Abstract: A transformation circuit includes a bidirectional switch, a capacitor, and a triac. A first electrode of the triac is for receiving an input alternating current (AC) voltage. A second electrode of the triac is for outputting a transformed AC voltage. A control electrode of the triac is electrically connected to one terminal of the bidirectional switch, and the other terminal of the bidirectional switch is electrically connected to one terminal of the capacitor, and the other terminal of the capacitor is electrically connected to the first electrode of the triac. When the capacitor is charged or discharged, the bidirectional switch is switched on or off, and the triac is triggered on or off accordingly, such that a duty cycle of the input AC voltage is changed to form the transformed AC voltage.