7.3 Electrical Components: Resistors, Capacitors, Inductors & Transformers

7.3 Electrical Components: Resistors, Capacitors, Inductors & Transformers

Quick Answer: Electronic circuits rely on fundamental passive components: Resistors limit current and drop voltage; Capacitors store charge in electrostatic fields and oppose voltage changes; Inductors store energy in magnetic fields and oppose current changes; and Transformers step AC voltage up or down using mutual electromagnetic induction. Mastering resistor color codes, component reactance ($X_C, X_L$), and transformer turns ratios ($N_p/N_s$) is essential for the AFCT EI test.

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1. Passive vs. Active Components Overview

• Passive Components: Electrical components that cannot generate energy or amplify power. They store, dissipate, or release electrical energy (e.g., Resistors, Capacitors, Inductors, Transformers).
• Active Components: Components that rely on an external power source to control electric current, amplify signals, or perform switching functions (e.g., Transistors, Diodes, Integrated Circuits).

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2. Resistors & Color Coding Systems

Resistors are designed to provide a specific amount of electrical resistance in a circuit. They are rated by two primary specifications:

1. Resistance Value (Ohms, $\Omega$): The opposition to current.
2. Power Rating (Watts, $W$): The maximum power the resistor can safely dissipate as heat without burning out ($1/8\text{W}, 1/4\text{W}, 1/2\text{W}, 1\text{W}, 5\text{W}$). Physical size determines wattage rating, not resistance value!

                  [ 4-Band Resistor Color Code Guide ]

         +-------------------------------------------------+
         |  [Band 1]  [Band 2]  [Band 3]      [Band 4]     |
         |   1st Digit 2nd Digit Multiplier    Tolerance   |
         +-------------------------------------------------+

The Standard Resistor Color Code Table

AFCT Mnemonic: Bad Boys Rob Our Young Girls But Violet Gives Willingly (Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White).

Step-by-Step Color Code Decoding Example

Determine the resistance value of a resistor with color bands: Yellow - Violet - Red - Gold.

• Band 1 (Yellow): First digit = 4
• Band 2 (Violet): Second digit = 7
• Band 3 (Red): Multiplier = $10^2 = 100$
• Band 4 (Gold): Tolerance = $\pm 5%$
• Calculation: $47 \times 100 = 4,700\Omega = 4.7\text{ k}\Omega \pm 5%$.
• Tolerance Range: $4,700 \times 0.05 = 235\Omega \implies 4,465\Omega \text{ to } 4,935\Omega$.

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3. Capacitors & Capacitance ($C$)

A Capacitor is an electrical component consisting of two parallel conductive plates separated by an insulating material called a dielectric (e.g., Air, Ceramic, Mica, Electrolytic, Mylar).

                    [ Capacitor Physical Construction ]

               Plate 1 (Conductive) ----+  +---- Lead 1
                                       |  |
                DIELECTRIC INSULATOR -->||  <-- Electrostatic Field
                                       |  |
               Plate 2 (Conductive) ----+  +---- Lead 2

Core Properties of Capacitors:

1. Capacitance Formula: $C = \frac{Q}{V}$ (measured in Farads, F; practical units are $\mu\text{F}$, $\text{nF}$, $\text{pF}$).
2. Energy Stored: Stores energy in an electrostatic field: $E = \frac{1}{2} C V^2$.
3. Behavior: Opposes sudden changes in voltage ($I = C \frac{dV}{dt}$).
4. DC vs AC: Blocks DC (after charging to full source voltage) and passes AC signals.

Capacitors in Series and Parallel (Opposite of Resistors!)

• Parallel Capacitors: Total capacitance increases (increases total plate surface area): $C_{\text{Total}} = C_1 + C_2 + C_3 + \dots + C_n$
• Series Capacitors: Total capacitance decreases (increases effective dielectric thickness): $\frac{1}{C_{\text{Total}}} = \frac{1}{C_1} + \frac{1}{C_2} + \dots \quad \implies \quad C_{\text{Total}} = \frac{C_1 \times C_2}{C_1 + C_2}$

Capacitive Reactance ($X_C$)

Capacitive Reactance is the opposition a capacitor offers to alternating current, measured in Ohms ($\Omega$): $X_C = \frac{1}{2 \pi f C}$

• As AC frequency ($f$) increases, capacitive reactance ($X_C$) decreases.
• At $0\text{ Hz}$ (DC), $X_C = \infty \Omega$ (Open Circuit).

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4. Inductors & Inductance ($L$)

An Inductor (also called a coil, choke, or reactor) consists of a conductive wire wrapped around a core (air, iron, or ferrite).

Core Properties of Inductors:

1. Inductance ($L$): Measured in Henries (H) (or $\text{mH}$, $\mu\text{H}$).
2. Energy Stored: Stores electrical energy in a magnetic field: $E = \frac{1}{2} L I^2$.
3. Lenz's Law & Back-EMF: Inductors oppose any change in current by inducing a counter-voltage (back-EMF).
4. DC vs AC: Passes DC easily (offering only wire resistance) and blocks high-frequency AC.

Inductors in Series and Parallel (Same as Resistors!)

• Series Inductors: $L_{\text{Total}} = L_1 + L_2 + L_3 + \dots + L_n$
• Parallel Inductors: $\frac{1}{L_{\text{Total}}} = \frac{1}{L_1} + \frac{1}{L_2} + \dots$

Inductive Reactance ($X_L$)

Inductive Reactance is the opposition an inductor offers to alternating current, measured in Ohms ($\Omega$): $X_L = 2 \pi f L$

• As AC frequency ($f$) increases, inductive reactance ($X_L$) increases directly.

5. Transformers & Electromagnetic Induction

A Transformer is a static electrical device that transfers electrical energy between two or more AC circuits through mutual electromagnetic induction.

                  [ Ideal Transformer Schematic ]

               Primary Coil (Np)    Iron Core    Secondary Coil (Ns)
                 ---( )---            ||            ---( )---
        Vp AC    ---( )---            ||            ---( )---    Vs AC
                 ---( )---            ||            ---( )---

The Transformer Equations:

\text{Turns & Voltage Ratio: } \frac{V_p}{V_s} = \frac{N_p}{N_s} \text{Power Conservation (Ideal 100% Efficient): } P_{\text{Primary}} = P_{\text{Secondary}} \implies V_p \times I_p = V_s \times I_s $\text{Current Inverse Ratio: } \frac{I_s}{I_p} = \frac{N_p}{N_s} = \frac{V_p}{V_s}$

Types of Transformers:

1. Step-Down Transformer: $N_s < N_p \implies V_s < V_p$ (Voltage decreases, Current increases).
2. Step-Up Transformer: $N_s > N_p \implies V_s > V_p$ (Voltage increases, Current decreases).
3. Isolation Transformer: $N_s = N_p \implies V_s = V_p$ (Used to decouple grounded circuits for safety).

Transformer Calculation Worked Example

A step-down power transformer has $1,200\text{ turns}$ on its primary winding and $100\text{ turns}$ on its secondary winding. If $120\text{V AC}$ is applied across the primary coil, find the secondary voltage ($V_s$).

• Formula: $\frac{V_p}{V_s} = \frac{N_p}{N_s} \implies V_s = V_p \times \left( \frac{N_s}{N_p} \right)$
• Calculation: $V_s = 120\text{V} \times \left( \frac{100}{1,200} \right) = 120 \times \frac{1}{12} = 10.0\text{ Volts AC}$.

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6. Comprehensive Passive Component Summary Reference Table