Difference between DIAC and TRIAC : Working & Their 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 fine control over the flow of power in a system additionally; there is an extensive wastage of power. In the present day, such devices have been developed which can allow 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. Earlier we have studied the basic electrical and electronic components such as transistors, capacitors, diodes, etc. But, to understand the switching devices like SCR, DIAC and triac we have to know about the thyristor. A thyristor is one type of semiconductor device that includes three or more terminals. It is 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. The symbols of DIAC and TRIAC are shown below.


Difference between Diac and Triac
Difference between Diac and Triac

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 that 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.

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 fluorescent lamps.

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 the device for the triac. The basic construction of DIAC consists 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 diode. The DIAC can be performing for both the direction. Then the symbol of the DIAC looks like a transistor.

DIAC Construction
DIAC Construction

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 the region. This causes a reduction 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 are shown below.

The volt-ampere characteristic of a DIAC is shown in the figure. It looks like a letter Z due to symmetrical switching characteristics for each polarity of the applied voltage.

DIAC Characteristics
DIAC Characteristics

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 the transmission, the diac preserves an almost continuous –Ve resistance characteristic, which means, voltage reduces with the enlarge in the 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.

Construction and Operation of TRIAC

TRIAC 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 which means current can flow in both directions. The structure of TRIAC is shown in the below figure. Here, in the structure of triac, two SCRs are connected in the antiparallel and it will act as a switch for both 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 both the polarities of the voltage across the MT1 & MT2.

TRIAC Construction
TRIAC Construction

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.

TRIAC Characteristics
TRIAC Characteristics

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 function with either +Ve or –Ve gate control voltage but in typical operation generally, the gate voltage is +Ve in the first quadrant and -Ve in the 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.

Why DIAC is used with TRIAC?

The main purpose of using DIAC with TRIAC is, TRIAC device does not fire symmetrically thus there is a slight difference between the two halve of the device. The non-symmetrical firing, as well as resultant waveforms, can give an increase to the unnecessary harmonics generation. The less symmetrical waveform increases the harmonic generation level. To solve the issues which result from the nonsymmetrical process, a DIAC is frequently arranged in series through the gate.

This DIAC device assists in making the switching more for both halves of the cycle. So the switching characteristic of this device is far more as compared with the TRIAC. As the DIAC stops any gate current supply when the trigger voltage reaches a certain voltage within any direction, then this will make the TRIAC firing point more in both directions also. So, DIACs may be frequently used with the TRIAC gate terminal.

These are extensively used components in conjunction with TRIACs to balance their switching characteristics. So, when the switching AC signals are reduced. Then the level of harmonics will generate. Although, two thyristors are normally used for large applications. But the combination of DIAC/TRIAC is extremely helpful for lower-power applications like light dimmers, and many more

DIAC/TRIAC Power Control

The power circuit of DIAC/TRIAC is shown below. This circuit starts works when the capacitor starts charging throughout the +Ve half cycle. Once the capacitor gets charged upto Vc, then the DIAC component will start conduction. When the DIAC activates, it provides a pulse toward the gate terminal of the TRIAC because of where the TRIAC starts conduction as well as current supplies through RL
In the negative half cycle, the capacitor will charge in opposite polarity.

Power Control Circuit
Power Control Circuit

Once the charging of the capacitor is done upto Vc, the DIAC will start conducting to provide a pulse to the TRIAC, then the current will supply throughout the RL. We know that the DIAC working can be done on two polarities because the two connections of two diodes can be done in parallel with each other, so it conducts on both polarities. The DIAC output can be given to the gate terminal of the TRIAC which is used to make the TRIAC ON conduct so that the load like lamp will be turned ON.

Difference between DIAC and TRIAC

The difference between DIAC and TRIAC include the following.

DIACTRIAC
The acronym of the DIAC is “Diode for the alternating current”.

 

The acronym of the TRIAC is “Triode for the alternating current”.

 

DIAC includes two terminalsTRIAC includes three terminals

 

It is a bi-directional and uncontrolled device

 

 It is a bi-directional and controlled device.

 

This name is derived from the combination of DI + AC, where DI means 2 & AC means alternating current.This name is derived from the combination of TRI + AC, where TRI means 3 & AC means alternating current.
 It can control both positive and negative half cycles of AC signal input. DIAC can be switched from its off state to ON state for either polarity of the applied voltage.
The DIAC construction  can be done either in NPN otherwise PNP formThe construction of TRIAC can be done with two separate devices of SCR.
It has less power handling capacityIt has a high power handling capacity
It doesn’t have a firing angleThe firing angle of this device ranges from  0-180° & 180°-360°.
This device plays a key role inactivate the TRIACThis device is used to control the fan, light dimmer, etc.
It has three layersIt has five layers
The advantages of DIAC are, it can be activated by decreasing the level of voltage under its breakdown voltage. Triggering circuit using DIAC is cheapThe advantages of TRIAC are, It can work through the +Ve as well as -Ve polarity of pulses. It uses a single fuse for protection. A secure breakdown can be possible in both directions.
The disadvantages of DIAC are, it is a low-power device and doesn’t include a control terminal.

 

 The disadvantages of TRIAC are, it is not reliable. As compared with SCR, these have low- ratings. When operating this circuit, we need to be cautious as it can activate in any direction.
 The applications of DIAC mainly include different circuits like lamp dimmer, heater control, universal motor speed control, etc.The applications of TRIAC mainly include control circuits, fans controlling, AC phase control, switching of high-power lamps, and controlling AC power.

Controlling of AC Voltage through DIAC & TRIAC

A semiconductor device like a TRIAC is used to control the current supply. The operation of this is similar to two thyristors which are connected in reverse parallel through a gate connection. Therefore, it can be activated into conduction.

These are utilized in power control to provide full-wave control. It controls the voltage among zero as well as full power. In many industries, over-voltage as well as under voltage problems can occur. Thus it causes a huge impact on output. To overcome this, we should use voltage controllers for controlling the voltage. A device like TRIAC gives an extensive range of control within an AC circuit without using exterior components.

AC Voltage Control Circuit
AC Voltage Control Circuit

In this circuit, the lamp is used as a load. We can observe the change in the light by changing the variable resistor. So, the readings of the lamp like voltage as well as the current can be observed at different steps. In a cathode ray oscilloscope, we can observe the waveform. The phase angle variation can also be observed by changing the potentiometer.

The AC voltage controllers are available in two types based on input supply given to the circuit like single phase & three phases. The operating of single-phase controllers can be done using a single voltage supply like 230v at 50Hz, whereas in three phases, the supply voltage will be 400v at 50 Hz. So, the break over voltage of a DIAC device is at 30 volts range.

DIAC and TRIAC Applications

The applications of DIAC and TRIAC mainly include the following.

  • The major application of DIAC is, it can be used in a triggering circuit of TRIAC by connecting the TRIAC’s gate terminal. Once the voltage which is applied across the gate terminal decreases under a fixed value, then the voltage at the gate terminal turns to zero & therefore the TRIAC will be deactivated.
  • DIAC is used to build different circuits like lamp dimmer, heat control, the universal motor speed control circuit & starter circuits used in fluorescent lamps.
  • TRIAC is used in the control circuits such as motor control, fan speed controlling, light dimmers, switching of high-power lamps, controlling of AC power in domestic applications.

Thus, this is all about the difference between DIAC and TRIAC, working and its characteristics. After 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.

3 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

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