Electronics Basics: Understanding Triac ; Principle And Applications

In simple terms,TRIAC is an important electrical component that has two leads that are used to connect the AC supply and third lead used to start (trigger) the device. So what is special about Triac? Unlike some other electronic devices, such as transistors and diodes, TRIAC can conduct a current in both directions between it's two conductive leads. The trigger part of the device, called its Gate, turns the device partially on or off to varying degrees. Using a Gate in conjunction with the AC supply voltage, the TRIAC can be configured to pass only a portion of the AC signal through it. Therefore it is often used in power control and switching applications. We usually can see them in devices such as dimmer switches, fan speed controllers and motor speed controllers.

TRIAC symbol

The circuit symbol indicates the way in which the TRIAC operates. Seen from the outside it may be viewed as two back to back thyristors. We can see in below given figure, that, on the TRIAC symbol there are three terminals. These are the Gate and two other terminals are often referred to as an "Anodes" (Anode 1 and anode 2) or "Main Terminals."

It is worth noting that the TRIAC is an electronic component that is effectively based on the thyristor. Basically it provides AC switching for electrical systems. Like the thyristor, the TRIACs also has vast use in many electrical switching applications.

The word TRIAC, was originally created by merging a triode with AC. It was initially a trade name used by General Electric for its version of an AC switch based on gate-controlled silicon. However, this word has become a common name for all such devices, since its initial release. Actually, these devices are also known as bidirectional or two-way thyristors. Sometimes a device is simply called a thyristor, for the sake of convenience, but it is not entirely accurate, since the device is essentially a configuration of two thyristors. In order to understand Triac, we must know how a thyristor works.

A Thyristor is nothing but a specialized semiconductor device, usually consisting of four layers of silicon, connected together. In this four separate layers of silicon are processed such that they have alternating electrical charges of positive-negative-positive-negative or PNPN. For accessing the thyristor, each end of the layers serves as a connector lead. The positive end is the anode of the device and the negative end of it is called as cathode. The controlling gate connection is also fed to a positively charged layer which is sandwiched between two negatively charged layers.

Current–voltage characteristic of TRIAC

Under normal static conditions, alternating layers of a charge resist conduction of electric current through a thyristor. However, there is a certain limitation on the amount of voltage that can resist the device. If the voltage applied to the device exceeds this limit, the device will yield to the effect called an avalanche and begin to conduct an electric current. This limiting voltage is termed as breakdown voltage of thyristor.

To control the conduction of thyristor, a negative voltage is usually applied to its gate. This changes the charge in the positive layer of thyristor by a more negative slope, which can cause an avalanche or breakdown. By changing this controlling voltage at the gate, it is possible to vary the avalanche point of the thyristor, allowing the device to conduct an electric current only at a specified limit of voltage or higher.

It is known that the AC signals continuously alternate from the total positive voltage to the zero voltage, then to the total negative voltage, back to zero voltage, and then again to the full positive voltage and so on. Which means that the AC signal constantly changes the voltage level. As a result of this, by varying the gate voltage of the thyristor, it is possible to change and monitor the percentage AC voltage that can pass through a certain device.

However, thyristors can only conduct an electric current in one direction, due to which it will block half of the alternating voltage in the same way as a semiconductor diode. In order to use full AC voltage, a TRIAC is built from two thyristors. This is done by connecting the anode of one thyristor with the cathode of the other at one end, and the remaining cathode and anode at the other end. This way the two devices can conduct one alternating voltage in both directions. Two Gates, which are also interconnected, allow one control signal on the gate to control the AC signal passing through the TRIAC. Thus, TRIAC can provide any desired part of the AC voltage to a device, such as an motor. By varying this control gate voltage, the motor speed can be changed.

Applications

^{A typical dimmer circuit using Triac : Image credits}

For years, TRIACs are used in a number of applications. However in practice, they tend not to be used in high power switching applications. Triac has a non-symmetrical switching characteristics, due to which it has limited or negligible use in high power applications. For high power applications it creates a number of difficulties, especially with electromagnetic interference.

However TRIACs can be used for many electrical switching applications such as:

• Electric fan speed controls
• Small motor controls
• Domestic light dimmers
• Control of small AC powered domestic appliances

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References:

• Triac Tutorial
• Triac
• Thyristor
• Basic Types of Thyristors and Applications