ANALOG ELECTRONICS

A better device is the triac, bidirectional thyristor, which allows the full a.c. power to be controlled. It comprises two thyristors connected in parallel and working in opposition but controlled by the same gate. The trigger pulses to a triac are provided by a special device called diac, bidirectional breakdown diode. It is, in effect, two Zener diodes connected back to back. The diac conducts in either direction if the applied voltage reaches the breakdown voltage of about 30 V. This