Basics of Triac

Silicon controlled Rectifier or SCR is a unilateral device where the control of the supply to the load is only half waveform. In order to achieve full waveform control, one with bilateral control is used.

It consists of two SCR connected in inverse parallel where the gates are connected together. The connections are labelled as MT1(Main Terminal 1), MT2(Main Terminal 2) and G(Gate).

Current can flow in either direction between MT1 and MT2 terminals when a small gate current is applied between MT1 and the gate terminal. It is turned ON by triggering a positive or negative current between MT1 and the gate.

The holding current is the minimum current required to hold it on. As with SCR, the control of large load current can be triggered by miliampere gate current, hence making it a convenient switch for AC circuits. It can also do phase control where a certain percentage of power from the mains is supplied to the load.

It is used in applications such as light dimmers and other motor control devices.

The setback of using this device is that it causes electromagnetic interference due to the sudden rise in the line current when it is gated ON. This chopping of the mains supply causes harmonics and may cause disturbances to other devices nearby.

It could cause interference in radio and television receivers as well as microcontroller based equipment. As such, care must be taken to filter out these harmonics to an acceptable level.

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