Triac is bidirectional devices unlike SCR which is unidirectional. Triac can be activated using either negative or positive current applied to its gate unlike SCR which can only be activated using positive currents. Triac cannot be turned off using the gate once turned on. The bi-directionality makes TRIACs very convenient switches for AC circuits, also allowing them to control very large power flows with only milliampere-scale gate currents. In addition, applying a trigger pulse at a controlled phase angle in an AC cycle allows one to control the percentage of current that flows through the TRIAC to the load (phase control), which is commonly used, for example, in controlling the speed of low-power induction motors, in dimming lamps and in controlling AC heating resistors.