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.
Symbol
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.
n 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.
I-V characteristic
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.
Application
Generally, DIAC is used as a triggering device
of TRIAC. It is also used in the below control schemes.
2. Fan speed regulator
Disadvantages
1. Low power device
2. Does not have a control terminal
