TRIAC, TRIode for Alternating Current, a report from one of the topic in Electrical Technology

TRIode for Alternating Current

1. TRI ode for A lternating C urrent (TRIAC)

2. What is TRIAC? The name TRIAC comes from the words  TRI ode for  A lternating  C urrent . The TRIAC was discovered the time that SCR was discovered therefore those two elements have the same history. It is actually composed of two different SCRs connected in parallel and in opposite direction. The formal name of a TRIAC is bidirectional triode thyristor. The name itselfe indicates that a TRIAC can allow current to flow within it's leads in any direction. This makes a TRIAC more suitable than an SCR when AC voltage needs to be controlled.

3. What is TRIAC? The TRIAC can become conductive with both positive or negative pulses Someone would expect the TRIAC to become conductive with positive pulses. This is far from true. A TRIAC becomes conductive with bot positive or negative pulses. As mentioned in the previous point, it has no polarity and therefore the polarity off the pulse on the gate is irrelevant. It only needs to be greater than the gate threshold voltage to turn on the TRIAC.

4. What is TRIAC? The TRIAC turns into non-conductive state only when the voltage difference between the bias is zero volts (0V) As it is already known from the theory of operation for the SCRs, an SCR goes into a non-conductive state when the the voltage difference between the anode and the cathode of the SCR is equal or smaller than zero volts. TRIACs goes off (non-conductive) only when the voltage between the two leads is zero volts.

5. What is TRIAC? As mentioned before, a TRIAC is actually two SCRs connected in parallel and opposite. The theory of operation for a TRIAC is based on the  theory of operation of an SCR triode . The differences between those two elements are: The TRIAC has no forward or reverse polarity The TRIAC has no Anode or Cathode pin that needs to be connected reverse of forward in order to allow current to flow within the device. Once a pulse is applied to the gate of the TRIAC, it shall become conductive and shall allow current to flow from the bias no matter what polarity.

6. APPLICATIONS TRIACs are used in many applications such as light dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances. They can be used both into AC and DC circuits but the original design was to replace the use of two SCRs in AC circuits. TRIACS may not turn on reliably with non-resistive loads, because due to the phase shift the holding current may not be achievable at trigger time. Therefore, a pulse train is sent to the gate until the load is turned on. Because of the fact that the gate current does not need to be maintained throughout the entire conduction angle, current can be limited significantly and this is a benefit when drive capabilities are narrow.

7. APPLICATIONS A TRIAC is used in AC power control applications in order to switch high voltages over both parts of the AC waveform. As a result, TRIAC circuits are used in a number of applications that require power switching. The TRIAC was developed after the thyristor, which controls AC current over one half of the cycle. The TRIAC circuit can be visualized as two thyristors that are opposite, parallel , and have the two gates connected together with one device’s anode connected to another’s cathode. A common household device that uses the TRIAC circuit is a light dimmer.

8. TRIAC COMPOSITION The basic TRIAC circuit has bi-directional properties. Similar to a thyristor, it has three terminals, one of them being a gate acting as a trigger to turn the device on. The other two terminals are referred to as Main Terminal 1 (MT1) and Main Terminal 2 (MT2), which have similar properties due to the circuit’s bi-directional nature.

9. How does TRIAC work? A positive or negative voltage that is applied to a TRIAC’s gate electrode triggers it. Once the circuit is triggered, it will conduct electricity until the current drops below the designed threshold value. As a result, it permits the control of large power flows with very small control currents. TRIACs also control the percentage of current that flows through the circuit to the load, which is referred to as phase control. TRIACs normally have a DIAC included in the circuit when used as a control for electrical equipment, in order to dampen the level of harmonics that the circuit produces.

10. What are the Uses for a TRIAC? TRIACs are used in a number of small AC power applications. Examples of these uses include dimmer switches, speed controls for electric fans, computerized control circuits of household appliances, and in electric motor controls.

11. Advantage and Disadvantage of TRIAC ADVANTAGES It is easy to use and is much cheaper than using two SCR thyristors separately for lower power applications. It can be triggered into a conduction state with both positive or negative polarity gate voltage and current pulses. DISADVANTAGES  They are not as reliable as SCR. The change in voltage rating is much lower compared to the SCR.

12. Advantage and Disadvantage of TRIAC ADVANTAGES It only requires a single large size heat sink unlike a single SCR thyristor which requires two heat sinks but of smaller size. Only a single fuse is required to provide the TRIAC with protection. Since it is a bilateral device a safe breakdown voltage in either direction is possible, unlike the SCR which requires a diode connected in parallel to guarantee protection. DISADVANTAGES The triggering circuit needs to be carefully monitored at all times as it can be triggered into conduction in either direction.

13. Application TRIACs are widely used for domestic light dimmers, small motor controls, electrical fan speed controls, and controlling small AC-powered domestic applications.

14. AC Switching Applications TRIACS main purpose is to be used in the switching of AC voltages. The figure below shows an example of an application that makes use of the TRIAC circuit for AC switching. When the switch SW1 is open there will be no gate current supply which would result in no current flowing through the lamp. When the switch is closed then it would be turned ON. This would allow the current to flow through the resistor R and a pulse would be provided to the gate terminal which would initiate the conduction through the TRIAC circuit once the break over voltage is reached. Hence, AC voltage Vs will then be allowed to flow through the circuit which would light up the lamp.

15. TRIAC Phase Control TRIACS are used to control the average power provided to a load. The TRIAC is triggered in such a way that the supplied AC power for a load is controlled through a portion of each half-cycle. During the first half of the positive half cycle, it is switched off for a particular interval and then switched on during the remaining portion of the half-cycle. The portion of the positive cycle it is switched off is known as the delay angle and the portion it is switched on is known as the conduction angle. The action is similar for the other half of the AC waveform.

16. TRIAC Phase Control This type of phase control application makes use of two diodes to provide trigger pulses to the TRIAC gate. Diode (D1) conducts during the positive part of the half-cycle and the second diode (D2) conducts during the negative half cycle. The load resistance value of R1 sets the threshold value at which the TRIAC would trigger the conduction state.

17. Types of TRIAC Pin or standard Type: This type of TRIAC looks like a small IC which has three pins that are the MT1, MT2 and the gate with a heat sink on the top. They are commonly used in domestic appliances.

18. Types of TRIAC Capsule or disc Type: This type of TRIAC is in the shape of a disc or capsule. It has wires which are extended to the terminals. They have a high current carrying capability and are made of the ceramic seal. They are widely used in applications such as AC switching and fast motor control.

19. Types of TRIAC Stud Type: Stud type devices are used in high power applications. They comprise a screwed bottom that acts as the main terminal and has two terminals on the top. One terminal is the other main terminal and the second terminal on the top is the gate terminal. It is used in phase control applications such as converters, regulated power supplies, speed, wind or temperature-controlled circuits, power supply control, motor control, and lighting circuits.

20. Summary A TRIAC is a three-terminal, five-layered, bidirectional power electronic switching device. It acts like two anti-parallel SCRs connected for bidirectional operation. It has three terminals: MT1, MT2, and gate. MT1 and MT2 are interchangeable. The sensitivity of TRIACs is higher in the first quadrant and the second quadrant. TRIACs are commonly used in low-power AC control circuits. They are commonly used in dimmers, single-phase speed control, and heat control.