Solid State AC Relays Using Triacs
The most basic solid state relay (SSR) is shown above being a light source and a triac with a photosensitive gate. For more information on how triacs and SCRs operate see Basic Triac and SCR Projects and Circuits. A solid state relay (SSR) consists of four main parts:
Opto-Isolators with Diacs
An opto-isolator is a solid state device designed to provide electrical isolation between input and output. The input consists of a light emitting diode (LED) in a six or eight pin dip (IC) package depending on type. The output can be a photo transistor, photo diac, etc. There is no electrical contact between input and output. When the LED is turned on, the diac, transistor, etc. will conduct from the light emitted from the diode thus turning on the triac like a switch. The MOC3011 series is made to connect to triacs, the MOC301x types for 110 volts, and the MOC302x types for 240 volts. View the schematic.
A snubber circuit (usually of the RC type) is often used between MT1 and MT2. Snubber circuits are used to prevent premature triggering caused for example by voltage spikes in the AC supply or those produced by inductive loads such as motors. Also, a gate resistor or capacitor (or both in parallel) may be connected between gate and MT1 to further prevent false triggering. That could increase the required trigger current and perhaps a delay in turnoff as the capacitor discharges.
In this circuit above the "hot" side of the line is switched and the load connected to the cold or ground side. The 39-ohm resistor and 0.01µF capacitor are for snubbing of the triac, and the 470 ohm resistor and 0.05 µF capacitor are for snubbing the coupler. These components may or may not be necessary depending upon the particular and load used.
View the schematic.
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