 ohm resistor R3 limits the charging current. In the off state, the AC voltage across the triac is equivalent to the line voltage. In a conventional circuit, the voltage across the triac in the on state would only be a few volts. To develop a slightly higher voltage (up to 30 volts peak to peak) a 15 volt zener diode D1 is inserted in series with the SCR. The 15 volt zener diode 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). Only about 50uA of LED current in the isolator is needed to fully turn on the SCR.

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