What is a TRIAC : Working & Its Applications

We know that the unidirectional device like an SCR includes the characteristics of the reverse blocking current because it stops the flow of current in a reverse-biased state however for several applications, this kind of current control is necessary especially in AC circuits. So this can be attained through SCRs, where the connection of two SCRs should be done in anti-parallel for controlling over both positive as well as negative input half cycles. But, this arrangement can be changed through a special semiconductor device called TRIAC, used to achieve bidirectional control. This device controls the AC precisely and frequently used in controlling the speed of the motor, AC control devices, AC circuits, light dimmers, pressure control systems, etc.

What is a TRIAC?

TRIAC (Triode for AC) is a semiconductor device widely used in power control and switching applications. It finds applications in switching, phase control, chopper designs, brilliance control in lamps, speed control in fans, motors, etc. The power control system is designed to control the distribution level of AC or DC. Such power control systems can be used to switch power to appliances manually or when temperature or light levels go beyond a preset level.

TRIAC or Triode for Alternating Current
TRIAC or Triode for Alternating Current

It is equivalent to two SCRs connected in inverse parallel with the gates connected together. As a result, it functions as a Bidirectional switch to pass the current in both directions once the gate is triggered. It is a three-terminal device with the Main terminal1 ( MT1), Main terminal 2( MT2), and a Gate. The MT1 and MT2 terminals are used to connect the Phase and Neutral lines while the Gate is used to feed the triggering pulse. The Gate can be triggered either by a positive voltage or negative voltage.

When the MT2 terminal gets a positive voltage with respect to the MT1 terminal and the Gate gets a positive trigger, then the left SCR of the TRIAC triggers and circuit completes. But if the polarity of the voltage at the MT2 and MT1 terminals is reversed and a negative pulse is applied to the Gate, then the right SCR of Triac conducts. When the Gate current is removed, the TRIAC switches off. So a minimum holding current must be maintained at the gate to keep the TRIAC conducting.


The construction of TRIAC is shown below. It includes four layers as well as six doping regions. The designing of its gate terminal can be done with an Ohmic contact using two regions namely the P region and N region so that this device can activate through both the polarities. Even though it is a bidirectional device, where the current and voltage of this can be specified with MT1 like the reference to diminishing confusion.

In the case of SCR, the terminals of the TRIAC can be denoted with MT1 and MT2 like an anode and cathode, and the gate terminal can be represented through ‘G’ like the thyristor. The gate terminal ‘G’ is connected to both P2 & N4 regions through a metallic contact and it is close to the MT1 terminal.
The connection of MT1 can be done to both the regions of P2 & N2 whereas MT2 can be connected to the regions of both P1 & N3. Therefore, the two terminals like MT1&MT2 are connected to both the regions of P & N of the device. Thus, the current flow between these two terminals can be decided through the layers in the device.

MT2 is connected to positive by the gate open as compared to MT1 for a TRIAC which is connected in forward biased. Thus, TRIAC works in the mode of forward blocking until the voltage across the TRIAC is low as compared to forwarding break overvoltage. Likewise, the MT2 terminal is made negative when a TRIAC is connected in reverse bias with respect to the MT1 terminal through gate open, then this device works in the mode of reverse blocking mode. A TRIAC can be made conductive through either +ve otherwise -ve voltage on the gate terminal.

Operation of TRIAC

When the applied voltage in the TRIAC is equivalent to the breakdown voltage, then the TRIAC will go into the conduction state. However, the most selected method for switching ON a TRIAC is by offering either a positive gate signal or a negative gate signal.

If the flow of current at the gate terminal is high, then less voltage is required to switch ON the TRIAC & it has the capability to get switch ON through both the polarities toward the gate signal. The working of TRIAC can be done in four kinds of modes like the following.

  • MT2 terminal is +ve with respect to MT1 terminal through a positive gate polarity with respect to MT1 terminal.
  • MT2 terminal is +ve with respect to MT1 terminal through a negative gate polarity with respect to MT1.
  • MT2 terminal is negative with respect to MT1 terminal through a negative gate polarity with respect to MT1 terminal.
  • MT2 terminal is negative with respect to MT1 terminal through a positive gate polarity with respect to MT1 terminal.


In this mode, once the MT2 terminal is +ve with respect to the MT1 Terminal, then the current flow will be in the direction of P1-N1-P2-N2. Throughout this process, the junction among the layers like P1-N1 & P2-N2 is connected in forward biased while the Junction among N1-P2 is connected in reverse biased. Once the +ve signal is given to the gate terminal then the junction among P2-N2 is connected in forward biased & breakdown happens.


Once the MT2 terminal is +ve & the gate signal is -ve, then the flow of current will be in a similar way to the first mode of P1-N1-P2-N2, however here the junction among the P2-N2 can be connected in forward biased & the current carriers are added into the P2 layer.


Once the MT2 terminal is +ve & -ve signal can be provided toward the gate terminal, then the flow of current will be in the direction of P2-N1-P2-N2. Throughout this process, the junction among the two layers like P2-N1 & P1-N4 is connected in forward biased while the junction among the layers N1-P1 is connected in reverse biased. Thus, this TRIAC will operate within the region of negatively biased.


Once the MT2 terminal is -ve & the gate terminal is activated through a positive signal then the junction among P2-N2 is connected in forwarding biased & the carriers of current are added, therefore the TRIAC is switched ON. Usually, the TRIAC doesn’t work in this mode due to the drawback that it should not be used for high di/dt circuits.

The sensitivity of TRIAC triggering by using modes 2 & 3 is high. The negative gate signal can be used in case of a minor activating capacity. The activating of mode 1 is sensitive as compared to other modes like 2 & 3, however it uses a +ve gate signal for activating. The most frequently used modes are 2 & 3.

Working of TRIAC

A simple application circuit of TRIAC is shown. Generally, TRIAC has three terminals M1, M2, and gate. A TRIAC, lamp load, and a supply voltage are connected in series. When supply is ON at positive cycle then the current flows through lamp, resistors, and DIAC (provided triggering pulses are provided at pin 1 of optocoupler resulting in pin 4 and 6 starts conducting) gate and reaches the supply and then only lamp glows for that half cycle directly through the M2 and M1 terminal of the TRIAC.

In the negative half cycle, the same thing repeats. Thus the lamp glows in both the cycles in a controlled manner depending upon the triggering pulses at the optoisolator as seen on the graph below. If this is given to a motor instead of a lamp the power is controlled resulting in speed control.

TRIAC Circuit
TRIAC Circuit
TRIAC Wave Forms
TRIAC Wave Forms

Triggering a TRIAC

Usually, 4 modes of triggering are possible in TRIAC:

  1. A positive voltage at MT2 and a positive pulse at the gate
  2. A positive voltage at MT2 and a negative pulse at the gate
  3. A negative voltage at MT2 and a positive pulse at the gate
  4. A negative voltage at MT2 and a negative pulse at the gate

Different Types of TRIAC Packages

For the Convenience of usage and different applications, the TRIACs are designed in different packages like pin/standard type, Capsule/Disc Type, and Stud Type.

Pin or Standard Type

This kind of TRIAC looks like a tiny integrated circuit through three terminals like the MT1, MT2 & Gate, and a heat sink on the pinnacle. These TRIACS are mainly utilized in household electronic appliances. The common packages of standard type TRIAC include TMA36S-L, TMA54S-L, TMA124S-L, TMA84S-L, TMA126S-L, TMA106S-L, TMA206S-L, etc.

Capsule/Disc Type

The Capsule type otherwise Disc Type TRIACs will be in the disc shape through extensive wires toward the terminals. These types of TRIACs have a high current capacity & are designed through a ceramic seal.

The applications of capsule or disc type include fast controlling of a motor as well as AC switching. The common capsule type packages are KS200A, KS100A, KS500A, KS300A, KS600A, KS1000A, and also KS800A.

Stud Type

The stud type TRIAC is mainly used within high power applications because they include a screwed bottom to perform like the main terminals & includes two terminals on its summit which are the other major terminal as well as the gate terminal.

These are mainly used within the phase control applications like lighting circuits, converter, RPS, speed control & temperature of circuits, etc. The packages of stud type TRIAC include TO-93, TO-118, TO-94, TO-48, TO-48, RSD7 & TO-65.

Affecting Factors

Unlike SCRs, TRIACS requires proper optimization for its proper functioning. Triacs have inherent drawbacks like the Rate effect, Backlash effect, etc. So designing Triac based circuits need proper care.

Rate Effect Severely Affects the Working of TRIAC

There is an internal capacitance that exists between the MT1 and MT2 terminals of the Triac. If the MT1 terminal is supplied with a sharply increasing voltage, then it results in the gate voltage breakthrough. This triggers the Triac unnecessarily. This phenomenon is called the Rate effect. The Rate effect usually occurs due to the Transients in the mains and also due to high inrush current when heavy inductive loads switch on. This can be reduced by connecting an R-C network between the MT1 and MT2 terminals.


Backlash Effect is Severe in Lamp Dimmer Circuits:

The backlash effect is the severe Control Hysteresis that develops in the lamp control or speed control circuits using a Potentiometer to control the Gate current. When the resistance of the potentiometer increases to maximum, the brightness of the lamp reduces to a minimum. When the pot is turned back, the lamp never turns on until the resistance of the pot decreases to a minimum.

The reason for this is the discharging of the capacitor in the Triac. The lamp dimmer circuits use a Diac to give a triggering pulse to the gate. So when the capacitor inside the Triac discharges through the Diac, the Backlash effect develops. This can be rectified by using a Resistor in series with the Diac or by adding a capacitor between the Gate and the MT1 terminal of Triac.

Backlash Effect
Backlash Effect

Effect of RFI on TRIAC

Radio Frequency Interference severely affects the functioning of Triacs. When the Triac switches on the load, the load current increases sharply from zero to a high value depending on the supply voltage and resistance of the load. This results in the generation of pulses of RFI. The strength of RFI is proportional to the wire connecting the load with the Triac. An LC-RFI suppressor will rectify this defect.

VI Characteristics

The VI characteristic of TRIAC is discussed below. These characteristics are related to SCR however it is appropriate for both positive & negative voltages of TRIAC. Its operation can be reviewed in four quadrants which are discussed below.

In the first quadrant, the voltage at the MT2 terminal is positive as compared to the MT1 terminal as well as the voltage at the gate terminal also positive than the first terminal

In the second quadrant, the voltage at the second terminal like MT2 is positive than MT1 & the voltage at the gate terminal is negative than terminal 1 like MT1.

In the third quadrant, the voltage at terminal 1 like MT1 is positive than terminal 2 like MT2 & the voltage at the gate terminal is negative.

In the fourth quadrant, the voltage at terminal 2 like MT2 is negative than the terminal1 MT1 & the voltage at the gate terminal is positive.

What is TRIAC Dimming?

In many lighting systems, TRIAC Dimmers play an essential role. Dimmers are mainly used to fix the lighting level so that energy can be conserved. Once the dimmer is connected through an LED light source, then this energy conserving can be fairly considerable.

The most common dimming controllers are phase-cut dimmers which are commonly known as TRIAC dimmers. Making the LED lamps using the TRIAC dimmer is quite complicated in the past, but now LED drivers using TRIAC dimming is quite simple.

TRIAC dimming mainly works as a switch with high-speed, used to manage the amount of electrical energy flowing through the bulb. Trigger orders what end the device begin to perform the electricity, basically chopping up the voltage signal, discontinuing the voltage from the supply at full load.

Once a TRIAC dimmer is used through an LED light & sourcing a LED driver with TRIAC dimming is essential to make sure the device is a TRIAC semiconductor device. These dimmers are mainly designed for resistive loads, so it is significant to obtain this correct. If the false TRIAC dimming LED driver can be sourced the lights will not act like expected, reducing the LED lifetime.

TRIAC is Unidirectional or Bidirectional?

TRIAC is a unidirectional device because it can switch both halves of an AC signal. It is achievable to analyze the TRIAC’s operation by placing the thyristors back to back. The thyristor symbol identifies the way where the TRIAC functions. From the outside, it looks like the thyristors are connected back to back.

The TRIAC is a perfect device for AC switching applications as it can regulate the flow of current over both bisects of an alternating series. A thyristor simply controls them above one half of a series. Throughout the remaining half, no conduction happens & consequently, simply half the signal can be utilized.


TRIAC BT136 is a family of TRIAC, it has a current rate of 6AMPs. We already have seen an application of TRIAC using BT136 above.

Features of BT136

  • Direct triggering from low power drivers and logic ICs
  • High blocking voltage capability
  • Low holding current for low current loads and lowest EMI at commutation
  • Planar passivated for voltage ruggedness and reliability
  • Sensitive gate
  • Triggering in all four quadrants

Applications of BT136:

  • Universally useful in motor control
  • General-purpose switching


TRIAC BT139 also comes under the TRIAC family, it has a current rate of 9AMPs. The main difference between BT139 and BT136 is the current rate and BT139 TRIACS are used for high power applications.

The features of BT139 include the following.

  • Direct triggering from low power drivers and logic ICS
  • High blocking voltage capability
  • Planar passivated for voltage ruggedness and reliability
  • Sensitive gate
  • Triggering in all four quadrants

The applications of BT139 include the following.

  • Motor control
  • Industrial and domestic lighting
  • Heating and static switching

What is the Difference between Thyristor and TRIAC?

The difference between SCR and TRIAC include the following.



A thyristor is also known as SCR or Silicon Controlled RectifierIt stands for  triode for alternating current
It is a unidirectional deviceIt is a bidirectional device
SCR or Thyristor includes four terminalsIt includes three terminals
It is reliableIt is less reliable
Thyristor uses toe heat sinksIt needs simply one heat sink
The rating of Thyristor is largeThe rating of TRIAC is small
SCR can be triggered through UJTIt can be triggered through a DIAC
A thyristor is used to control DC powerIt controls AC as well as DC power
In thyristor, one mode of operation is possibleIt includes four different modes of operations
A thyristor operates in simply one quadrant VI characteristicsIt operates simply in two quadrants VI characteristics
A thyristor can be activated through positive gate voltage simply.It can be activated through whether the positive gate voltage or negative gate voltage
It has high current capabilitiesIt has low current capabilities


The advantages of TRIAC include the following.

  • It uses a heat sink that is slightly large in size or slightly larger size, whereas for SCR two small size heat sinks must be used.
  • A secured breakdown in any direction can be possible however for SCR protection must be given through a parallel diode.
  • In DC applications, SCR needs to connect through a parallel diode for protecting from reverse voltage, while TRIAC may function without using a diode because a safe breakdown in any direction is achievable.
  • Once the voltage is decreased to zero then the TRIAC will be turned OFF.
  • It can be activated through positive otherwise negative polarities of gate signals
  • It can be protected with a single fuse.


The disadvantages of TRIAC include the following.

  • As compared to SCR, these are not reliable
  • As compared to SCR, reliability is low.
  • It will be activated in any direction so must be cautious while switching ON the circuit.
  • The switching delay  is high
  • The rating of dv/dt is extremely less than SCR
  • TRIAC will have fewer ratings as compared to the silicon-controlled rectifiers.
  • It is not applicable in DC applications

Applications of TRIAC

TRIACs are used in numerous applications such as light dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of numerous household small and major appliances. They can be used both in AC and DC circuits however the original design was to replace the utilization of two SCRs in AC circuits. There are two families of TRIACs, which are mainly used for application purposes, they are BT136, BT139.

Thus, this is all about an overview of TRIAC which is known as a triode for alternating current, construction, working, packages, differences with SCR, advantages, disadvantages, and applications. Here is a question for you, what is the function of SCR?

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