Clase magistral. Aprende analizar circuito regulador voltaje variable 100V (clase 93.3)

Understanding Linear Power Supply Circuits

Introduction to Linear Power Supplies

• The speaker introduces the topic of linear power supply circuits, focusing on those based on operational amplifiers and integrated voltage regulators.

• Acknowledges limitations in output voltage regulation for these types of circuits, particularly when higher voltages (60V, 80V, or 90V) are required.

Overview of Transistor-Based Regulators

• The video will explain a regulator circuit using only transistors that includes short-circuit protection and stabilization features.

• The circuit consists of three transistors, a Zener diode, resistors, and a potentiometer for adjusting output voltage.

Circuit Functionality

• Input voltage is supplied from an unregulated power source with some ripple; the goal is to achieve a constant output voltage with minimal ripple.

• The circuit adjusts the collector-emitter voltage of transistor T1 to maintain a steady output despite variations in input voltage.

Voltage Regulation Mechanism

• As input voltage increases or decreases, corresponding adjustments occur in the base-emitter voltage of transistor T2 to keep the output stable.

• The relationship between input current through R1 and its effect on T1's operation is discussed; if insufficient current flows through R2, T2 will not conduct.

Detailed Circuit Analysis

• Current flow dynamics are explained: part goes through R1 while another part reaches the base-emitter junction of T1.

• Discussion about ensuring sufficient base-emitter current for T2 to operate effectively; otherwise, it remains off leading to zero collector current.

Zener Diode Characteristics

• Introduces the Zener diode (1N5371), which has a nominal breakdown voltage of 60V but can vary by ±3V according to specifications.

• Explains how total control voltage at point C combines both Zener and base-emitter voltages resulting in approximately 60.7 volts at this point.

Output Voltage Calculation

Understanding Voltage Control in Circuits

Voltage Variations with Potentiometers

• The circuit can achieve a theoretical output voltage of 126V by adjusting the cursor position on the potentiometer, affecting the voltage across two resistors.

• When the cursor is positioned higher, the output voltage decreases to around 63-64V; conversely, lowering it increases the output voltage.

• To achieve smaller variations in output voltage, a 10k resistor combined with a 1k potentiometer is recommended for finer adjustments.

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Effects of Input Voltage Changes

• An increase in input voltage leads to an increase in output voltage; however, this effect is regulated by transistors within the circuit.

• As input voltage rises, it causes an increase in collector-emitter voltage (point C), which subsequently raises base-emitter current due to its diode-like behavior.

Transistor Behavior Under Load Changes

• Increased base-emitter current results in greater collector current; thus, if collector current rises significantly, base current may decrease as a consequence.

• A rise in collector current from transistor 3 leads to decreased base current for transistor 1. This dynamic illustrates how changes propagate through connected components.

Regulation Mechanism Explained

• The system stabilizes output voltage despite fluctuations: when input increases leading to higher output voltages, compensatory mechanisms ensure that overall stability is maintained.

• If load resistance decreases (demanding more intensity), initial conditions may cause a drop in output until transistors adjust accordingly to meet demand.

Response to Load Resistance Changes

• A decrease in load resistance prompts an immediate response where the system attempts to maintain constant output despite increased demand for intensity.

Understanding Output Voltage Stabilization

Current Relationships in the Circuit

• The emitter current and collector current are interconnected; as the emitter current increases, the collector current also rises. However, the output current does not increase proportionally.

• A stabilization effect is observed where a decrease in load leads to an increase in output voltage, maintaining a constant output despite fluctuations.

Effects of Resistance Changes

• If resistance (R) decreases significantly, it causes a substantial increase in output current. This relationship is crucial for understanding circuit behavior under varying loads.

• When base-emitter voltage reaches approximately 0.66V due to increased currents, the transistor begins conducting more effectively.

Current Limiting Mechanisms

• As R2 increases, it limits further increases in output current. The circuit's design ensures that while one part of the circuit draws more current, another part compensates by limiting total flow.

• The interplay between different currents within the circuit prevents excessive increases in output current, stabilizing overall performance.

Power Limitations and Simulation Insights

• If resistance continues to decrease beyond certain thresholds, output voltage will start to drop even if output current can rise slightly up to 4A.

• The theoretical explanation provided is supported by simulation results which validate these behaviors before physical implementation with real components.

Practical Implementation Considerations

• For practical applications involving transistors, it's essential to ensure that input voltages exceed minimum operational levels (e.g., at least 128V for linear operation).

• Ripple effects from capacitors must be accounted for when designing circuits; variations should be minimized for stable operation.

Circuit Configuration and Measurements

• A specific circuit configuration was tested using a transformer and rectifier setup; measurements indicated an average DC voltage of 167V after filtering.

• Key measurements include collector-emitter voltage differences and power dissipation calculations based on observed currents through various components.

Final Observations on Transistor Behavior

• Measured values indicate low base-emitter voltages (around 736mV), suggesting that under certain conditions, transistors may operate near cutoff points with minimal conduction.

Analysis of Circuit Behavior Under Load Changes

Observations on Voltage and Current Adjustments

• As the cursor is adjusted, the voltage decreases while current increases; at 105V, the current reads 420 mA with a slight leakage of 5 mA.

• The output voltage stabilizes around 70.6V when adjusting to a base-emitter voltage of 0.8mV, indicating a deviation from theoretical expectations.

Impact of Load Variations on Circuit Performance

• Modifying the load results in increased current demand; as load resistance decreases, current rises to 862 mA while maintaining an output voltage near 104V.

• With a load reduced to 50 ohms (10%), output voltage slightly drops to 103V but remains relatively stable despite increased current.

Limitations Encountered with Further Resistance Decrease

• When resistance is further decreased to about 5%, the circuit begins conducting more significantly, pushing current above 3 A while causing output voltage to decline.

• At very low resistances (15 ohms), significant drops in output voltage are observed, highlighting limitations in power and current handling capabilities.

Conclusion on Transistor Capacity and Recommendations