As a high-power lamp switch. Fig. 21.11 shows the use of a triac as an a.c. on/off switch. When switch S is thrown to position 1, the triac is cut off and the output power of lamp is zero. But as the switch is thrown to position 2, a small gate current (a few mA) flowing through the gate turns the triac on. Consequently, the lamp is switched on to give full output of 1000 watts.