What is a TRIAC? Working, Diagram, Construction, Characteristics & Applications

The major drawback of the SCR is that as a single SCR cannot be used to control the a.c. power in load. However, in a.c. systems, it is often desirable and necessary to exercise to control over both positive and negative half cycles. For this purpose, a semiconductor power electronics device called TRIAC is used. The TRIAC is an acronym that has been coined to identify the TRIode AC semiconductor switch. Thus TRIAC is an abbreviation of Triode AC Switch. The word Tri’ indicates that the device has three terminals and the word ‘AC’ means that the device controls the alternating current, or can conduct current in either direction.

In thyristor family, after SCR, TRIAC is the most widely used power electronics device for a.c. power control. TRIAC is a three terminal semiconductor or bilateral switching device which can control alternating current in a load, when triggered either by a positive or negative gate pulse irrespective of the polarity of the voltage applied across its main terminals. In operation, TRIAC is similar to two SCRs connected in antiparallel. The TRIACs are now available with the reasonably large current ratings upto 25 A and voltage ratings upto 500 V.

Construction of TRIAC

TRIAC is yet another thyristor family device which is used extensively for the control of power in a.c. circuits. TRIAC is the word derived by combining the capital letters from the words TRIode and AC. TRIAC is a bidirectional switching semiconductor device. Fig. 1 shows a cross-sectional view of a TRIAC. The TRIAC is usually turned ON by applying a positive or negative voltage to the gate with respect to MT₁.

 

Fig. 1: Cross-sectional view of a TRIAC

(a) Basic structure

(b) Equivalent structure

(c) Equivalent circuit

(d) Symbol

Fig. 2: Structure of a TRIAC

Principle of Operation of TRIAC

The operation of a TRIAC may be explained from Fig. 3. The series resistor R_L may be connected as a load in series with the terminal MT₂. The terminal MT₂ may be made positive or negative with respect to MT₁.

Fig. 3: Operating principle of a TRIAC

The TRIAC is said to be forward biased when terminal MT₂ is positive with respect to MT₁, and the terminal G is open i.e. V_g = 0. The SCR T₁ will be forward biased whereas the SCR T₂ will be reverse biased. As the forward voltage V_T is less than the forward breakover voltage V_BO, the SCR T₁ will not conduct. Also SCR T₂ will not conduct. Hence, the TRIAC will be in OFF state. As the forward voltage approaches the bleakover voltage, the SCR T₁ will conduct the current and it will be in ON state. So, the TRIAC will be in ON state. If a positive or negative trigger pulse is applied at gate G, the SCR T₂ will conduct the current and it will be in ON state, so the TRIAC will be in ON state.

The TRIAC is said to be reverse biased when the terminal MT₂ is negative with respect to MT₁ and the terminal G is open, i.e. V_g = 0. The SCR T₁ will be reverse biased whereas the SCR T₂ will be forward biased. As the reverse voltage V_T is less than the reverse breakover voltage (- V_BO), the SCRs T₂ as well as T₁ will not conduct. Hence, the TRIAC will be in OFF state. As the reverse voltage approaches the breakover voltage, the SCR T₂ will conduct the current and it will be in ON state. So the TRIAC will be in ON State. If a positive or negative gate pulse is applied at gate G, the SCR T₂ will conduct the current and it will be in ON state. So, the TRIAC will be in ON state. It may be noted that a gate current pulse of specified amplitude and of either polarity (positive or negative) will trigger the TRIAC into conduction (ON) State, assuming that TRIAC is in OFF state. Also the TRIAC conducts in both directions for either polarity (positive or negative) of voltage on terminals MT₁ and MT₂.

Modes of Operation of TRIAC

The TRIAC can be turned ON by applying positive or negative gate current pulse at a gate by keeping the TRIAC in either forward or reverse biased conditions. The turn ON process of TRIAC in four modes can be explained as follows:

Mode 1 (Quadrant I):

Consider the structure of a TRIAC as shown in Fig. 4. When the terminal MT₂ is positive with respect to the terminal MT₁, the junctions P₁N₁ and P₂N₂ ate forward biased but the junction N₁P₂ is reverse biased.

Fig. 4: TRIAC in Mode 1

When the positive gate current is applied at the gate G, the gate current I_g flows through the junction P₂N₂. The TRIAC starts conducting through the layers P₁N₁P₂N₂ due to positive regenerative action. The TRIAC is more sensitive in this mode of operation as compared to other modes of operation. This is the recommended method of triggering the TRIAC, if its conduction is desired in first quadrant. The TRIAC can be turned ON at a lower gate current at a fixed forward voltage V_T, i.e. V_T < V_BO.

Mode 2 (Quadrant I):

Consider the structure of a TRIAC as shown in Fig. 5.

Fig. 5: TRIAC in Mode 2

When the terminal MT₂ is positive with respect to the terminal MT₁, the junctions P₁N₁ and P₂N₂ are forward biased, but the junction N₁P₂ is reverse biased. When the negative gate current is applied at the gate G, the gate current I_g flows through P₂N₃. The TRIAC starts conducting through P₁N₁P₂N₃ layers initially. The left hand side of layer P₂ is more positive as compared to its right hand side as the terminal MT₁ is negative on right hand side. Because of this potential gradient, a current is established in P₂ layer which is shown by dotted line in Fig. 5. As a result, the right hand side of TRIAC starts conducting through P₁N₁P₂N₂ layers and stays in ON State. TRIAC is less sensitive in this mode of operation as it requires more gate current to turn ON at the same forward voltage V_T as compared to Mode 1. The turn ON delay is also more in this Mode 2. This mode of operation is called junction gate operation, which involves high switching losses and therefore this mode of operation is not normally used.

Mode 3 (Quadrant II) :

Consider the structure of a TRIAC as shown in Fig. 6.

Fig. 6: TRIAC in Mode 3

The terminal MT₂ is negative with respect to the terminal MT₁ and the positive gate current pulse is applied at the gate G. The junction P₂N₂ is forward biased by the external positive gate current. Because the terminal MT₁ is connected to layer N₂, the layer N₂ injects electrons into P₂ layer as shown by dotted arrows. This causes the reverse biased junction N₁P₁ to breakdown. As a result, the TRIAC starts conducting through P₂N₁P₁N₄ layers and stays it in ON state. TRIAC is less sensitive in this mode of operation.

Mode 4 (Quadrant III) :

Consider the structure of a TRIAC as shown in Fig. 7.

Fig. 7: TRIAC in Mode 4

The terminal MT₂ is negative with respect to the terminal MT₁ and the gate current pulse is also negative. Here the layer N₃ acts as a remote gate. The external negative gate current pulse of gate G causes the gate current to flow through the junction P₂N₃. This causes the junction P₂N₃ to become forward biased. The TRIAC starts conducting through the layers P₂N₁P₁N₄ and stays in ON state. TRIAC is more sensitive in this mode of operation as compared to that of Mode 3. In all the four modes of operation, the sensitivity of a TRIAC is highest in first quadrant (Mode 1) with positive gate current and also in third quadrant (Mode 3) with negative gate current. The degree of sensitivity is indicated by the magnitude of gate current. The TRIAC operates in four modes of operation as explained above. Thus the triggering modes of a TRIAC are as given below:

MT2	MT1	Gate	Quadrant	Mode
Positive	Negative	Positive	I	1
Positive	Negative	Negative	I	2
Negative	Positive	Positive	III	3
Negative	Positive	Negative	III	4

V-I Characteristics of TRIAC

It is the curve between the voltage applied across the two main terminals and the current flowing through the TRIAC at constant gate current. The TRIAC has two types of V-I characteristics, namely forward and reverse characteristics.

Experimental setup for forward characteristics of a TRIAC:

The experimental set-up for plotting the forward characteristic of a TRIAC is as shown in Fig. 8. Initially, adjust the positive gate voltage to zero.

Fig. 8: Experimental setup for forward characteristics of a TRIAC

Then increase the applied voltage across the TRIAC in small suitable steps. At each step, record the value of TRIAC current and the voltage across it. Now, if we plot a graph with the positive voltage across a TRIAC along the horizontal (X) axis and the TRIAC current along the vertical (Y) axis, we obtain a curve in 1^st quadrant as shown in Fig. 9. A similar procedure may be used to obtain curves marked I_g, I_g1, and I_g2 for different values of gate current.

Fig. 9: Static V-I characteristics of a TRIAC

Experimental setup for reverse characteristics of a TRIAC:

The experimental set-up for plotting the reverse characteristics of a TRIAC is shown in Fig. 10. Initially, adjust the negative gate voltage to zero. Then increase the applied negative voltage across the TRIAC in some suitable steps. At each step, record the value of TRIAC current and the negative voltage across it.

Fig. 10: Experimental setup for reverse characteristics of a TRIAC

Now, if we plot a graph with negative voltage across a TRIAC along the horizontal (X) axis and the negative TRIAC current along the vertical (Y) axis, we obtain curves for different values of negative gate currents as shown in 3^rd quadrant of Fig. 9.

Forward and reverse characteristics:

The forward and reverse V-l characteristics Of a TRIAC are as shown in Fig. 9. The following are the important points that are observed from the forward characteristics

1. The curve is identical to the forward characteristics Of SCR.
2. The TRIAC is OFF until the applied voltage exceeds the breakover voltage.
3. AS the applied voltage exceeds the breakover voltage, the TRIAC turns ON and the voltage drop across the TRIAC decreases to a low value of holding voltage. The TRIAC forward current increases to a value determined by the supply voltage and the load resistance.
4. As the value of gate current is increased above zero, the breakover voltage is lowered like SCR. The TRIAC is never operated with the zero gate current. When gate current of a suitable value is applied, usually in the form of pulses, the TRIAC turns ON at much lower breakover voltage.

Note: The following are the important points that are observed from reverse characteristics:

• The curve is identical to the reverse characteristics of SCR.
• The TRIAC is OFF until the applied negative voltage exceeds the break-over voltage.
• As the applied negative voltage exceeds the breakover voltage, the TRIAC current decreases to a low value of holding current. The TRIAC reverse current increases to a value determined by the supply voltage and load resistance.
• If the value of gate current is increased above zero, the breakover voltage is lowered. Like SCR, the TRIAC is never operated with the zero gate current. When the negative gate current of a suitable value is applied usually in the form of pulses, the TRIAC turns ON at much lower breakover voltage.

Applications of TRIAC

The TRIAC has an important property that it can conduct current in either forward or reverse directions, depending upon the polarity of the voltage across its main terminals. This property makes the TRIAC very useful for industrial applications. Some of the important applications of a TRIAC are as given below.

1. as static switch.
2. for phase control.
3. for speed control Of AC motor.
4. for light-dimming control.
5. for thermostat/heater control.
6. for liquid level control.
7. for AC power control.
8. for electronic change over of transformer tap.
9. as a high power lamp switch.
10. in battery charger cum emergency light system.
11. in flasher circuit.
12. as a proximity detector.

Advantages of TRIAC

The advantages of a TRIAC are as given below :

1. It needs a single heat sink of slightly larger size, whereas the anti-parallel SCR pair needs two heat sinks of slightly smaller sizes, but due to the clearance total space required is more for SCRs.
2. It needs a single fuse for protection and this also simplifies the construction.
3. It can be triggered with positive or negative polarity gate voltages.
4. In some d.c. applications, SCR is required to be fitted with a parallel diode or protect against reverse voltage, a TRIAC used may work without a diode as a safe breakdown in either direction is possible.
5. It can conduct current in both directions.
6. It can control a.c. power as well as d.c. power.
7. It can be triggered by negative or positive gate voltage.
8. It controls more power than that of SCR.
9. It needs a single heat sink of slightly larger size than that of a SCR.

Disadvantages of TRIAC

The disadvantages of TRIAC are as given below :

1. It has low (di/dt) rating as compared to SCRs.
2. It is not available in higher ratings.
3. Its trigger circuit needs careful consideration as it can be triggered in either direction.
4. Its reliability is less than that of SCR.
5. It cannot be used for power controlling of inductive circuits or motor loads.
6. It is costly than that of SCR.
7. It has low (dv/dt) ratings.