Diode for alternating current is a trigger diode which conducts current only after the DIAC reach the breakdown voltage. The breakdown voltage of DIAC is 30 V. It has construction like an open base NPN transistor. The DIAC can be used in many application such as motor speed controls and light dimmers circuit. Figure below shows the DIAC block construction, schematic symbol and a AC Phase Control Circuit :
DIAC Basic Operation circuit schematic
Since its like transistor structure, the DIAC generate a high impedance blocking state up to a voltage breakover point(Vbo) when it enters a negative resistance region. The DIAC characteristic is generate bidirectional pulsing oscillator in a resistor-capacitor AC. [Circuit's schematic diagram source: littlefuse.com]