Difference Between DIAC and TRIAC – Working and Characteristics

There are several applications where it is preferred to regulate the power fed to a load. For instance: using electrical methods controlling the speed of a motor or fan. But, these methods do not allow a fine control over the flow of power in a system additionally; there is an extensive wastage of power. In the present days, such devices have been developed which can allow a fine control over the flow of large blocks of power in a system. These devices perform as controlled switches and can complete the duties of controlled rectification, regulation and inversion of power in a load. The essential semiconductor switching devices are UJT, SCR, DIAC and TRIAC.

Difference between Diac and Triac
Difference between Diac and Triac

Earlier we have studied about the basic electrical and electronic components such as transistor, capacitor, diode, etc. But, to understand the switching devices like scr, diac and triac we have to know about the thyristor. A thyristor is a one type of semiconductor device includes three or more terminals. It is a unidirectional similar to a diode but switched like a transistor. Thyristors are used to control high voltages and currents in the motors, heating and lighting applications.

Difference between Diac and Triac

The differences between diac and triac mainly include what are a diac and triac, construction of triac and diac, working, characteristics and applications.

What are Diac and Triac?

We know that, thyristor is a half wave device like a diode and that will supply only half power. A Triac device comprises of two thyristors that are connected in opposite direction but in parallel but, it is controlled by the same gate. Triac is a 2-dimensional thyristor which is activated on both halves of the i/p AC cycle using + Ve or -Ve gate pulses. The three terminals of the Triac are MT1; MT2 & gate terminal (G). Generating pulses are applied between MT1 and gate terminals. The ‘G’ current to switch 100A from triac is not more than 50mA or so.

TRIAC
TRIAC

The DIAC is a bi-directional semiconductor switch that can be switched on in both polarities. The full form of the name DIAC is diode alternating current. Diac is connected back to back using two zener diodes and the main application of this DIAC is, it is widely used to help even activating of a TRIAC when used in AC switches, dimmer applications and starter circuits for florescent lamps.

DIAC
DIAC

Construction and Operation of DIAC

Basically, the DIAC is a two terminal device; it is a combination of parallel semiconductor layers that allows activating in one direction.This device is used to activating device for the triac. The basic construction of diac consist of two terminals namely MT1 and MT2. When the MT1 terminal is designed +Ve with respect to the terminal MT2, the transmission will take place to the p-n-p-n structure that is another four layer diaode. The diac can be performing for both the direction. Then symbol of the diac look like a transistor.

Construction of DIAC
Construction of DIAC

The DIAC is basically a diode that conducts after a ‘break-over’ voltage, selected VBO, and is exceeded. When the diode surpasses the break-over voltage, then it goes into the negative dynamic resistance of region. This causes in a reduce in the voltage drop across the diode with rising voltage. So there is a quick increase in the current level that is mannered by the device.

The diode leftovers in its transmission state until the current through it falls below, what is termed the holding current, which is usually chosen by the letters IH. The holding current, the DIAC reverts to its non-conducting state. Its behavior is bidirectional and thus its function takes place on both halves of an alternating cycle.

Characteristics of DIAC

V-I characteristics of a diac is shown below

Volt-ampere characteristic of a diac is shown in figure. Its looks like a letter Z due to symmetrical switching characteristics for each polarity of the applied voltage.

The diac performs like an open-circuit until its switching is exceeded. At that position the diac performs until its current decreases toward zero. Because of its abnormal construction, doesn’t switch sharply into a low voltage condition at a low current level like the triac or SCR, once it goes into transmission, the diac preserves an almost continuous –Ve resistance characteristic, that means, voltage reduces with the enlarge in current. This means that, unlike the triac and the SCR, the diac cannot be estimated to maintain a low voltage drop until its current falls below the level of holding current.

Characteristics of DIAC
Characteristics of DIAC

Construction and Operation of TRIAC

Traic is a three terminal device and the terminals of the triac are MT1, MT2 and Gate. Here the gate terminal is the control terminal. The flow of current in the triac is bi directional that means current can flow in both the directions. The structure of triac is shown in the below figure. Here, in the structure of triac, two SCRs are connected in the anti parallel and it will acts like a switch for both the directions. In the above structure, the MT1 and gate terminals are near to each other. When the gate terminal is open, the triac will obstruct the both the polarities of the voltage across the MT1 & MT2.

Construction of TRIAC
Construction of TRIAC

To know more about TRIAC please follow the below link: TRIAC – Definition, Applications & Working

Characteristics of TRIAC

The V-I characteristics of TRIAC are discussed below

The triac is designed with two SCRs which are fabricated in the opposite direction in a crystal. Operating characteristics of triac in the 1st and 3rd quadrants are similar but for the direction of flow of current and applied voltage.

The V-I characteristics of triac in the first and third quadrants are basically equal to those of an SCR in the first quadrant.

It can be functioned with either +Ve or –Ve gate control voltage but in typical operation generally the gate voltage is +Ve in first quadrant and -Ve in third quadrant.

The supply voltage of the triac to switch ON depends upon the gate current. This allows utilizing a triac to regulate AC power in a load from zero to full power in a smooth and permanent manner with no loss in the device control.

Characteristics of TRIAC
Characteristics of TRIAC

Thus, this is all about the difference between diac and triac, working and its characteristics. After the all the discussion in the above finally we can conclude that diac and triac are very useful for the applications of power electronics for the purpose of controlling. We hope that you have got a better understanding of this concept. Furthermore any queries regarding this concept or electrical and electronics projects, please gives your valuable suggestions by commenting in the comment section below.

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4 Comments

  1. There is a confusion left in my mind.

    1. Tarun Agarwal says:

      Hi Sanchit
      Please be specify your query so that I can clarify your confusion

  2. Krutika shelar says:

    what is mean by triac

    1. Tarun Agarwal says:

      Hi Krutika Shelar
      Thanks for your feedback and if you want to know more about triac definition and its applications please refer to the link.
      For customization of projects please email us on team@elprocus.com

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