AC107 Breakdown Devices PDF

AC107: BREAKDOWN DEVICES Introduction to Breakdown Devices dimmer switches) or control motor speeds without the inefficiencies of I. Definition of Breakdown Devices resistive methods...

AC107: BREAKDOWN DEVICES Introduction to Breakdown Devices dimmer switches) or control motor speeds without the inefficiencies of I. Definition of Breakdown Devices resistive methods. What are Breakdown Devices? Advantages of Threshold Switching: o Definition: Breakdown devices are o Reduced Power Loss: Since these electronic components engineered to devices don’t start conducting until switch on or conduct current only the voltage or current reaches a when certain voltage or current specific threshold, they minimize conditions are met. power loss when off, making them more o Threshold Activation: These devices efficient. rely on specific “threshold” o Increased Safety: With controlled conditions to operate. They remain in switching, they prevent unintended a non-conducting state until a set power surges and safeguard sensitive voltage (known as the “breakdown equipment. voltage”) or current is reached. 3. Real-World Relevance of Breakdown o Examples of Thresholds: Devices ▪ Voltage Threshold: A point at which Common Applications: the device “breaks down” and begins to conduct, such as in the case of a o Light Dimmers: By adjusting when in DIAC, which turns on only after the the AC cycle the device switches on, voltage surpasses a certain level. TRIACs can reduce the effective voltage reaching a light bulb, ▪ Current Threshold: Some devices, dimming it. like SCRs, will only continue conducting as long as the current o Appliance Speed Control: Fans and remains above a minimum level, certain appliance motors utilize ensuring they turn off safely if the phase control (enabled by TRIACs or current drops. SCRs) to vary speed without mechanical wear. 2. Why Breakdown Devices are Essential o Power Supply Units: Breakdown Controlled Switching Capabilities in devices like SCRs help in rectifying Power Electronics: AC to DC in power supplies, delivering controlled power output o Power Control: These devices are to devices. critical for applications where we need precise control over AC or DC The Role of Breakdown in Device Operation power. By switching at defined points, they allow us to adjust 1. Explanation of Electrical Breakdown power levels to suit various applications. What is an Electrical Breakdown? o Safe and Efficient Power o Definition: Electrical breakdown Management: In power systems, occurs when an insulating material breakdown devices provide safe suddenly begins to conduct due to an switching without sparking or applied voltage exceeding its arcing, reducing wear and breakdown voltage. In general prolonging device life. contexts, this can lead to failure, but in breakdown devices, it’s controlled to allow purposeful conduction. Applications of Breakdown Devices: o Natural and Controlled Breakdown: o AC vs. DC Control: Breakdown devices While breakdown is often considered work in both AC and DC applications, a failure mode in materials, allowing for versatile use in power breakdown devices use it electronics. For instance, TRIACs intentionally to enable switching. are preferred for AC control due to their ability to conduct in both Controlled Breakdown in Devices: directions, while SCRs are more suited to DC or one-way AC control. o Examples: o Efficient Power Adjustment: They ▪ SCRs conduct when a gate trigger allow controlled power flow, meaning voltage is applied, causing a they can adjust light levels (as in controlled breakdown that initiates conduction across the device. AC107: BREAKDOWN DEVICES ▪ DIACs and TRIACs trigger similarly by reaching a breakdown voltage, initiating a conduction path. o Why Control is Key: By controlling breakdown, these devices switch in a predictable, repeatable manner, making them reliable components in circuits needing precise on-off control. 2. Controlled vs. Uncontrolled Breakdown o Operation: Conduct only in one Uncontrolled Breakdown: direction (typically DC or o Description: Occurs unintentionally unidirectional AC control) and are in devices when the insulation fails turned on by a gate signal. under excessive voltage, SCR (Silicon Controlled Rectifier): potentially leading to damage. o Structure: Four-layer, three-junction o Example: A surge that exceeds the (PNPN) limits of a transistor might permanently damage the component by creating an unintended conduction path. Controlled Breakdown: o Description: Triggered at a precise voltage or current, which has been designed as a switching mechanism. o Characteristics: Conducts upon gate o Example: In an SCR, the gate voltage triggering and remains on until triggers conduction, creating a current falls below a certain level. controlled breakdown that enables power flow in a safe and managed o Common Use: Rectifiers in DC motor way. control and power regulation. Why This Matters: TRIAC (Triode for Alternating Current): o Safe Power Switching: Controlled o Definition: A bidirectional device breakdown allows these devices to that conducts in both directions, function reliably in high-power ideal for AC applications. applications where typical switches would struggle due to arcing. o Circuit Protection: Devices like SCRs are used in over-voltage protection circuits, where they can quickly trigger to shunt excess current, protecting sensitive components. Classifications of Breakdown Devices o Operation: Controlled by a gate trigger and typically works well in 1. Overview of Device Types light dimming, motor speed control, and AC power control. Thyristors: DIAC (Diode for Alternating Current): o Definition: A family of semiconductor devices that include SCRs and are o Structure: A two-terminal, generally used for high-power, high- bidirectional device without a gate. voltage applications due to their robustness and efficiency. AC107: BREAKDOWN DEVICES 2. Summary of Device Characteristics o Each device serves a specific type of power control: ▪ SCRs and Thyristors: Unidirectional, high-power DC applications. ▪ TRIACs and DIACs: Bidirectional, AC applications. o Operation: Conducts when a specific breakover voltage is reached. ▪ UJT and PUT: Timing and triggering in control circuits. o Use: Primarily used for triggering TRIACs in AC circuits. Foundational Understanding: UJT (Unijunction Transistor): o Understanding each device’s unique properties provides a foundation for o Definition: A three-layer, single PN selecting the appropriate component junction device with a unique based on application requirements. triggering characteristic. Engagement Questions: What electronic devices have you used where you can adjust power levels (like dimming or speed control)? Why might controlled breakdown be safer than other forms of switching? Examples/Analogies: Compare SCRs to valves in a water pipe system that only allow flow when a threshold is reached. o Characteristics: Known for its simplicity and reliability, often Use a light dimmer as a familiar used in timing circuits. example to illustrate phase control achieved through TRIACs. o Application: Used in relaxation oscillators and triggering circuits Overview of Breakdown Devices for SCRs. 1. Definition of Breakdown Devices Breakdown Devices and Threshold PUT (Programmable Unijunction Switching: Transistor): o Recap Definition: Reinforce that o Definition: A more versatile breakdown devices operate by unijunction transistor where the reaching a certain threshold voltage breakdown voltage can be set by the or current, which triggers their user. switch from a non-conductive to a conductive state. o Purpose in Circuits: Emphasize that this threshold switching allows precise control in circuits where direct mechanical switching would be less effective or too bulky. This makes them invaluable in high-power and phase-control applications. Practical Use of Threshold Operation: o Use: Typically found in applications o Reliability and Control: Explain needing precise control of timing that breakdown devices don’t conduct and phase, like in phase control until conditions are just right, circuits. allowing predictable switching that AC107: BREAKDOWN DEVICES can be integrated into timed or ▪ DIACs do not have a gate but conduct triggered circuits. when a certain voltage threshold is reached, making them ideal for o Typical Applications: Used for triggering other components like applications where gradual TRIACs. adjustment or specific timing is required, such as in motor speed controllers, dimmers, and power supply regulators. 3. Operational Principles of Each Device SCRs and Thyristors: 2. Breakdown Device Categories o Trigger/ Firing Mechanism: The SCR remains in an off state until a gate o Thyristors: A class of devices signal is applied, after which it including SCRs, designed for high- conducts until current drops below a power switching in one direction specific holding level. (typically in DC or rectified AC). o SCR (Silicon Controlled Rectifier): A specific type of thyristor that turns on with a gate pulse and stays on as long as current flows above a certain threshold. o TRIAC (Triode for Alternating Current): A bidirectional device that can conduct in both directions, ideal for AC applications. o DIAC (Diode for Alternating Current): A two-terminal device that conducts in both directions once a certain breakover voltage is reached, often used to trigger TRIACs. o UJT (Unijunction Transistor): A transistor that serves as a triggering device, often used in timing circuits due to its predictable switching characteristics. o PUT (Programmable Unijunction Transistor): A more advanced UJT where the breakdown voltage can be set, providing flexibility in timing and control applications. Unique Characteristics of Each Device: o Each device type is constructed to perform specific roles in power and phase control applications. o Applications: Commonly used in DC o Unidirectional vs. Bidirectional applications, rectifiers, and Operation: For instance, SCRs and situations where high-voltage Thyristors conduct in one direction, control is needed without switching while TRIACs conduct in both, making polarity. them versatile for AC power control. TRIACs: o Trigger Mechanisms: o Bidirectional Conduction: Unlike ▪ SCRs require a gate signal to start SCRs, TRIACs can conduct in both conducting. directions, allowing for AC control. ▪ TRIACs are triggered similarly but can conduct both ways, suitable for AC loads. AC107: BREAKDOWN DEVICES o 4. Device Functions and Applications o Triggering: Triggered by a gate pulse, and conduct regardless of Thyristors and SCRs: polarity, making them versatile in AC applications. o Primary Use: Typically used for DC control or unidirectional AC o Use in AC Phase Control: TRIACs are applications where power needs to be widely used in dimmers, as they switched on and off without reversal. control the portion of the AC cycle that reaches the load, adjusting o Applications: DC motor controls, AC power smoothly. rectifiers, and power regulators. DIACs: TRIACs: o Triggering: DIACs conduct only once o Bidirectional Control for AC: their breakover voltage is reached, Perfect for devices where we need and this conduction is power to flow in both directions, bidirectional. such as in AC power systems. o Applications: AC dimmers, fan speed controls, and heaters. DIACs: o Used with TRIACs: Often found in circuits where triggering a TRIAC at specific AC points is needed. o Applications: Dimmer switches, especially in consumer electronics o Function in AC Circuits: Commonly and lighting systems. paired with TRIACs, DIACs help UJT and PUT: control the timing of the TRIAC’s conduction phase, allowing smooth o Timing and Triggering: Both UJT and phase control. PUT are frequently used in circuits where timing is crucial, like in UJT and PUT: oscillators and pulse circuits. o UJT: Known for its ability to o Applications: Oscillators, timing switch at precise intervals, making circuits, and pulse triggering for it suitable for timing applications SCRs in control applications. like oscillators and trigger circuits. Engagement Questions What might be a disadvantage of a device like an SCR that only conducts in one direction? o Potential Answer: SCRs can’t control AC power in both directions, limiting them to unidirectional DC or rectified AC, meaning they are less flexible for full AC power o PUT: A more advanced UJT where users applications. can set the triggering voltage, adding flexibility in applications Can you think of a situation where you needing precise control over timing would need a device that can switch and phase. AC rather than DC? AC107: BREAKDOWN DEVICES o Potential Answer: AC dimmer switches o Gate Terminal: The gate serves as the for lighting and speed control in control point. A small current or fans are applications that require voltage applied to the gate initiates bidirectional (AC) control to the thyristor’s switch, allowing it effectively adjust power levels. to conduct between the anode and cathode. o Gate Control: By managing the gate Examples and Analogies signal, we control when the thyristor turns on, allowing for precise power Analogy for SCR and TRIAC (Traffic regulation. Flow): o SCR as One-Way Traffic: Imagine an SCR as a one-way traffic flow—current can 2. Operation of a Thyristor only travel in one direction, which makes it efficient for DC power Gate Triggering Mechanism: control. o Switching On: The thyristor remains o TRIAC as Two-Way Traffic: TRIACs, on off (non-conductive) until a small the other hand, work like two-way voltage or current pulse is applied traffic, making them ideal for AC to the gate. This pulse causes the control, as they can handle current central PN junction (J2) to break in both directions. down, initiating conduction. Example: DIAC and TRIAC in Dimmers: o Latching Characteristic: Once the thyristor switches on, it “latches” o DIAC as a Starter: Explain that the and continues conducting, even if the DIAC acts like a starter for the gate signal is removed. This latching TRIAC, allowing it to turn on at the effect continues until the current right time in the AC cycle. This flowing through the device falls triggers the TRIAC to regulate power below a specific holding current, at flow to the light bulb smoothly, which point it switches off. adjusting brightness levels. How Latching Works: o Current Flow: Once latched, the Thyristors thyristor will stay in the on state 1. Thyristor Structure as long as a minimum current, known as the holding current, flows through Understanding the PNPN Structure: it. This property makes it highly reliable for continuous conduction. o Four-Layer Design: A thyristor has four alternating layers of P and N- o Importance of Holding Current: If the type semiconductor materials, current drops below this level (e.g., creating a structure that looks like in an AC cycle when the waveform this: P-N-P-N. reaches zero), the thyristor automatically switches off and will o Three PN Junctions: The four layers only turn back on if the gate create three PN junctions (labeled receives another pulse. J1, J2, and J3) inside the device, each with distinct roles in switching o Unidirectional Control: Since the and conduction. thyristor conducts only in one direction, it is typically used for o Analogy: Imagine the structure as a DC or rectified AC applications. For layered sandwich, where each layer AC power control, it can only control interacts with its neighbor. In this half of the cycle (positive or case, the structure controls how and negative), depending on how it’s when current flows, with J2 being the oriented in the circuit. central junction controlling switching. Key Terminals: Switching Characteristics: o Anode and Cathode: Current flows from o Speed and Efficiency: Thyristors are the anode (positive) to the cathode fast-acting devices, enabling them (negative) once the device is to handle large power loads triggered. efficiently. AC107: BREAKDOWN DEVICES o Control Limitations: While they work o Potential Answer: The latching well for DC and rectified AC, their characteristic is advantageous unidirectional nature makes them because it ensures the thyristor less versatile for full AC power remains on, maintaining power flow control, where bidirectional devices even if the gate pulse is removed. like TRIACs are more suitable. However, this can be a disadvantage in applications where we need precise on-off control, as the thyristor 3. Applications of Thyristors cannot turn off until the current drops below the holding level. AC Power Control: Why might thyristors be more suited to o Rectifier Circuits: Thyristors are high-power DC control than for AC commonly used in rectifier circuits, applications? where they convert AC to DC. Their ability to control high voltages and o Potential Answer: Thyristors conduct currents makes them suitable for only in one direction, making them industrial AC to DC conversion. ideal for DC or rectified AC applications where power flows in one o Phase Control: Thyristors can be used direction. In AC applications where to control the phase angle in an AC bidirectional control is needed waveform, determining when in the AC (like full-wave AC control), TRIACs cycle they begin conducting. This are often more practical. allows them to modulate the output DC power for applications like charging, heating, and power Examples and Analogies supplies. Mechanical Latch Analogy: Motor Speed Control: o Explanation: Think of a thyristor o Voltage Control in DC Motors: In DC like a mechanical latch. Once you push motors, thyristors control the a door with a latch, it stays closed voltage supplied to the motor, which until you actively release it. in turn controls its speed. By Similarly, a thyristor, once turned adjusting the timing of the gate on, stays latched in the on state pulse, we can vary the output voltage until the current drops below the and thus motor speed, allowing for holding level. smooth and efficient motor operation. o Advantages: Using thyristors in Home Dimmer Switch Example: motor control is more efficient than using resistive methods, as they o Explanation: In a home light dimmer, avoid significant power losses. the thyristor controls light brightness by adjusting the phase Dimmer Circuits: angle. When the thyristor delays conduction in each AC cycle, it lets o Light Dimming: Thyristors are also only a portion of the waveform key components in light dimmers. By through, dimming the light. By adjusting when in the AC cycle the advancing the conduction point, it thyristor switches on, they control allows more current to reach the the amount of current flowing to the bulb, increasing brightness. light bulb, thus varying brightness. Silicon Controlled Rectifier (SCR) o Smooth Control: This method allows for a gradual adjustment of light 1. Construction of SCR - The SCR is intensity, making thyristors common essentially a type of thyristor with a in household and commercial lighting four-layer PNPN structure, similar to applications. the basic thyristor but refined for specific high-power applications. The SCR has three main terminals: Engagement Questions Anode (A): The positive terminal, How might the latching characteristic where the current flows into the SCR. of a thyristor be both an advantage and a disadvantage? Cathode (K): The negative terminal, where the current exits. AC107: BREAKDOWN DEVICES Gate (G): The control terminal, which Overvoltage Protection: initiates the SCR’s conduction. Surge Suppression: SCRs can act as a Comparison to Thyristor: The SCR is failsafe, diverting excess current designed to handle higher currents and is during an overvoltage event to typically optimized for rectification protect other sensitive components. purposes, making it a staple in DC and high-power applications. Example: SCRs in circuit breakers act like an automatic switch, 2. Operation of SCR disconnecting power during faults. Triggering Mechanism Gate Pulse: When a small pulse is TRIAC (Triode for Alternating Current) applied to the gate, the SCR switches on, allowing current to flow between 1. Construction of TRIAC: Dual SCR the anode and cathode. Configuration: A TRIAC essentially behaves like two SCRs in parallel but Latching: Once triggered, the SCR facing opposite directions, allowing remains in the on state even if the bidirectional conduction. gate pulse is removed, as long as the current remains above a certain Three-Terminal Device: holding level. This latching property Main Terminals (MT1 and MT2): These is a defining feature, ensuring terminals handle the bidirectional reliable conduction until conditions current. change. Gate (G): The gate initiates conduction in either direction, depending on the polarity. Switching Off: Versatility: This construction makes Current Threshold: The SCR turns off TRIACs ideal for handling AC, as they automatically when the current drops allow current to flow in both directions. below the holding level. This is particularly useful in rectification 2. Operation of TRIAC and motor control, where load fluctuations naturally create this Bidirectional Conduction: drop, resetting the SCR for the next Gate Triggering: A gate pulse cycle. triggers the TRIAC, allowing it to Unidirectional Conduction: conduct in either direction, depending on the polarity of the DC and Rectified AC Use: Since the applied voltage. SCR conducts only in one direction, it’s ideal for controlling rectified AC Cycle Conduction: TRIACs can turn AC (converted to unidirectional on at any point in the AC waveform, pulses) and DC. allowing phase control in both halves of the AC cycle, giving full control 3. Applications of SCR over power delivery. Rectification in DC Motor Drives: Phase Control: SCRs control the DC voltage applied to motors, making them suitable for Variable Conduction Angle: By speed regulation in industrial DC adjusting when in the AC cycle the motors, elevators, and machinery. gate is triggered, TRIACs control the amount of power reaching the load, Example: By adjusting the timing of making them suitable for dimmers and gate pulses, SCRs vary the output speed controllers. power, controlling motor speed smoothly. Smooth Control of AC Power: This phase control ability allows TRIACs to Role in AC-DC Conversion: SCRs are deliver power gradually, which is used in inverters to convert DC back beneficial in applications requiring into AC power by controlling the adjustable output, like dimmable output waveform shape. lighting. Application in UPS Systems: SCRs in inverters help maintain power supply during outages by converting battery-stored DC power to AC. AC107: BREAKDOWN DEVICES 3. Applications of TRIAC Triggering TRIACs: DIACs are often paired with TRIACs to provide a precise Light Dimmers: triggering point. When used together, the DIAC ensures that the TRIAC conducts AC Control for Lighting: TRIACs only when the breakover voltage is control the brightness of lights by reached, giving fine control over phase adjusting the conduction phase in the angles. AC cycle. When the gate triggers later in the cycle, less power 3. Applications of DIAC reaches the bulb, reducing brightness. Triggering TRIACs in Light Dimming Circuits: Fan Speed Controls: Controlled Phase Angle for Smooth Variable Speed Control: In AC fans, Dimming: In a dimmer circuit, DIACs TRIACs modulate the voltage, enable TRIACs to trigger at a specific allowing smooth adjustment of fan point in the AC cycle, providing smooth speed without mechanical switches, control over the light intensity. saving energy and reducing wear. Reliable Switching: Using a DIAC with a AC Power Control: TRIAC ensures that the TRIAC fires at the right time in each cycle, allowing Broad Use in Household Appliances: for stable dimming and reducing flicker. TRIACs are commonly found in appliances like heaters, which need AC Power Control: gradual power regulation to control Timing and Precision: In AC control temperature. circuits, DIACs ensure that TRIACs only switch on at precise points, helping to manage power for applications that DIAC (Diode for Alternating Current) require reliable control without complex circuitry. 1. Construction of DIAC Engagement Questions Structure: For SCRs: What might be the advantages Two-Terminal Device: Unlike TRIACs or of an SCR’s latching characteristic in SCRs, DIACs have only two terminals and motor control applications? Answer: no gate, meaning they conduct without an This latching makes SCRs ideal for motor external trigger signal. control, as they maintain a stable on- Symmetrical PNPN Construction: DIACs state for consistent power without are designed to conduct in either continuous gate pulses, leading to direction, making them bidirectional efficient speed control. devices suitable for AC applications. For TRIACs: Why do you think TRIACs are Breakover Voltage Mechanism: The DIAC more popular than SCRs for household AC remains non-conductive until a specific applications like dimmers? Answer: breakover voltage is reached, at which TRIACs can control power in both AC point it switches on and allows current directions, making them suitable for to flow. devices powered by AC, like light dimmers, whereas SCRs control only one 2. Operation of DIAC direction. Triggering Mechanism: For DIACs: What might be the benefit of a DIAC’s breakover voltage in dimmer Breakover Voltage: DIACs do not conduct circuits? Answer: The breakover voltage until the voltage across them exceeds a allows for predictable switching, certain threshold (breakover voltage). enabling precise TRIAC control, which Once this voltage is reached, they creates smoother dimming effects and switch on and conduct until the current reduces flickering. drops below a minimum level. Examples and Analogies Bidirectional Conduction: SCR in DC Motor Control:Think of the SCR AC Compatibility: DIACs work well in AC as a gate valve in a water pipe: once circuits as they can switch in both opened, it allows the water (or current) positive and negative cycles, providing to flow freely until the supply pressure controlled conduction based on voltage drops. This stability is beneficial for levels. motor control, where maintaining steady Use with TRIACs: speed is essential. AC107: BREAKDOWN DEVICES TRIAC in Light Dimming:Use the analogy This behavior leads to a sharp rise in of a dial on a stereo. As you adjust the current flow through the device, stereo dial, you control the volume. making it function as a relaxation Similarly, the TRIAC adjusts the amount oscillator—a circuit that turns on and of current going to the light bulb, off periodically. controlling brightness smoothly. Self-Regulating Cycle: DIAC Triggering a TRIAC: After the initial conduction, the UJT Imagine the DIAC like a spring-loaded automatically returns to its off state switch. Only when a certain level of once the emitter current drops, pressure is applied does it snap on, allowing for repetitive on/off allowing current to flow. This action, cycling. This property is particularly when combined with a TRIAC, provides useful in oscillators and timing reliable timing in dimming circuits. applications. 3. Applications of UJT Triggering SCRs: UJT (Unijunction Transistor) Pulse Generation: The UJT’s sharp 1. Construction of UJT switching action generates short pulses that can trigger other devices like Structure: SCRs. In a circuit, the UJT can o The UJT has a three-layer structure repeatedly send pulses to keep an SCR with a single PN junction, making it in the on state, ideal for continuous different from typical transistors. power regulation. It consists of an N-type silicon bar with a P-type region diffused in the middle. Timing Circuits: o Terminals: The UJT has three Precise Timing with Relaxation terminals: Emitter (E), Base 1 (B1), Oscillators: The relaxation oscillator and Base 2 (B2). property makes UJTs suitable for timing applications, such as setting intervals ▪ Emitter (E): The control terminal or delays. Commonly used in blinking that initiates the switching of the lights or controlled pulse delays. UJT. Oscillators: ▪ Base 1 (B1) and Base 2 (B2): These form a resistive path through the N- Waveform Generation: UJTs are commonly type bar, with B1 being closer to the used in oscillator circuits to create emitter than B2. periodic waveforms, which are important in timing, frequency generation, and Intrinsic Standoff Ratio: This is a audio applications. unique property of the UJT, defined by the internal resistances between B1 and B2. It determines the voltage at which PUT (Programmable Unijunction Transistor) the UJT turns on, making the device stable for timing and oscillation. 1. Construction of PUT 2. Operation of UJT Structure: The PUT resembles the UJT in appearance and operation, but it has Triggering Mechanism: added functionality that allows its When a voltage is applied to the voltage threshold to be adjusted or Emitter (E), the UJT initially remains programmed. non-conductive due to the intrinsic Terminals: The PUT has three main standoff ratio. This ratio prevents terminals: current from flowing through the emitter until a certain threshold is Anode (A): Where current enters the reached. PUT. Relaxation Oscillator: Cathode (K): The output terminal. When the emitter voltage exceeds the Gate (G): The programmable control standoff ratio, the UJT “switches on,” terminal that sets the device’s causing a sudden drop in resistance switching threshold. between the emitter and B1. AC107: BREAKDOWN DEVICES Programmable Voltage Threshold: The Answer: It allows the circuit to switch unique feature of the PUT is the ability on and off in a predictable pattern, to adjust the gate voltage, effectively ideal for creating time-based pulses. setting the voltage at which the device turns on. For PUT: What advantages does a programmable threshold provide in a 2. Operation of PUT timing or phase control circuit? Threshold Triggering: Answer: It allows precise control over when the device triggers, which is The PUT remains in its off state until essential for timing accuracy and for the anode voltage exceeds the gate fine-tuning phase angles in AC voltage. Once the anode voltage applications. surpasses this threshold, the PUT switches on, allowing current to flow Analogies and Examples from anode to cathode. UJT in Timing: Imagine the UJT like a ticking clock Programmability: that “ticks” each time it reaches a voltage level, creating a rhythmic pulse Adjustable Triggering Point: By for timing control. adjusting the gate voltage, the triggering threshold can be fine-tuned, PUT in Phase Control: making the PUT highly versatile in applications that require specific Compare the PUT to a thermostat: just as triggering points. a thermostat switches on when the temperature hits a set point, the PUT Controlled Switching: Once triggered, switches on when the voltage reaches its the PUT remains in the on state until set threshold, providing flexible the current through it drops below a timing for precise control. certain holding level, similar to the operation of SCRs. 3. Applications of PUT DC Phase Control Circuit Timing Circuits: 1. Objective of DC Phase Control Precision Timing: The ability to Purpose: The goal of DC phase control adjust the triggering voltage allows circuits is to manage the power delivered PUTs to create accurate timing to a load by controlling the phase angle intervals, making them suitable for (or timing) at which the current is applications requiring variable timing applied. This technique is particularly adjustments, such as in digital timers useful for regulating output voltage and, or pulse delay circuits. consequently, power supplied to devices like heating elements and motors in DC Triggering Circuits: circuits. Controlled Pulse Generation: PUTs are 2.Components Used in DC Phase Control often used to trigger SCRs in phase control circuits for AC power SCR (Silicon Controlled Rectifier): management, similar to how UJTs The SCR is a key component in DC phase function but with added precision due control circuits due to its ability to to the adjustable threshold. handle high power and control DC Phase Control Applications: current flow. Once the SCR is triggered, it remains on until the Adjustable Phase Shifting: PUTs play a current drops below a certain level. crucial role in controlling the phase of AC power by adjusting the point in UJT (Unijunction Transistor):The UJT the AC cycle at which they trigger, is commonly used as a triggering allowing for smooth power modulation. mechanism for the SCR. By generating This makes them ideal for variable- precise timing pulses, it determines speed motor drives and dimming when the SCR will conduct, applications. controlling the point at which DC power is applied to the load. Lecture Engagement Questions 3. How DC Phase Control Works For UJT: How does the relaxation oscillator function benefit a timing Phase Control through Firing Angle circuit? Adjustment: AC107: BREAKDOWN DEVICES Firing Angle: The firing angle is the precise timing for when the TRIAC begins point within each cycle at which the SCR conducting within each AC cycle. is triggered. By adjusting the timing of the UJT pulse, the firing angle of the 3. How AC Phase Control Works SCR is controlled, effectively varying Varying Trigger Points in the AC Cycle: the voltage reaching the load. Phase Angle Control: By adjusting the Output Control: When the SCR is point in the AC cycle at which the TRIAC triggered earlier in the cycle, more is triggered, the power supplied to the voltage is applied to the load, load can be regulated. This delay in increasing the power output. triggering changes the effective Conversely, triggering the SCR later in voltage reaching the load, controlling the cycle reduces the power to the load. the power output. Benefits: Bidirectional Conduction: Since the Allows fine control over output voltage TRIAC can conduct in both directions, it and power delivered to DC loads, allows for a smooth control over AC enabling applications like motor speed power, providing continuous adjustments control and temperature regulation in over power levels throughout the AC heating systems. cycle. 4. Engagement Questions Benefits: Why is an SCR preferred over other Smooth AC Power Control: Ideal for devices in DC phase control circuits? applications that require gradual adjustments in AC power, such as light Answer: Its unidirectional control and dimming, motor speed variation, and fan latching capability make it ideal for controls. steady DC power management. 4. Engagement Questions What might be a potential downside of using SCR for phase control? Why would a TRIAC be preferred over an SCR in AC phase control circuits? Answer: It only controls power in one direction, making it less suited for AC Answer: The TRIAC’s bidirectional applications where bidirectional control allows it to handle both halves control is needed. of the AC cycle, unlike an SCR which only conducts in one direction. What role does the DIAC play in ensuring Phase Control Circuit accurate triggering of the TRIAC? 1. Objective of AC Phase Control Answer: It maintains a controlled trigger point, ensuring consistent and Purpose: The aim of an AC phase control precise timing for TRIAC conduction. circuit is to control the amount of power delivered to an AC load by altering the phase angle at which the current is allowed to flow. This enables Applications of Phase Control Circuits applications like dimming lights, 1. AC Phase Control Applications adjusting motor speeds, and controlling heating in AC-powered devices. Light Dimmers:By adjusting the TRIAC's firing angle, light dimmers can control 2. Components Used in AC Phase Control the brightness of lighting by varying TRIAC (Triode for Alternating Current): the average voltage supplied to the bulb. The TRIAC is essential in AC phase control because it can conduct in both Motor Speed Controllers:In AC motors, directions, making it suitable for the speed can be adjusted by controlling alternating current (AC) applications. It the amount of power supplied through allows bidirectional control, which is TRIAC phase control, making it useful in essential for handling AC power. fans, drills, and other speed-variable appliances. DIAC (Diode for Alternating Current): Heaters and Temperature Control:Phase The DIAC is a triggering component for control allows precise control over the TRIAC, ensuring that the TRIAC only heating elements by regulating the power conducts when a certain threshold voltage delivered, which is useful in electric is reached. This helps in creating heaters and thermostatic devices. AC107: BREAKDOWN DEVICES 2. DC Phase Control Applications Heating Control: By adjusting the firing angle of the SCR, DC heating systems can precisely control temperature, making this useful in industrial furnaces and other DC- powered heating systems. Regulated DC Power Supplies: DC phase control circuits enable regulated power supplies to adjust their output voltage by modifying the SCR’s conduction angle, allowing for adaptable DC output. 3. Analogies and Practical Examples AC Phase Control (Light Dimming):Similar to how a faucet controls water flow, TRIAC-based light dimmers allow you to adjust the "flow" of electricity to a light bulb, dimming or brightening it as desired. DC Phase Control (Heating):Think of a heater controlled by an SCR like an adjustable stove burner. The SCR controls the "burning" level by regulating the power reaching the heater, making it ideal for precise temperature settings.