DIACs and TRIACs: Bidirectional Semiconductor Devices

Introduction

Introduction to DIACs and TRIACs

Why Bidirectional Devices?

  • AC power systems require devices that can conduct in both directions

  • Unidirectional devices (like SCR) need complex arrangements for AC control

  • Need for simple, cost-effective AC power control solutions

Key Features

  • DIAC: DIode for Alternating Current - voltage-triggered switch

  • TRIAC: TRIode for Alternating Current - current-controlled switch

  • Both belong to the thyristor family of semiconductor switches

  • Commonly used together for AC phase control applications

  • Essential for residential and industrial AC power control

DIAC (DIode for Alternating Current)

What is a DIAC?

Basic Definition

  • A two-terminal semiconductor device with bidirectional switching capability

  • Also known as bidirectional avalanche diode or two-terminal AC switch

  • Terminals designated as Main Terminal-1 (MT1) and Main Terminal-2 (MT2)

  • Sometimes called Anode-I and Anode-II

Key Characteristics

  • No gate terminal - voltage-controlled only

  • Exhibits negative resistance in conduction region

  • Symmetric operation in both directions

  • Fixed breakover voltage (cannot be controlled)

DIAC: Construction and Structure

Physical Structure:

  • PNPNP structure - arrangement of semiconductor layers

  • Symmetric construction with interchangeable terminals

  • Can conduct current in either direction

  • Fabricated using semiconductor technology

Operation Principle:

  • Works as bidirectional avalanche diode

  • Switches from OFF to ON state when breakover voltage is reached

DIAC Construction, Symbol and Equivalent Circuit
DIAC Construction, Symbol and Equivalent Circuit

DIAC: V-I Characteristics

Operating Regions:

  • Blocking state: When \(|V| < V_{\mathrm{BO}}\), only leakage current flows

  • Conduction state: When \(|V| \geq V_{\mathrm{BO}}\), device conducts with low resistance

  • Negative resistance: Current increases while voltage decreases during switching

  • Operations in quadrant I and III only

V-I Characteristics of a DIAC
V-I Characteristics of a DIAC

Key Parameters

  • Breakover voltage (\(V_{\mathrm{BO}}\)): Typically around \(30\ \mathrm{V}\)

  • Voltage drop during conduction: About \(3\ \mathrm{V}\)

  • Current limiting: Amplitude limited by external resistance

DIAC: Switching Mechanism

Turn-ON Process

  1. Applied voltage increases across DIAC terminals

  2. When \(|V| \geq V_{\mathrm{BO}}\), avalanche breakdown occurs

  3. Device switches to low-impedance state (negative resistance)

  4. Current amplitude is limited by external resistance

Turn-OFF Process

  • Device turns OFF when current drops below holding current

  • No gate control available - purely voltage and current dependent

  • Natural turn-OFF at current zero crossing in AC circuits

Important Note

  • DIAC has fixed switching characteristics

  • Cannot be controlled once breakdown voltage is reached

  • Firing angle is fixed for a given supply voltage

DIAC Circuit Operation

Basic DIAC Circuit Operation

  • DIAC turns ON when supply voltage reaches breakover voltage

  • During positive half-cycle: Conducts when \(v > V_{\mathrm{BO}}\)

  • During negative half-cycle: Conducts when \(v < -V_{\mathrm{BO}}\)

  • Fixed firing angle based on \(V_{\mathrm{BO}}\) and supply voltage

DIAC Circuit and Current Waveforms
DIAC Circuit and Current Waveforms

Applications

  • Primarily used as triggering device for TRIAC

  • Two-terminal AC switch applications

  • Oscillator circuits

  • Simple AC control circuits

TRIAC (TRIode for Alternating Current)

What is a TRIAC?

Definition and Purpose

  • Three-terminal semiconductor device for bidirectional AC control

  • Equivalent to two SCRs connected anti-parallel with common gate

  • Enables variable power control in AC circuits

  • Can handle both directions of current flow

Advantages over SCR

  • Single device instead of two anti-parallel SCRs

  • Simplified gate drive circuitry

  • Reduced component count and cost

  • Better thermal management

Terminal Configuration

  • MT1: Main Terminal 1 (reference terminal)

  • MT2: Main Terminal 2

  • G: Gate (control terminal near MT1)

TRIAC: Structure and Operating Principle

Internal Structure

  • Complex semiconductor structure

  • Multiple current paths enable bidirectional conduction

  • Gate located near MT1 for control

  • Asymmetric structure leads to different sensitivities

TRIAC Structure, Symbol and Equivalent Circuit
TRIAC Structure, Symbol and Equivalent Circuit

Operating Principle

  • Works like two SCRs sharing a common gate

  • Gate current triggers the device based on MT2 polarity

  • Once triggered, gate loses control (latching behavior)

  • Device turns OFF at natural current zero crossing

TRIAC: V-I Characteristics

Quadrant Operation

  • First quadrant: MT2 positive with respect to MT1

  • Third quadrant: MT2 negative with respect to MT1

  • Can be triggered with or without gate signal

  • Multiple characteristic curves for different gate currents

TRIAC V-I Characteristic Curves
TRIAC V-I Characteristic Curves

Characteristic Features

  • Breakover voltage decreases with increasing gate current

  • Forward voltage drop during conduction is small

  • Gate control allows variable firing angle

  • Current ratings: 1 A to 300 A, Voltage ratings: up to 1200 V

TRIAC: Four Operating Modes

Four Modes of TRIAC Operation
Mode MT2 Gate Sensitivity
1 Positive Positive Most sensitive
2 Positive Negative Less sensitive
3 Negative Negative More sensitive
4 Negative Positive Least sensitive

Practical Considerations

  • Modes 1 and 3 are predominantly used (higher sensitivity)

  • Mode 2 and Mode 4 are less commonly used

  • Gate current requirement varies with operating mode

  • Commercial TRIACs optimized for efficient operation

TRIAC Operating Mode Details

Mode 1: MT2 Positive, Gate Positive

  • Most sensitive mode of operation

  • Gate current flows from gate to MT1

  • Structure operates similar to normal SCR

  • Minimum gate current required for triggering

Mode 3: MT2 Negative, Gate Negative

  • Second most sensitive mode

  • Remote gate effect through auxiliary structure

  • Slightly higher gate current needed than Mode 1

  • Good sensitivity for practical applications

Less Used Modes

  • Mode 2: MT2 positive, Gate negative - Less sensitive

  • Mode 4: MT2 negative, Gate positive - Least sensitive, rarely used

DIAC-TRIAC Firing Circuits

Basic DIAC-TRIAC Phase Control Circuit

Circuit Components

  • \(R_1\): Fixed resistor for protection

  • \(R_2\): Variable resistor for firing angle control

  • \(C\): Timing capacitor

  • \(R_3\): Gate current limiting resistor

Control Principle

  • Capacitor charges through \((R_1 + R_2)\)

  • When capacitor voltage = DIAC breakover voltage, DIAC conducts

  • Capacitor discharges through DIAC and TRIAC gate

  • TRIAC turns ON and conducts until current zero

DIAC-TRIAC Firing Circuit
DIAC-TRIAC Firing Circuit

Control Range

  • Practical firing angle range: \(10^\circ\) – \(170^\circ\)

  • Variable \(R_2\) controls charging time and thus firing angle

Circuit Operation Analysis

Step-by-Step Operation

  1. Charging phase: Capacitor \(C\) charges through \((R_1 + R_2)\)

  2. Trigger point: When \(v_C = V_{\mathrm{BO}}\) of DIAC, DIAC conducts

  3. Gate pulse: Capacitor rapidly discharges through DIAC to TRIAC gate

  4. TRIAC conduction: TRIAC turns ON and conducts until current zero

  5. Reset: Process repeats for next half-cycle

Resistance Control Effect

  • Small \(R_2\): Fast charging → Early firing → High power output

  • Large \(R_2\): Slow charging → Late firing → Low power output

  • Charging time constant: \(\tau = (R_1 + R_2) \times C\)

Waveforms for Different Control Settings
Waveforms for Different Control Settings

TRIAC Firing Circuit Waveforms

For Minimum \(R_2\) (Early Firing)

  • Capacitor charges quickly

  • DIAC fires early in the half-cycle

  • TRIAC conducts for longer duration

  • Higher power delivered to load

For Maximum \(R_2\) (Late Firing)

  • Capacitor charges slowly

  • DIAC fires late in the half-cycle

  • TRIAC conducts for shorter duration

  • Lower power delivered to load

Waveform Asymmetry Issue

  • Basic circuit produces unsymmetrical waveforms

  • Due to TRIAC characteristics and capacitor hysteresis

  • Capacitor retains some charge when input voltage becomes zero

  • Additional components needed for symmetrical operation

Improved TRIAC Firing Circuit

Problems with Basic Circuit

  • Asymmetric triggering due to capacitor hysteresis

  • Different positive and negative half-cycle behavior

  • Unequal power delivery in both half-cycles

Improved TRIAC Control Circuit
Improved TRIAC Control Circuit

Circuit Improvements

  • Additional \(R_3\) and \(C_1\): Provide symmetrical discharge path

  • Snubber circuit: R-C network for protection against voltage spikes

  • Better waveform symmetry: Equal conduction in both half-cycles

  • Commercial circuits: Include EMI filters and protection elements

Protection and Practical Considerations

TRIAC Protection Requirements

Snubber Circuit

  • Purpose: Limit \(dv/dt\) to prevent false triggering

  • Components: Small R-C network across TRIAC

  • Protection: Against voltage transients and spikes

TRIAC Circuit with Protection Elements
TRIAC Circuit with Protection Elements

Additional Protection Elements

  • Fuses: Overcurrent protection for safety

  • Snubber circuits: Essential for inductive loads

  • EMI suppression: Filters to reduce electromagnetic interference

  • Thermal management: Heat sinks for high-power applications

Applications

Applications of DIAC-TRIAC Circuits

Residential Applications

  • Light dimmers: Variable lighting control

  • Fan speed controllers: Ceiling fans and ventilation

  • Heating control: Electric heaters and cooking appliances

  • Motor control: Single-phase AC motors

Industrial Applications

  • Temperature controllers: Process heating control

  • Motor soft starters: Reduced inrush current

  • Static AC switches: Contactless switching

  • Heat control systems: Industrial heating applications

Load Compatibility

  • Resistive loads: Excellent performance (heaters, lamps)

  • Inductive loads: Requires snubber circuits (motors, transformers)

  • Universal motors: Good performance for speed control

Summary and Comparison

DIAC vs TRIAC: Complete Comparison

Device Comparison
Parameter DIAC TRIAC
Number of terminals 2 (MT1, MT2) 3 (MT1, MT2, Gate)
Control method Voltage-triggered only Gate current controlled
Triggering Fixed at \(V_{\mathrm{BO}} \approx 30V\) Variable with gate signal
Power handling Low (mW range) High (up to several kW)
Primary function Trigger pulse generation AC power control
Turn-off method Current below holding value Current zero crossing
Applications TRIAC triggering Motor control, heating, lighting
Voltage rating Around 30V Up to 1200V
Current rating Few hundred mA Up to 300A

Key Learning Points

DIAC Characteristics

  • Two-terminal bidirectional avalanche diode

  • Fixed breakover voltage around 30V

  • Primarily used for triggering TRIACs

  • No control over switching once breakover voltage is reached

TRIAC Characteristics

  • Three-terminal bidirectional thyristor

  • Gate-controlled switching for variable power control

  • Four operating modes with different sensitivities

  • Widely used for AC power control applications

Circuit Design Considerations

  • DIAC-TRIAC combination provides simple AC control

  • Protection circuits essential for reliable operation

  • Asymmetry issues require careful circuit design

  • Snubber circuits needed for inductive loads