Difference Between DIAC and TRIAC

Main Differences Between DIAC & TRIAC

Regulating and controlling of the power supplied to a load is very important in order to reduce the wasting of energy and efficiently use of energy. Electrical and electronics play a major role in doing so through the use of semiconductor devices. DIAC and TRIAC are semiconductor devices used for regulation of such power supplied to the load.

These semiconductor devices are used to control the AC power supplied to load such as in controlling the speed of motors used in any application. Semiconductor devices such as SCR, DIAC, TRIAC & other thyristor family components having compact & small size are used due to their fine power control & high efficiency.

Difference Between DIAC and TRIAC

In order to understand DIAC & TRIAC better, we have to study SCR since they are the modified version of SCR.

SCR

SCR or Silicon Controlled Rectifier is a semiconductor component that belongs to the thyristor family. It is a four-layer device made of alternating P & N-layers PNPN. There are 3 PN junctions. It is a unidirectional controlled switch used for rectification and regulation of alternating current

SCR Symbol & StructureIt has 3 terminals: Anode (A), Cathode (C) & Gate (G). The anode & cathode are the main terminals used for the conduction of current while the Gate terminal is the control terminal used for triggering or firing the SCR.

The Anode lead is connected with the outermost P-layer while the Cathode lead is connected with the outermost N-layer. While the gate lead is connected with the middle P-layer.

SCR works in three modes: Forward Blocking, Forward Conduction & Reverse Blocking Mode. In forward blocking mode, the SCR is connected in forward bias without any triggering pulse at the gate. In this mode, the SCR does not conduct.

Thyristor & Silicon Controlled Rectifier (SCR) operationIn reverse blocking mode, the SCR is connected in reverse bias as shown in fig (c). The SCR does not conduct in this mode even if there is a gate signal.

In forward conduction mode, the SCR is connected in forward bias as shown in fig (b) & it is triggered by applying a triggering pulse at its gate terminal. Forward conduction also occurs if the voltage exceeds its breakdown voltage.

The operation of SCR is very simple. When the anode is connected to a higher voltage than the cathode, the two PN junctions at the ends become forward biased while the middle junction is in reverse bias. The reverse junction does not allow the flow of current. A positive voltage pulse at the gate triggers the SCR into conduction by switching the middle junction into forward bais.

When the SCR is in forward conduction mode, removing the gate pulse will not switch it off. But the voltage between anode & cathode must be brought down so that the current falls below the “holding current” limit.

A thyristor a unidirectional switch that only allows current in one direction. It can only allow a half-wave of alternating current. it cannot conduct current in the reverse direction. It is triggered by applying only a positive gate pulse.

SCR’s Key Points

DIAC

A DIAC is an acronym that stands for “Diode for Alternating Current”. As the name suggests, it is a diode for conduction alternating current.  It a bidirectional semiconductor uncontrolled switch capable of conducting current in both directions. It starts conduction when the applied voltage increases above the break-over voltage VBO. The main function of a DIAC is to help in activating TRIAC to perform symmetrical switching.

DIAC Symbol & StructureIt is a two-terminal device made from a combination of two antiparallel SCRs without the gate terminals. The two terminals of DIAC are named Main Terminal 1 (MT1) & Main Terminal 2 (MT2). The DIAC is designed in such a way that it is symmetrical from both sides having equal regions.

DIAC is like a diode that switches ON when the applied voltage exceeds a certain limit except it can switch ON in both directions. It does not conduct when there in either direction when the voltage is below break over voltage VBO which is 30v in most cases. Once the voltage exceeds VBO, the DIAC switches ON & the voltage across DIAC falls down to 5 volts.   

DIAC’s Key Points

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TRIAC

TRIAC stands for “Triode for Alternating Current”. It is a semiconductor controlled switch that can conduct in both directions. It is made from a combination of two SCR in an antiparallel configuration. TRIAC is used for regulating power in AC circuits & it can regulate both halves of the AC cycle.

TRIAC Symbol & StructureSince it is made by the combination of two antiparallel SCR, it has 4 layers & three terminals: Main terminal 1(MT1), Main terminal 2 (MT2) & gate. There is no Anode & cathode because it can conduct in both directions. The gate terminal is connected to both the P & N regions of each SCR.

The TRIAC can be triggered by a positive or negative gate pulse. But generally, the positive pulse is used for operation in the 1st quadrant & the negative pulse is used for operation in the 3rd quadrant. In the 1st quadrant, both the current and voltage across the TRIAC is positive while in the 3rd quadrant both are negative. These two quadrants imply the two halves of AC waves positive half & negative half. Although there is a slight difference in the firing due to the different gate regions (P & N region) in each half.

TRIAC’s Key Points

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Key Differences Between DIAC & TRIAC

DIAC TRIAC
DIAC is an acronym for “Diode for Alternating Current”. TRIAC is an acronym for “Triode for alternating current”.
It is a diode for AC that allows current in both directions when the applied voltage increase above break overvoltage. It is a bidirectional SCR that conducts in both directions when it is triggered using gate pulse.
DIAC has 4 alternating layers with equal regions. it is made of 4 layers of alternating layers.
It has 2 terminals: Main terminal 1 (MT1) & main terminal 2 (MT2). It has 3 terminals: Main terminal 1(MT1) & Main terminal 2 (MT2) & Gate.
It is an uncontrolled bidirectional switch. It is a controlled bidirectional switch.
There is no firing angle. TRIAC has a firing angle from 0° to 180° for the positive half & from 180° to 360° for the negative half.
It is made by the combination of two SCRs in antiparallel without any gate terminal. It is made from a combination of two SCR with a common gate terminal.
It has a symmetrical structure i.e. same structure in both current directions. TRIAC has a slight non-symmetrical structure due to different gate regions in both halves.
It does not have any control input or gate terminal. It has a separate control input used for triggering the TRAIC.
It cannot block the voltages above 30 volts. It can block very high voltage in the range of 600 to 1000 V.
It has very low power ratings. It has a very high power rating in the range of 15KW.
It is triggered by increasing the voltage above break over voltage usually 30V. It is triggered by applying an either positive or negative voltage pulse.
DIAC offers a very symmetrical response in triggering. TRIACs triggering is not symmetrical & require DIAC to help it in firing symmetrically.
The DIAC has very low power ratings therefore it is mainly used for triggering TRIAC. The TRIAC is used in supplying controlled power to AC loads, Speed control of fans & motors, dimmers in lamps & lights, etc.

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Properties & Characteristics of DIAC & TRIAC

The following different properties differentiate both DIAC and TRIAC having different characteristics and applications.

Structure

Terminals

Triggering

The triggering refers to the switching ON of a device into a conduction state.           

Characteristics Curve

The symmetry of the characteristic curve is very important for bidirectional current devices. A component having a symmetrical structure has a symmetrical characteristic curve

Control

Power Ratings

Application

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