The circuit shown in the figure uses a TRIAC and an SBS for triggering, with a diode bridge and an optical coupler. The diode bridge causes the signals from the optical coupler to be applied to the TRIAC gate, through the SBS in the two semi cycles. Thus, when the optical coupler emitter is illuminating the phototransistor, the TRIAC gate is grounded and no trigger signal reaches that component. When the phototransistor stops receiving light, the trigger signal for the triac can reach him via SBS and this leads to his conduction with the load feed. Also, in this case, the TRIAC taken as an example is of the type of low tripping current. For higher current TRIACs, in the order of 10 mA, the resistor must have its value reduced. Values between 10 k ohms and 22 k ohms can be experienced. Depending on the trigger current of the TRIAC, the coupler must be more sensitive.
