TRIAC POWER CONTROL MECHANISM

Schedule TRIAC in power control system

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.


Usually 4 modes of triggering is possible in TRIAC:
1. A positive voltage at MT2 and a positive pulse at the gate
2. A positive voltage at MT2 and a negative pulse at the gate
3. A negative voltage at MT2 and positive pulse at the gate
4. A negative voltage at MT2 and a negative pulse at the gate
Unlike SCRs, Trics 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.
Rate Effect Severely Affects the Working of TRIAC

There is an internal capacitance exists between the MT1 and MT2 terminals of the Triac. If the MT1 terminal is supplied with a sharply increasing voltage, then it results in the gate voltage break through. This triggers the Triac unnecessarily. This phenomenon is called Rate effect. The Rate effect usually occurs due to the Transients in the mains and also due to high inrush current when heavy inductive loads switch on. This can be reduced by connecting an R-C network between the MT1 and MT2 terminals.

Backlash Effect is Severe in Lamp Dimmer Circuits:

Back lash effect is the severe Control Hysteresis that develops in the lamp control or speed control circuits using a Potentiometer to control the Gate current. When the resistance of the potentio meter increases to maximum, the brightness of the lamp reduces to minimum. When the pot is turned back, the lamp never turns on until the resistance of the pot decreases to minimum. The reason for this is the discharging of the capacitor in the Triac. The lamp dimmer circuits use a Diac to give triggering pulse to the gate. So when the capacitor inside the Triac discharges through the Diac, the Back lash effect develops. This can be rectified by using a Resistor in series with the Diac or by adding a capacitor between the Gate and the MT1 terminal of Triac


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Effect of RFI on TRIAC

Radio Frequency Interference severely affects the functioning of Triacs. When the Triac switches on the load, the load current increases sharply from zero to a high value depending on the supply voltage and resistance of the load. This result in the generation of pulses of RFI. The strength of RFI is proportional to the wire connecting the load with the Triac. An LC-RFI suppressor will rectify this defect.



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