Triac

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Triac

A triac is made up of 2 thyristors placed head-to-tail in order to be bidirectional, usable in AC.It is also called solid state relay and it is the basis of dimmers.

Is an active component increasingly used in today's assemblies. Its direct use on the sector makes it possible to switch large loads from much more modest circuits.


Triac Symbol
Triac Symbol

The three electrodes of the triac are called gate (control electrode), and MT1 and MT2 (for Anodes 1 and 2). These last two electrodes ensure the main conduction.

Operation

Once triggered by an impulse on the gate, the triac lets the current flow until this current falls below a critical threshold (holding current). Due to this structure, the triac is used to control the passage of the two halfwaves of an alternating current (while the thyristor only conducts during one halfwave).

simple trigger with triac

The circuit above is very simple trigger. The phase shifter network Rc charges the capacitor. when it reaches a voltage at the terminals, greater than the reversal voltage of the diac, this device provides a positive impulse on the trigger of the triac, which is put into conduction.

By making R adjustable, the time constant of the circuit and the switching moment are modified with respect to the sinusoid of the sector.

Triggering takes place in modes 1 and 3 of the triac. The discharge by the Diac of a 0.1 microfarad capacitor allows the triggering of all common triacs.

This controls the power switched by the triac for the installation.

this circuit is phase-controlled and is therefore affected by a control hysteresis and uncertain operation on inductive load.

Using the triac as a dimmer

For use of the triac as a dimmer, an RC network is often used, if necessary associated with a diac, a bidirectional diode making it possible to obtain an even greater phase shift. The intensity in the load is varied by means of the variable resistor.

Using the triac as a dimmer

Two important points: in the case of an inductive load, it is necessary to add a triac protection circuit, by connecting a resistor and a capacitor in parallel, and an anti-interference circuit, comprising a self with or without capacitors. . In addition, as soon as the power exceeds 100 W, the triac must be equipped with a heat sink.

It should never be forgotten that the triac is directly connected to the mains and that all the necessary precautions should therefore be taken in this regard, starting with perfect insulation of the assembly.

The applications

The triac is above all intended to drive loads connected to the 230 V mains, for example a bulb (non-inductive load) or an electric motor (inductive load). It is used to make light dimmers (dimmers), speed variators for the motors of household or DIY appliances.

It also allows, associated with a sensor (photoresistor, etc.) to control a switching or regulation device (heating, solenoid valve, etc.) by opening or closing the circuit.

Warning! It should be emphasized that the triac, even if it is very safe to operate, is a component to be handled with great care, since it is connected to the mains. As soon as an assembly is connected to the sector, think about safety.


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