TRIAC – Definition, Applications & Working

Definition

TRIAC

TRIAC

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:

TRIAC-SYMBOL

TRIAC-SYMBOL

  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

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.

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.

RATE EFFECT

RATE EFFECT

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.

Backlash Effect

Backlash Effect

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.

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.

TRIAC Circuit

TRIAC Circuit

TRIAC Wave Forms

TRIAC Wave Forms

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.

TRIAC BT136:

TRIAC BT136 is a family of TRIAC, it has current rate of 6AMPs. We already have seen an application of TRIAC using BT136 above.

Features of BT136:

  • Direct triggering from low power drivers and logic ICs
  • High blocking voltage capability
  • Low holding current for low current loads and lowest EMI at commutation
  • Planar passivated for voltage ruggedness and reliability
  • Sensitive gate
  • Triggering in all four quadrants

Applications of BT136:

  • Universally useful in motor control
  • General purpose switching

TRIAC BT139:

TRIAC BT139 is also comes under TRIAC family, it has current rate of 9AMPs. The main difference between BT139 and BT136 is current rate and BT139 TRIACS are used for high power applications.

Features of BT139:

  • Direct triggering from low power drivers and logic ICs
  • High blocking voltage capability
  • Planar passivated for voltage ruggedness and reliability
  • Sensitive gate
  • Triggering in all four quadrants

Applications of BT139:

  • Motor control
  • Industrial and domestic lighting
  • Heating and static switching

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