L33 Diac Triac UJT

Silicon Controlled Rectifier (SCR) Overview

Introduction to SCR

• The session begins with a recap of the previous discussion on Silicon Controlled Rectifiers (SCR), highlighting its importance as a power device.

• Unlike normal rectifiers, SCRs have a control terminal that regulates the rectification process.

Characteristics of SCR

• An SCR consists of two transistors: one PNP and one NPN, which work together in its operation.

• Once turned off, an SCR cannot return to its off state naturally; it requires forced methods like anode current interruption or forced commutation to turn off.

Operational Regions

• In the first quadrant, an SCR operates in a forward blocking region when off, showing distinct voltage characteristics before switching on.

• The behavior of an SCR can be manipulated by increasing gate current, which reduces forward breakover voltage.

Introduction to DIAC

Understanding DIAC Functionality

• A DIAC is defined as a diode for alternating current (AC), functioning without a control terminal and allowing triggering in both directions.

• It features breakover voltage characteristics similar to those of an SCR but applies in both quadrants, enhancing AC applications.

Basic Construction and Operation

• The construction involves semiconductor layers arranged such that they can trigger under specific conditions without needing external control.

• When analyzing two diodes connected with opposing biases, only one diode becomes forward biased while the other remains reverse biased until sufficient voltage is applied.

Current Flow Dynamics

Diode Behavior Under Voltage Changes

• As external voltage increases across the diodes, D1 becomes forward biased while D2 remains reverse biased initially.

• Current flow is absent until the supply voltage reaches levels that allow D1 to conduct fully.

Understanding Diodes and Their Applications

Breakdown of Diode D2

• The diode D2 undergoes breakdown when reverse bias increases, leading to a sudden surge in reverse current, resembling a short circuit.

• This behavior can be represented graphically, showing the relationship between applied voltage and current flow.

Current Flow Characteristics

• Initially, only a small current flows due to minority carriers; as voltage increases across the depletion region, there is a sudden increase in current.

• Reversing polarity results in diode D1 being reverse biased while D2 is forward biased, demonstrating that both diodes can conduct under different conditions.

Diak Structure and Functionality

• The combination of two diodes allows for bidirectional current flow; however, controlling this flow is not possible with just diaks.

• A diak's structure resembles two connected diodes (anode 1 and anode 2), enabling current flow in both directions but lacking control over which quadrant the current flows through.

Applications of Diaks

• Diaks are primarily used in AC switches where bidirectional current is needed. They also find applications in dimmer circuits for LED lighting and starter circuits for fluorescent lamps.

Introduction to Triacs

Differences Between Diaks and Triacs

• Unlike diaks, triacs allow precise control over switching actions. A triac can manage currents with respect to either anode terminal effectively.

• A triac is defined as a diak with an additional gate terminal that controls turn-on conditions for bilateral devices.

Understanding TRIAC and Uni Junction Transistor

TRIAC Basics

• A TRIAC can be created by connecting a DIAC with a control terminal, allowing it to function similarly to an SCR (Silicon Controlled Rectifier).

• The construction of a TRIAC involves multiple layers; the speaker notes that there are seven layers in total, which may differ from some textbooks stating six layers.

• The additional layer in the TRIAC's structure is meant to illustrate that the gate can connect at two different points, simplifying its operation.

• Current flow through the device occurs via PN junctions; if one gate layer is activated positively, it influences current direction through the device.

• Students are encouraged to consult textbooks or online resources for further understanding of both DIAC and TRIAC working principles.

Introduction to Uni Junction Transistor (UJT)

• The UJT is introduced as a special type of transistor characterized by having only one junction between P and N regions.

• Unlike standard bipolar transistors that have three layers and two junctions, UJTs operate with just one junction, making them unique.

• The construction includes an aluminum rod acting as the emitter terminal placed on an N-type silicon slab, creating a single junction.

• The positioning of the aluminum rod near one end of the silicon slab creates internal resistance differences essential for transistor action.

• In contrast to traditional transistors with three terminals (emitter, base, collector), UJTs feature two bases (B1 and B2) without a collector.

Understanding the Uni Junction Transistor (UJT)

Overview of UJT Structure

• The UJT consists of an aluminium rod and a silicon end-type slab, with two base regions featuring ohmic contacts.

• A lightly doped n-type silicon slab has two base contacts at both ends, forming a PN junction at the boundary with the aluminium rod, which is referred to as a uni-junction.

• The aluminium rod is alloyed to the silicon slab closer to one base contact (base 2), which is made positive relative to base 1 by applying Vbb volts.

Circuit Representation

• The circuit symbol for the UJT shows the emitter leg tilted due to its alignment closer to base 2 than base 1.

• The arrowhead in the symbol indicates conventional current flow due to hole movement when in forward-biased active state.

Electrical Equivalent and Characteristics

• To analyze characteristics, start from the emitter side; it includes a diode formed by the PN junction between the aluminium rod and n-type material.

• Two resistances (RB1 and RB2) are associated with each base region, with voltage Vbb applied across them.

Switching Conditions

• The UJT switches on or off based on conditions related to voltages across components. If emitter voltage (V) is less than diode drop plus VB1, it remains off; if greater, it turns on.

Characteristic Regions of UJT

• Three characteristic regions are identified: cutoff region (very low current), negative resistance region (current decreases as voltage increases), and saturation region (current stabilizes).

• In this context, confusion may arise regarding graph parameters; however, it's crucial that current versus emitter voltage relationships are understood clearly.

This structured summary provides insights into key concepts surrounding Uni Junction Transistors while linking directly back to specific timestamps for further exploration.

Understanding Power Devices: Diac, Triac, and Uni Junction Transistor

Characteristics of Current in Cut-off Region

• The current in the cut-off region is described as extremely small or almost zero. This indicates that the device is not conducting significantly during this phase.

• As voltage increases in the cut-off region, there is a gradual increase in current until it reaches saturation. This behavior is crucial for understanding how these devices operate under different conditions.

Graphical Representation of Device Characteristics

• When voltage and current characteristics are swapped, the graph's representation also changes accordingly. This highlights the interdependence of voltage and current in power devices like diacs, triacs, and uni junction transistors.

Overview of Key Power Devices

• The session covered three important power devices:

• Diac: A device used for switching applications.

• Triac: A bidirectional device useful for controlling AC power.

• Uni Junction Transistor (UJT): Known for its unique characteristics in triggering circuits.

• Understanding these devices is essential as they are significant topics within the syllabus. Students should focus on their basic working principles, construction, and characteristics.

Conclusion of Unit 4

• The discussion wraps up with an emphasis on the importance of mastering these three devices before moving on to subsequent units focused on power systems. This marks the completion of Unit 4 content. Students are encouraged to review these concepts thoroughly before progressing to new material in future classes.