TRIAC and DIAC

The TRIAC is a bidirectional thyristor that conducts current in both directions when triggered. It is primarily used in AC power control applications like fan regulators and light dimmers.

Core Concept

A TRIAC behaves like two inverse parallel SCRs with a common gate. It operates in four distinct quadrants depending on the polarities of MT2 and the gate terminal relative to MT1.

Quadrant I positive gate and Quadrant III negative gate are the most sensitive triggering modes. Popular devices like the BT136 are rated for 4A RMS continuous current.

A DIAC is often used in series with the TRIAC gate to provide sharp symmetrical triggering pulses. The DIAC breaks over at around 32V allowing a sudden surge of gate current.

Key Formulas

RMS output voltage for an AC voltage controller depends on the firing angle alpha. Vrms equals Vs multiplied by the square root of 1 minus alpha divided by pi plus sine of two alpha divided by two pi.

Given: Vs = 230 V, alpha = 90 degrees (pi/2)
Formula: Vrms = Vs * sqrt(1 - (pi/2)/pi + sin(pi)/(2pi))
Steps: Vrms = 230 * sqrt(1 - 0.5 + 0)
Final Answer: 162.6 V

Exam Tip: TRIACs have poor dv/dt ratings compared to SCRs. They are rarely used in highly inductive load applications without a heavy snubber network.

Performance Parameters

• RMS current rating: up to 40 A
• Blocking voltage: up to 800 V
• Gate trigger current Igt: 5 mA to 50 mA
• DIAC breakover voltage: typically 32 V
• Commutating dv/dt: 5 to 20 V per microsecond

Quick Revision

• Bidirectional AC conduction
• Gate triggers in either direction
• Four modes of operation
• DIAC ensures symmetrical firing
• Ideal for domestic AC dimmers
• Exam trap: Assuming TRIACs handle high frequency switching well