ACST12-7SG

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Thyristors - TRIACs

A TRIAC is a three-terminal, solid-state AC switch. It is a type of thyristor or silicon-controlled rectifier and is basically a two-terminal ac switch but with two main terminals connected in parallel. TRIACs perform the function of two SCRs connected in inverse parallel, so that when a command current is supplied to the Thyristor it will switch a load on or off when the correct amount of current is crossed. TRIACs are well suitable for applications such as lamp dimming, light dimming, small motor control, heating control and AC switching.

When a TRIAC is on, it operates just like two SCRs connected in inverse parallel, that is, it will stay on when a steady small current is applied and it will not shut down until the current properly drops to zero. It can be turned off by switching off the AC power or by resetting the AC voltage. The TRIAC is the most common device used in AC switching circuits. It is used in applications such as lamp dimming, light dimming, small motor control, heating control and AC switching.

In order to understand its working principle we need to consider its three main terminals. The main terminals are the main gate (MTG), the main anode (MA) and the main cathode (MC). The main gate is connected to the AC source, so when a voltage is applied to this terminal, a current can flow through the MTG, MA and MC terminals. If the current is higher than a certain amount (called the Holding Current) the TRIAC will conduct, even if the voltage is small. When a voltage higher than the Holding Current is applied to the MTG terminal, the TRIAC will start to conduct and turn on. This is known as the Firing Phase.

Once the TRIAC has been turned on, the device will remain in the on state even if the firing threshold is not crossed. The maximum current that the TRIAC can carry depends on the device characteristics and the maximum peak current potential (IPM) is determined by the design of the TRIAC. The TRIAC is in an \'on\' state until the voltage across the MA and MC terminals drops below the Holding Current.

The TRIAC can also be used to dim lamps, or to control the speed of a motor. This is done by controlling the firing angle of the TRIACs. The angle is defined as the angle between the moment at which the TRIAC turns on and the moment it turns off. Therefore, by controlling the angle of the TRIACs, we can control the brightness of the lamp, or the speed of the motor, depending on the application.

The application field for a TRIAC is very wide and covers a lot of aspects of electrical engineering. It is an invaluable tool for controlling AC circuits and is also used in computerized control systems and in some industrial applications such as AC welding machines. It is also used in automotive, railway and robotics control systems. TRIACs are also used in dimmers and in home appliances such as washing machines, ovens and air conditioners.

In summary, the TRIAC is a very useful device for switching AC currents. Its main uses are in AC switching circuits, dimmers and in home appliances. Its main terminal structure consists of the MTG, MA and MC terminals and its firing threshold is determined by the Holding Current. Its application field is very wide ranging from computerized control systems to AC welding machines.

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