V- I Characteristics
The operation of the diac can be explained by imagining it as two diodes connected in series. When applied voltage in either polarity is small ( less than break over voltage) a very small amount of current called the leakage current flows through the diac. Leakage current caused due to the drift of electrons and holes in the depletion region is not sufficient to cause conduction in the device. The device remains in non conducting mode. How ever when T1 is positive with respect to T1 the layers P-N-P-N starts conducting only when applied voltage of T1 exceeds break over voltage V boi . once the conduction starts the current through the diac becomes very large and has to be limited by the external resistance in the circuit.
When T2 positive with respect to Tp the layers P-N-p-N conduct. This happens when the applied voltage of T2 exceeds break over voltage V Bo2. In both the cases the current during blocking regions are small leakage current . the behaviour in both the directions are is similar because doping level is same in all the layers in tow directions. The break over voltage for commonly used disc is about 30v.
When T1 is positive and voltage is less than Vbo1 only a small leakage current flows through the device. When voltage exceeds Vboi it starts conducting and current becomes large. As the current increase the voltage drop across diac decrease. Thus it exhibit negative resistance characteristics. The characteristics of diac in reverse direction when T2 is positive lies in the third quadrant and is exactly similar to that in the first quadrant. The break over voltage VBOi and V Bo2 are exactly equal in magnitude. In both the cases the device exhibits negative resistance behaviours during conduction region. Diac is mainly used for triggering triacs.