TRIAC [Triode for Alternating Current]

 TRIAC

What is a Triac?

A Triac is defined as a three-terminal AC switch which is different from the other silicon-controlled rectifiers in the sense that it can conduct in both directions that is whether the applied gate signal is positive or negative, it will conduct. Thus, this device can be used for AC systems as a switch.

This is a three-terminal, four-layer, bi-directional semiconductor device that controls AC power. The triac of maximum rating of 16 kw is available in the market.

The figure shows the symbol of triac, which has two main terminals MT1 and MT2 connected in inverse parallel and a gate terminal.

Construction of Triac

Two SCRs are connected in inverse parallel with the gate terminal as common. Gate terminals is connected to both the N and P regions due to which gate signal may be applied which is irrespective of the polarity of the signal. Here, we do not have anode and cathode since it works for both polarities which means that device is bilateral. It consists of three terminals namely, main terminal 1(MT1), main terminal 2(MT2), and gate terminal G.

Figure shows the construction of a triac. There are two main terminals namely MT1 and MT2 and the remaining terminal is gate terminal.

Operation of Triac

The triac can be turned on by applying the gate voltage higher than break over voltage. However, without making the voltage high, it can be turned on by applying the gate pulse of 35 micro seconds to turn it on. When the voltage applied is less than the break over voltage, we use gate triggering method to turn it on.

There are four different modes of operations, they are-

1. When MT2 and Gate being Positive with Respect to MT1
   When this happens, current flows through the path P1-N1-P2-N2. Here, P1-N1 and P2-N2 are forward-biased but N1-P2 is reverse biased. The triac is said to be operated in positively biased region. Positive gate with respect to MT1 forward biases P2-N2 and breakdown occurs.
2. When MT2 is Positive but Gate is Negative with Respect to MT1
   The current flows through the path P1-N1-P2-N2. But P2-N3 is forward biased and current carriers injected into P2 on the triac.
3. When MT2 and Gate are Negative with Respect to MT1
   Current flows through the path P2-N1-P1-N4. Two junctions P2-N1 and P1-N4 are forward biased but the junction N1-P1 is reverse biased. The triac is said to be in the negatively biased region.
4. When MT2 is Negative but Gate is Positive with Respect to MT1
   P2-N2 is forward biased at that condition. Current carriers are injected so the triac turns on. This mode of operation has a disadvantage that it should not be used for high (di/dt) circuits. Sensitivity of triggering in mode 2 and 3 is high and if marginal triggering capability is required, negative gate pulses should be used. Triggering in mode 1 is more sensitive than mode 2 and mode 3.

Characteristics of a Triac

The triac characteristics is similar to SCR but it is applicable to both positive and negative triac voltages. The operation can be summarized as follows-

First Quadrant Operation of Triac

The voltage at terminal MT2 is positive with respect to terminal MT1 and gate voltage is also positive with respect to first terminal.

Second Quadrant Operation of Triac

The voltage at terminal 2 is positive with respect to terminal 1 and gate voltage is negative with respect to terminal 1.

Third Quadrant Operation of Triac

The voltage of terminal 1 is positive with respect to terminal 2 and the gate voltage is negative.

Fourth Quadrant Operation of Triac

The voltage of terminal 2 is negative with respect to terminal 1 and gate voltage is positive.

When the device gets turned on, a heavy current flows through it which may damage the device, hence in order to limit the current a current-limiting resistor should be connected externally to it. By applying proper gate signal, firing angle of the device may be controlled. The gate-triggering circuits should be used for proper gate triggering. We can use diac for triggering the gate pulse. For firing of the device with proper firing angle, a gate pulse may be applied up to a duration of 35 microseconds.

Single-Phase Motor Starters

Often, a capacitor-start or split-phase motor must operate where arcing of a mechanical cut-out start switch is undesirable or even dangerous. In such cases, the mechanical cut- out start switch can be replaced by a TRIAC. See Figure.

 

Figure. A mechanical cut-out start switch may be replaced by a TRIAC.

 A TRIAC is able to operate in such dangerous environments because it does not create an arc. The gate and cut-out signal are given to the TRIAC through a current transformer.

As the motor speeds up, the current is reduced in the current transformer, and the transformer no longer triggers the TRIAC. With the TRIAC turned off, the start windings are removed from the circuit.

Testing Procedures for TRIACs

TRIACs should be tested under operating conditions using an oscilloscope. A DMM may be used to make a rough test with the TRIAC out of the circuit. See Figure.

To test a TRIAC using a DMM, the following procedure is applied:

1. Set the DMM on the Ω scale.
2. Connect the negative lead to the main terminal 1.
3. Connect the positive lead to the main terminal 2. The DMM should read infinity.
4. Short-circuit the gate to main terminal 2 using a jumper wire. The DMM should read almost 0 Ω. The zero reading should remain when the lead is removed.
5. Reverse the DMM leads so that the positive lead is on the main terminal 1, and the negative lead is on the main terminal 2. The DMM should read infinity.
6. Short-circuit the gate of the TRIAC to main terminal 2 using a jumper wire. The DMM should read almost 0 Ω. The zero reading should remain after the lead is removed.

Advantages of Triac

1. It can be triggered with positive or negative polarity of gate pulses.
2. It requires only a single heat sink of slightly larger size, whereas for SCR, two heat sinks should be required of smaller size.
3. It requires single fuse for protection.
4. A safe breakdown in either direction is possible but for SCR protection should be given with parallel diode.

Disadvantages of Triac

1. They are not much reliable compared to SCR.
2. It has (dv/dt) rating lower than SCR.
3. Lower ratings are available compared to SCR.
4. We need to be careful about the triggering circuit as it can be triggered in either direction.

Uses of Triac

1. They are used in control circuits.
2. It is used in High-power lamp switching.
3. It is used in AC power control.

Frequently Asked Questions (FAQs) 

Q. State the full form of the TRIAC.

Answer: Triode AC switch

Q. Why the Triac is called as bidirectional?

Answer: The conduction of the Triac is done either polarity of the supply voltage therefore it is called as bidirectional.

Q. How many terminals the Triac consists of?

Answer: Three: MT1, MT2 and gate

Q. Which semiconductor device requires more gate current in order to turn it on? Triac or SCR

Answer: SCR
Q. Whether the Triac is turned on by positive gate current or negative gate current?

Answer: Either positive or negative gate current

Q. Give reason: The Triac is rarely operated in the first quadrant with negative gate current and third quadrant with positive gate current.

Answer: Due to less sensitive

Q. In which quadrant of the operation, the Triac has greatest sensitivity?

Answer: First quadrant with gate positive gate current and third quadrant with negative gate current.

Multiple Choice Questions and Answers

Q1. A Triac has three terminals viz ………………

1. Drain, source, gate
2. Two main terminal and a gate terminal
3. Cathode, anode, gate
4. None of the above

Answer: 2

Q2. A triac is equivalent to two SCRs …………..

1. In parallel
2. In series
3. In inverse-parallel
4. None of the above

Answer : 3

Q3. A triac is a …………. switch

1. Bidirectional
2. Unidirectional
3. Mechanical
4. None of the above

Answer: 1

Q4. The V-I characteristics for a triac in the first and third quadrants are essentially identical to those of ………………. in its first quadrant

1. Transistor
2. SCR
3. UJT
4. none of the above

Answer: 2

Q5. A triac can pass a portion of …………… half-cycle through the load

1. Only positive
2. Only negative
3. Both positive and negative
4. None of the above

Answer : 3

Q6. A diac has  ………….. terminals

1. Two
2. Three
3. Four
4. None of the above

Answer : 1

Q7. A triac has …………….. semiconductor layers

1. Two
2. Three
3. Four
4. Five

Answer : 3

Q8. A diac has …………… pn junctions

1. Four
2. Two
3. Three
4. None of the above

Answer : 2

Q9. The device that does not have the gate terminal is ……………….

1. Triac
2. FET
3. SCR
4. Diac

Answer : 4

Q10. A diac has ……………… semiconductor layers

1. Three
2. Two
3. Four
4. None of the above

Answer : 1

Q11. A UJT has ……………….

1. Two pn junctions
2. One pn junction
3. Three pn junctions
4. None of the above

Answer : 2

Q12. The normal way to turn on a diac is by ………………..

1. Gate current
2. Gate voltage
3. Breakover voltage
4. None of the above

Answer : 3

Q13. A diac is …………………. switch

1. An c.
2. A d.c.
3. A mechanical
4. None of the above

Answer : 1

Q14. In a UJT, the p-type emitter is ……………. doped

1. Lightly
2. Heavily
3. Moderately
4. None of the above

Answer : 2

Q15. Power electronics essentially deals with control of a.c. power at …………

1. Frequencies above 20 kHz
2. Frequencies above 1000 kHz
3. Frequencies less than 10 Hz
4. 50 Hz frequency

Answer : 4

Q16. When the emitter terminal of a UJT is open, the resistance between the base terminal is generally ………………..

1. High
2. Low
3. Extremely low
4. None of the above

Answer : 1

Q17. When a UJT is turned ON, the resistance between emitter terminal and lower base terminal …………….

1. Remains the same
2. Is decreased
3. Is increased
4. None of the above

Answer : 2

Q18. To turn on UJT, the forward bias on the emitter diode should be …………… the peak point voltage

1. Less than
2. Equal to
3. More than
4. None of the above

Answer : 3

Q19. A UJT is sometimes called …………. diode

1. Low resistance
2. High resistance
3. Single-base
4. Double-base

Answer : 4

Q20.  When the temperature increases, the inter-base resistance (RBB) of a UJT ………….

1. Increases
2. Decreases
3. Remains the same
4. None of the above

Answer : 1

Q21. This question will be available soon

 

Q22. When the temperature increases, the intrinsic stand off ratio ……….

1. Increases
2. Decreases
3. Essentially remains the same
4. None of the above

Answer : 3

Q23. Between the peak point and the valley point of UJT emitter characteristics we have ………….. region

1. Saturation
2. Negative resistance
3. Cut-off
4. None of the above

Answer : 2

Q24. A diac is turned on by …………………

1. A breakover voltage
2. Gate voltage
3. Gate current
4. None of the above

Answer : 1

Q25. The device that exhibits negative resistance region is ………………..

1. Diac
2. Triac
3. Transistor
4. UJT

Answer : 4

Q26. The UJT may be used as ……………….

1. Am amplifier
2. A sawtooth generator
3. A rectifier
4. None of the above

Answer : 2

Q27. A diac is simply ………………

1. A single junction device
2. A three junction device
3. A triac without gate terminal
4. None of the above

Answer : 3

Q28. After peak point, the UJT operates in the ……………. region

1. Cut-off
2. Saturation
3. Negative resistance
4. None of the above

Answer : 3

Q29. Which of the following is not a characteristic of UJT?

1. Intrinsic stand off ratio
2. Negative resistance
3. Peak-point voltage
4. Bilateral conduction

Answer : 4

Q30. The triac is …………….

1. Like a bidirectional SCR
2. A four-terminal device
3. Not a thyristor
4. Answers (1) and (2)

Answer : 1