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 shown in the below image.
Construction of DIAC
The DIAC is a two-terminal device. It is a bidirectional device. Hence, it can operate in both directions 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 terminals 1 & 2). The terminal MT1 connects 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 the P2-N3 layer.
When the MT1 terminal is positive regarding the MT2 terminal, the conduction occurs through the layers P1-N2-P2-N3. This operation is similar to a single SCR. Similarly, when MT2 is positive regarding the MT1 terminal, the conduction occurs through the layers P2-N2-P1-N1. This operation is also identical to another single SCR.
This is similar as shown in the symbol that two SCRs connect back to back. The arrows indicate the direction of the current. From the 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 break-over voltage, a small current will flow the device. This current is known as the leakage current.
The leakage current produces due to the 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 begins 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 gets in the third quadrant. The IV characteristic is as shown in the below figure.
As shown in the above figure, when the applied voltage is less than the break-over voltage, a minimal current will flow through the device, once the applied voltage increases from the break-over voltage, the current increases, and the voltage decrease.
The break-over voltage of a DIAC is between 28V to 42V and the typical turn ON time is 50 to 500 msec. Turn OFF time is around 100 nsec. The DIAC has a power handling capacity between 300mW to 1W.
Generally, DIAC is used as a triggering device of TRIAC. It is also used in the below control schemes.
- Lamp dimmer
- Fan speed regulator
- Temperature controller
- Low power device
- Does not have a control terminal
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