DIAC means DIode Alternating Current. Generally, DIAC uses to trigger the TRIAC. Sometimes it is also known as a TRIAC without GATE terminal. The DIAC is nothing but the combination of two SCRs connected back to back. The symbol of DIAC is as shown in below image.
Construction of DIAC
The DIAC is a two-terminal device. It is a bidirectional device. Hence, it can operate in both direction and current flow in both directions. It is a parallel inverse combination of semiconductor layers that permits triggering in either direction. The arrangement of layers is as shown in the below figure.
As we can see in the construction and symbol, the DIAC has two terminals. These terminals are MT1 and MT2 (main terminal 1 & 2). The terminal MT1 connect with the P1 layer and N1 layer. The terminal MT2 connect with the P2 and N3 layer. There is a junction (N2 layer) between the P1-N1 layer and P2-N3 layer.
When MT1 terminal is positive with respect to the MT2 terminal, the conduction takes place through the layers P1-N2-P2-N3. This operation is similar to a single SCR. Similarly, when MT2 is positive with respect to the MT1 terminal, the conduction takes place through the layers P2-N2-P1-N1. This operation is also similar to another single SCR.
This is similar as shown in the symbol that two SCRs connect back to back. The arrows indicate the the direction of the current. From above two figures it is clear that the DIAC can conduct in both directions.
Working of the DIAC
DIAC can be turned on by either the positive and negative half cycle of the AC supply voltage. If the applied voltage is less than the forward breakover voltage, the small amount of current will flow the device. This current is known as the leakage current. The leakage current produces due to drift of electrons and holes at the depletion region. Because of the small current, it is not sufficient to cause the conduction.
In this condition, the DIAC remains in a non-conducting mode. This mode is also known as the blocking mode. When the applied voltage reaches the break-over voltage, the device starts conducting. The current flowing through the device starts increasing. Hence, the voltage across it starts decreasing.
This region is known as the conduction region or conduction state. The break-over voltage of the DIAC remains unchanged because of the absence of the GATE terminal.
For the positive half cycle, the characteristic obtains in the first quadrant. For the negative half cycle, the characteristic obtains in the third quadrant. The IV characteristic is as shown in the below figure.
As shown in above figure, when the applied voltage is less than the break-over voltage, a very small amount of current will flow through the device. Once, the applied voltage increases from the break-over voltage, the current is increasing and the voltage is decreasing.
The break-over voltage of a DIAC is between 28V to 42V and typical turn ON time is 50 to 500 msec. turn OFF time is around 100 nsec. The DIAC has power handling capacity between 300mW to 1W.
Generally, DIAC use as a triggering device of TRIAC. It is also use in below control schemes.
- Lamp dimmer
- Fan speed regulator
- Temperature controller
- Low power device
- Does not have a control terminal