4.3 Passive Components & Schematic Symbols

Passive Components & Schematic Symbols

Electronic components are broadly divided into two categories: active and passive. Active components, like transistors and integrated circuits, require an external power source to perform their primary function (such as amplification or switching) and can inject power into a circuit. Passive components, on the other hand, do not require external power to operate. They cannot amplify a signal, though they can attenuate (weaken) it, store it, or shift its phase.

The four primary passive components are resistors, capacitors, inductors, and transformers. Mastering their functions and recognizing their schematic symbols is critical for understanding circuit diagrams and repairing equipment.

Resistors

The most common component in any electronic circuit is the resistor. A resistor is specifically manufactured to provide a precise, calculated amount of opposition to the flow of current. When current passes through a resistor, the energy restricted by the resistance is dissipated as heat.

Types of Resistors

1. Fixed Resistors: These have a single, unchangeable resistance value. They are made from various materials, including carbon composition, metal film, and wirewound.
2. Variable Resistors (Potentiometers): Also known as "pots," these allow the user to manually adjust the resistance. The volume knob on a radio is typically a potentiometer. It contains a resistive track and a wiper that slides along it; as the wiper moves, the resistance between the terminals changes.

Power Ratings

Resistors not only have a resistance value (in ohms) but also a power rating (in watts). The power rating dictates how much heat the resistor can safely dissipate before it physically burns up. Common ratings for small electronics are 1/8 watt, 1/4 watt, and 1/2 watt. If you need to dissipate 2 watts of power (calculated via P = I²×R), a 1/4-watt resistor will quickly smoke and fail; you must use a resistor rated for at least 2 watts, preferably more.

Reading Resistor Color Codes

Because many fixed resistors are too small to have numbers printed on them, they use a standard color-code system of painted bands.

• 4-Band Resistors:
  • Band 1: First digit
  • Band 2: Second digit
  • Band 3: Multiplier (number of trailing zeros)
  • Band 4: Tolerance (gold = ±5%, silver = ±10%)
• 5-Band Resistors (higher precision):
  • Bands 1-3: First three digits
  • Band 4: Multiplier
  • Band 5: Tolerance

The color number values are: Black (0), Brown (1), Red (2), Orange (3), Yellow (4), Green (5), Blue (6), Violet (7), Gray (8), White (9). Example: A 4-band resistor colored Yellow(4) - Violet(7) - Red(2 zeros) is a 4,700 ohm (4.7kΩ) resistor.

Resistors in Series and Parallel

When resistors are connected end-to-end (in series), their total resistance adds up (R_total = R1 + R2 + R3). When connected side-by-side (in parallel), the total resistance decreases, as there are now multiple paths for the current to flow. The total resistance of a parallel circuit is always less than the value of the smallest resistor in the group.

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Capacitors

A capacitor is a device that stores electrical energy in an electric field. Physically, a basic capacitor consists of two conductive plates separated by an insulating material known as a dielectric. When voltage is applied across the plates, electrons crowd onto one plate (making it negative) and are pulled from the other (making it positive), creating an electric field between them.

Types of Capacitors

Capacitors are generally categorized by the dielectric material used between their plates:

• Ceramic Capacitors: Small, disc-shaped, cheap, and excellent for high-frequency RF bypass applications.
• Electrolytic Capacitors: These are polarized (they have a specific positive and negative lead) and offer very high capacitance values in a small package. They are typically used in power supply filters to smooth out voltage ripples.
• Mica and Tantalum Capacitors: Used in applications requiring high stability and precision.
• Variable Capacitors: These contain a set of fixed plates (stator) and a set of movable plates (rotor) that mesh together without touching. Rotating the shaft changes the overlapping surface area, changing the capacitance. They are widely used for tuning radio receivers and transmitters.

Capacitance Values and Voltage Ratings

Capacitance is measured in Farads (F), but a 1-Farad capacitor is enormous. In radio work, we deal with microfarads (µF, one-millionth of a farad) and picofarads (pF, one-trillionth of a farad).

Capacitors also have a voltage rating, specifying the maximum voltage that can be applied before the dielectric breaks down, causing a short circuit. An electrolytic capacitor rated for 16V will explode if subjected to 50V.

Note on wiring: Unlike resistors, capacitors wired in parallel add their capacitance together (C_total = C1 + C2). When wired in series, their total capacitance decreases.

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Inductors

An inductor (often called a coil or a choke) stores energy in a magnetic field. It is typically constructed by winding a length of conductive wire into a coil. When current flows through the wire, it generates a magnetic field around the coil. If the current changes, the expanding or collapsing magnetic field induces a counter-voltage that opposes the change in current. Therefore, an inductor easily passes direct current (DC) but resists alternating current (AC).

Types of Inductors

Inductance is measured in Henries (H), though radio circuits mostly use millihenries (mH) or microhenries (µH).

• Air Core Inductors: Wire coiled around an empty space or plastic tube. Common in high-frequency RF applications.
• Ferrite or Iron Core Inductors: Wire wrapped around a piece of magnetic material (like iron or ferrite). The core dramatically increases the magnetic field strength, providing much higher inductance in a smaller package.

Like resistors, inductors in series add their values together. Inductors in parallel result in a lower total inductance.

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Transformers

A transformer consists of two or more inductors (coils) that share the same magnetic core. When an AC voltage is applied to the first coil (the primary winding), it creates a fluctuating magnetic field in the core. This magnetic field cuts across the second coil (the secondary winding), inducing an AC voltage in it. Transformers only work with AC, not DC.

Step-up, Step-down, and Turns Ratio

The voltage induced in the secondary winding depends on the turns ratio—the ratio of wire turns on the primary coil to turns on the secondary coil.

• If the secondary has more turns than the primary, it is a step-up transformer, increasing the voltage.
• If the secondary has fewer turns, it is a step-down transformer, decreasing the voltage.

Transformers are used in power supplies to drop 120V AC wall power down to a safer 12V AC or 24V AC for conversion to DC. They are also widely used in radio frequency circuits to match impedances between different amplifier stages or between a transmitter and an antenna.

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Fuses and Circuit Breakers

While not complex components, fuses and circuit breakers are critical safety devices. They are designed to be the weakest link in a circuit, placed in series with the power source. If excessive current flows (due to a short circuit or component failure), the fuse wire melts and physically breaks the circuit, stopping the flow of electricity and preventing fires or further damage. Circuit breakers perform the same function but can be manually reset after they "trip."

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Schematic Symbols

A schematic diagram is a map of an electronic circuit. Instead of drawing pictures of the physical components, engineers use standard symbols. Familiarizing yourself with these is a major part of the Technician exam.