TRIAC Full Form – triode for alternating current

TRIAC

A three terminal semiconductor device called a TRIAC is used to regulate current. The word TRIode, which stands for Alternating Current, gives it its name.

It is essentially a development of the SCR or thyristor, but the TRIAC is a bidirectional device as opposed to the thyristor which can only conduct in one direction.

The TRIAC’s mode of operation is recognised by the circuit symbol. From the outside, it appears to be two thyristors placed back to back, and this is what the circuit symbol denotes.

TRIAC symbol

On the TRIAC symbol there are three terminals. These are the Gate and two other terminals are often referred to as an “Anode” or “Main Terminal”. As the TRIAC has two of these they are labelled either Anode 1 and Anode 2 or Main Terminal, MT1 and MT2.

TRIAC Working

A part that is essentially based on the thyristor is the TRIAC. It offers electrical systems AC switching. The TRIACs are utilised in numerous electrical switching applications, just like the thyristor. They specifically use circuits in light dimmers and other devices that allow for the use of both AC cycle halves.

This makes them more efficient in terms of the usage of the power available. While it is possible to use two thyristors back to back, this is not always cost effective for low cost and relatively low power applications.

It is possible to view the operation of a TRIAC in terms of two thyristors placed back to back.

The fact that the TRIAC does not switch symmetrically is one of its flaws. It frequently has an offset, switching for each half of the cycle at a different gate voltage. This leads to more harmonics being produced, which is bad for EMC performance and also causes an imbalance in the system.

In order to improve the switching of the current waveform. And ensure it is more symmetrical is to use a device external to the TRIAC to time the triggering pulse. A DIAC placed in series with the gate is the normal method of achieving this.

TRIAC Operation

With switch S open, there will be no gate current and the triacs is cut off. Even with no current the triacs can be turned on provided the supply voltage becomes equal to the breakover voltage.

When switch S is closed, the gate current starts flowing in the gate circuit. Breakover voltage of triacs can be varied by making proper current flow. Triacs starts to conduct whether MT2 is positive or negative w.r.t MT1.

If terminal MT2 is positive w.r.t MT1 the TRIAC is on and the conventional current will flow from MT2 to MT1. If terminal MT2 is negative w.r.t MT1 the TRIAC is again turned on and the conventional current will flow from MT1 to MT2.

VI Characteristics

Triac’s Ist and IIIrd quadrant V-I curves are nearly identical to SCR’s Ist quadrant V-I curve. Although either a positive or negative gate control voltage can be used to operate the triac, in typical operation the gate voltage is typically positive in quadrant I and negative in quadrant III. Gate current affects the supply voltage at which the triac turns on. the higher gate current and lower supply voltage required to turn on a triac. This enables smooth, continuous control of a.c. power in a load from zero to full power with no loss in the controlling device using a triac.

Advantages and disadvantages

When requiring to switch both halves of an AC waveform there are two options that are normally considered. One is to use a TRIAC, and the other is to use two thyristors connected back to back – one thyristor is used to switch one half of the cycle and the second connected in the reverse direction operates on the other half cycle. As there are two options the advantages and disadvantages of using a TRIAC must be weighed up.

Advantages

1. Can switch both halves of an AC waveform
2. Single component can be used for full AC switching

Disadvantages

1. A TRIAC does not fire symmetrically on both sides of the waveform
2. Switching gives rise to high level of harmonics due to non-symmetrical switching
3. More susceptible to EMI problems as a result of the non-symmetrical switching
4. Care must be taken to ensure the TRIAC turns off fully when used with inductive loads.

Applications

TRIACs are used in a number of applications. However they tend not to be used in high power switching applications – one of the reasons for this is the non-symmetrical switching characteristics. For high power applications this creates a number of difficulties, especially with electromagnetic interference.

However TRIACs are still used for many electrical switching applications:

1. Domestic light dimmers
2. Electric fan speed controls
3. Small motor controls
4. Control of small AC powered domestic appliances