TRIAC (Triode for AC) is the semiconductor device widely used in power control and switching applications. It finds applications in switching, phase control, chopper designs, brilliance control in lamps, speed control in fans, motors etc. The power control system is designed to control the distribution level of AC or DC. Such power control systems can be used to switch power to appliances manually or when temperature or light levels go beyond a preset level.
TRIAC is equivalent to two SCRs connected in inverse parallel with the gates connected together. As a result, the TRIAC functions as a Bidirectional switch to pass the current in both directions once the gate is triggered. TRIAC is a three terminal device with a Main terminal1 ( MT1), Main terminal 2( MT2) and a Gate. The MT1 and MT2 terminals are used to connect the Phase and Neutral lines while the Gate is used to feed the triggering pulse. The Gate can be triggered either by a positive voltage or negative voltage. When the MT2 terminal gets a positive voltage with respect to the MT1 terminal and the Gate gets a positive trigger, then the left SCR of the TRIAC triggers and circuit completes. But if the polarity of the voltage at the MT2 and MT1 terminals is reversed and a negative pulse is applied to the Gate, then the right SCR of Triac conducts. When the Gate current is removed, the TRIAC switches off. So a minimum holding current Ih must be maintained at the gate to keep the TRIAC conducting.
Triggering a TRIAC
Usually 4 modes of triggering is possible in TRIAC:
Factors Affecting Working of TRIAC
Unlike SCRs, TRIACs require proper optimization for its proper functioning. Triacs have inherent drawbacks like Rate effect, Backlash effect etc. So designing of Triac based circuits need proper care.
Working of TRIAC
A simple application circuit of TRIAC is shown. Generally, TRIAC has three terminals M1, M2 and gate. A TRIAC, lamp load and a supply voltage are connected in series. When supply is ON at positive cycle then the current flows through lamp, resistors and DIAC (provided a triggering pulses are provided at pin 1 of opto coupler resulting in pin 4 and 6 start conducting) gate and reaches the supply and then only lamp glows for that half cycle directly through the M2 and M1 terminal of the TRIAC. In negative half cycle the same thing repeats. Thus the lamp glows in both the cycles in a controlled manner depending upon the triggering pulses at the opto isolator as seen on the graph below. If this is given to a motor instead of lamp the power is controlled resulting in speed control.
Applications of TRIAC:
TRIACs are used in numerous applications such as light dimmers, speed controls for electric fans and other electric motors and in the modern computerized control circuits of numerous household small and major appliances. They can be used both into AC and DC circuits however the original design was to replace the utilization of two SCRs in AC circuits.There are two families of TRIACs, which are mainly used for application purpose, they are BT136, BT139.