DIAC Working with Application Circuit Simulation
The most popular and commonly used Power Electronic Switching Devices are the BJT, MOSFET , and IGBT. But when it comes to switching AC waveforms, we can frequently notice the TRIAC being used to switch current in both directions. Now, since TRIACs cannot fire/trigger symmetrically it is accompanied by a support component called DIAC. A DIAC is a two-terminal device that can act as a switch based on the voltage applied across it. In this article, we will learn more about DIAC, its construction, working and application.
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.
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.
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.
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.
VI Characteristics of DIAC
The V-I characteristic curve of the DIAC will be in the shape of a Z and the curve will be lying on the first and third quadrants because they conduct in both the positive and negative polarity. The First quadrant represents the positive half cycle where the current will be flowing from MT1 to MT2 and the second quadrant represents the negative half cycle where the current will be flowing from MT2 to MT1.
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.
How to use a DIAC?
The DIACs are mostly used within the TRIAC circuits because the TRIAC does not fire a circuit symmetrically, due to the slight difference between the two halves of the device. The non- symmetrical firing and the resulting waveform will generate unwanted harmonics in the output, the lesser the symmetrical of the waveform the greater the harmonics will be.
The DIAC is connected in series with the gate of the TRIAC in order to resolve the problems caused due to the non-symmetrical firing. DIAC can help in switching symmetrically in both the halve cycle since it has a more even switching characteristics than the TRIAC. The DIAC can prevent any gate current flowing until the applied voltage reaches a certain voltage in any of the direction, hence the firing point of the TRIAC will be more even in both directions.
Applications of DIAC
If you want a TRIAC to conduct you need to provide a positive or negative pulse to the gate, in order to provide symmetric firing the DIAC are mostly used along with the TRIAC circuit.
1. DIACs are used for triggering TRIAC or other kinds of thyristors,
2. DIACs are used as a trigger device in various applications such as Phase control circuits of motor speed control.
3. Light dimmers,
4. Heat controls,
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