The DIAC (Diode for Alternating Current) is a bidirectional semiconductor switch that can be turned on in both directions when the applied voltage exceeds its breakover voltage. It is used to trigger TRIACs and in thyristor-based circuits. DIACs come in various packages and have two main terminals labeled A1 and A2. They conduct current in either direction through a five-layer structure and have no gate terminal. Common applications include phase control circuits for motor speed and light dimmers.

1. DIAC
Rashmi Patil

2. What is DIAC
The term DIAC stands for the DIode for Alternating Current
(DIAC), it is a bidirectional semiconductor switch that can be
turned ON in both forward and reverse direction. The device is
a member of the Thyristor family and it is mostly used in
triggering TRIAC and other Thyristor based circuits. The DIAC
starts conducting electric current if the applied voltage goes
beyond its break-over voltage.
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3. DIAC Packages
DIACs are available in different types of DIAC
packages such as discrete components in small leaded
packages, surface-mount packages, large packages that are
bolted to chassis and various other packages. Most of the
time the DIAC and TRIAC are used together, so they are
available in integrated packages also.
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4. DIAC - Symbol
DIAC is given by the symbol of two Diodes connected in parallel
and opposite to one another and has two terminals. Since the
DIAC is bidirectional, we can’t name those terminals as anode
and cathode, the terminals of DIAC are simply called A1 and A2
or MT1 and MT2 where MT stands for Main terminals. Hence
the pinouts of DIAC are reversible just like a resistor or ceramic
capacitor.
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5. DIAC - Symbol
• You could have noticed, although it belongs to the
thyristor family it does not have a controlling gate
terminal because they can be turned on or off by
simply reducing the voltage level below
the avalanche breakdown voltage and it can be
done in both the polarities.
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6. The above diagram shows the typical construction of the DIAC. As mentioned earlier the
DIAC has two terminals namely MT1 and MT2 and it can deliver current flow in both
directions. The DIAC is made of a five-layered structure; the layers closer to the terminals
are the combination of both positive and negative layers. When the voltage is passed to the
terminals the layer with respective polarity to the voltage gets activated, this combination of
both the polarities helps in operating the DIAC in both the directions
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8. DIAC working Principle
• The above image shows the clear operation of the DIAC with
respective to the polarities. Consider the MT1 terminal to be
positive, then the P1 layer near MT1 will be activated, so the
conduction will be taking place in the order of P1-N2-P2-N3.
When the current is flowing from MT1 to MT2 the junction
between P1-N2 and P2-N3 are Forward Biased and the
junction between N2-P2 is reverse biased.
• Similarly, if we consider MT2 terminal to be positive, then the
P2 layer near MT2 will be activated and the conduction will be
taking place in the order of P2-N2-P1-N1. The current will be
flowing from MT2 to MT1 and the junctions between P2-N2
and P1-N1 are forward biased and the junction Between N2-
P1 is reverse biased. Hence the conduction will be possible in
both the directions.

10. VI Characteristics of DIAC
Initially, the resistance of the DIAC will be higher because of the Reverse Bias
junction between the layers so there will be small leakage current flowing
through the DIAC, it is mentioned as the blocking state in the curve. Once the
applied voltage reaches the breakdown voltage the resistance of the DIAC drops
abruptly and then it starts conducting which leads to a sharp decrease in voltage
and the current starts increasing, which is mentioned as a conduction state in the
curve. Most of the DIACs will be having the breakdown voltage around 30 Volts,
the exact breakdown voltage will be based on the type of the device. The DIAC
will be in the conducting state until the current reaches the particular value called
the holding current, where holding current is the minimum current that required
for a device to keep it in the ON state.

11. Applications of DIAC
The DIACs are used for triggering TRIAC or other kinds of
thyristors, apart from this they do not possess many
applications. The DIACs are used as a trigger device in various
applications such as Phase control circuits of motor speed
control, light dimmers, heat controls, and many other control
circuits. Let’s look into the examples of Light Dimmer and heat
control circuits.
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