In electronic circuits, resistors are typically labeled with the letter "R" followed by a number, such as R1 for resistor number 1. Resistors serve multiple functions in a circuit, including shunting, current limiting, voltage division, biasing, and more.
**Resistor Parameters and Identification**
The unit of resistance is the ohm (Ω), with common units like kiloohms (kΩ) and megaohms (MΩ). The conversion is: 1 MΩ = 1000 kΩ = 1,000,000 Ω. There are three main methods to identify resistor values: direct marking, color coding, and numerical notation.
- **Numerical Notation**: Used mainly on small surface-mount components. For example, 472 means 47 × 10² = 4.7 kΩ, and 104 means 10 × 10ⴠ= 100 kΩ.
- **Color Coding**: The most widely used method. A four-band resistor has two significant digits, one multiplier, and one tolerance band. A five-band resistor adds a third significant digit for precision. For example:
| Color | Digit | Multiplier | Tolerance (%) |
|--------|-------|------------|----------------|
| Silver | - | x0.01 | ±10 |
| Gold | - | x0.1 | ±5 |
| Black | 0 | x1 | - |
| Brown | 1 | x10 | ±1 |
| Red | 2 | x100 | ±2 |
| Orange | 3 | x1000 | - |
| Green | 5 | x100000 | ±0.5 |
| Blue | 6 | x1,000,000 | ±0.2 |
| Purple | 7 | x10,000,000| ±0.1 |
| Gray | 8 | x100,000,000| - |
| White | 9 | x1,000,000,000| - |
**Capacitors**
Capacitors are denoted with the letter "C" followed by a number, such as C13. They consist of two conductive plates separated by an insulator. Their primary function is to store electrical energy and block DC while allowing AC to pass through.
The capacitive reactance (XC) determines how much AC is blocked and is calculated as: XC = 1 / (2Ï€fC), where f is the frequency and C is the capacitance. Common types include electrolytic, ceramic, chip, tantalum, and polyester capacitors.
Capacitance values are marked using direct labeling, color codes, or numerical notation. The standard unit is farad (F), with subunits like millifarad (mF), microfarad (μF), nanofarad (nF), and picofarad (pF). For example, 10 μF/16V indicates a 10 microfarad capacitor rated at 16 volts. Small capacitors may use letters or numbers, such as 1P2 = 1.2 pF or 1n = 1000 pF.
**Diodes**
Diodes are labeled with "D" followed by a number, like D5. They allow current to flow in one direction only. This property makes them useful for rectification, voltage regulation, and signal control.
Common diode types include rectifier diodes (e.g., 1N4004), switching diodes (e.g., 1N4148), Schottky diodes (e.g., BAT85), LEDs, and Zener diodes. To identify polarity, small diodes often have a colored band indicating the cathode. LEDs can be identified by their longer leg (anode) and shorter leg (cathode).
When testing a diode with a multimeter, the red probe should touch the anode, and the black probe the cathode. If the reading shows a low resistance, the diode is functioning properly.
**Zener Diodes**
Zener diodes are used for voltage regulation and are labeled with "ZD" followed by a number, like ZD5. They maintain a stable voltage across their terminals when reverse-biased beyond the breakdown point.
Faults in Zener diodes include open circuits, short circuits, or unstable voltage output. Common models include 1N4728 (3.3V), 1N4733 (5.1V), and 1N4750 (27V).
**Inductors**
Inductors are labeled with "L" followed by a number, such as L6. They store energy in a magnetic field and oppose changes in current. Inductors are often used in oscillating circuits with capacitors.
Inductance is measured in henrys (H), with common subunits like millihenrys (mH) and microhenrys (μH). They are marked using either direct labeling or color coding, similar to resistors. For example, brown-black-gold-gold represents 1 μH with 5% tolerance.
**Varactor Diodes**
Varactor diodes are used in high-frequency modulation circuits. Their internal junction capacitance varies with applied voltage, making them ideal for frequency modulation.
Failures in varactors usually involve leakage or poor performance, which can cause distortion in the transmitted signal. Replacement with a similar model is recommended.
**Transistors**
Transistors are labeled with "Q" followed by a number, such as Q17. They come in NPN and PNP types and are used for amplification, switching, and signal processing.
Common transistor types used in telephones include A92, 9015 (PNP), and 9014, 9018, 9013, 9012 (NPN). Transistors can be connected in common emitter, common collector, or common base configurations, each with different impedance and gain characteristics.
**Field Effect Transistors (FETs)**
FETs are known for high input impedance and low noise, making them ideal for input stages in many devices. They are divided into junction-type (JFET) and insulated-gate (MOSFET) types.
Unlike BJTs, FETs are voltage-controlled, making them suitable for low-current applications. They also offer greater flexibility due to the interchangeability of source and drain. FETs are widely used in large-scale integrated circuits.
Understanding transistor parameters is essential for designing and analyzing circuits. For instance, calculating the base resistor (Rb) for a transistor in saturation mode involves knowing its saturation current. If the saturation current is 100 mA, then Rb = (U - 0.7) / 0.1, where U is the operating voltage and 0.7 is the base-emitter voltage drop. Amplification calculations follow similar principles.
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