
Introductory Circuit Analysis (13th Edition)
13th Edition
ISBN: 9780133923605
Author: Robert L. Boylestad
Publisher: PEARSON
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
Transcribed Image Text:### Electrical Circuit Analysis: Step-by-Step Evaluation
**Question: In STEP 1, before SW1 is closed, where is current flowing in this circuit?**
1. **No current is flowing. The switch is open and the transistor is not allowing any current to flow.**
2. **Before the switch is closed, current flows through the transistor base to the emitter. After the switch is closed, current flows from the collector to the emitter, allowing it to flow through D1, which will then light.**
3. **Before the switch is closed, current bypasses the transistor, so D1 cannot light.**
4. **Since D1 is reversed, no current can flow unless SW1 is pressed, triggering the transistor base to allow current to flow back through the LED.**
---
### Explanation of Circuit Components:
- **SW1 (Switch 1)**: This component controls the flow of current within the circuit. When SW1 is open, it prevents current from flowing.
- **Transistor**: A semiconductor device used to amplify or switch electronic signals. It has three terminals: base, collector, and emitter.
- **D1 (Diode 1)**: Typically an LED (Light Emitting Diode) in this context, which will light up when current flows through it in the correct direction.
In Step 1, an analysis of the circuit indicates that no current is flowing because SW1 is open. This state prevents the transistor from allowing any current to pass, ensuring that no part of the circuit is active.
For further study, it would be beneficial to understand the role of each component under different conditions, such as when the switch is closed and how the transistor influences current flow.

Transcribed Image Text:**Title: Basic Transistor Switch Circuit Diagram**
**Introduction:**
This page provides a detailed explanation of a basic transistor switch circuit, ideal for students and enthusiasts learning about electronics.
**Circuit Description:**
The circuit diagram shown represents a simple transistor switch used to control an LED. The components and their values are outlined as follows:
**Components:**
1. **Power Supply Block (PSB):** 5V power source.
2. **Resistor R1:** 10 kΩ resistor connected in series with the base of the transistor.
3. **Resistor R2:** 1 kΩ resistor connected in series with the LED.
4. **SW1:** Switch used to control the base current to the transistor.
5. **D1:** Green LED.
6. **Q1:** NPN transistor (2N2222).
**Circuit Operation:**
1. **Power Source:** The 5V power supply (PSB) provides the necessary voltage to the circuit.
2. **Base Resistor (R1):** When switch SW1 is closed, a voltage is applied to the base of the transistor Q1 through a 10 kΩ resistor (R1). This resistor limits the base current to a safe level to avoid damaging the transistor.
3. **Switch (SW1):** Controls the connection of the base of transistor Q1 to the power supply.
4. **Transistor (Q1):** The 2N2222 NPN transistor acts as a switch. When SW1 is closed, the base current flows through R1 into the base of the transistor, turning it on. This allows current to flow from the collector to the emitter.
5. **Collector Resistor (R2):** The 1 kΩ resistor (R2) in series with the LED (D1) limits the current flowing through the LED.
6. **LED (D1):** The green LED illuminates when current flows through it, indicating that the transistor Q1 is conducting.
**Summary:**
When SW1 is closed, the base current activates transistor Q1, allowing current to flow from the collector to the emitter. This current passes through the 1 kΩ resistor (R2) and the green LED (D1), causing the LED to light up. Thus, this circuit demonstrates the basic operation of a transistor switch, used to control the illumination of an LED.
By understanding this simple configuration, students can grasp the fundamental principles
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