Electrical Circuit Assembly Worksheet
Objective
To assemble the components of a given electrical circuit and understand the principles of circuit design, component connection, and electrical measurements.
Materials Required
- Breadboard 1
- Resistors (various values: 220Ω, 1kΩ, 10kΩ) 3 each
- Capacitors (various values: 0.1μF, 1μF, 10μF) 2 each
- LED (various colors) 5
- Transistors (NPN, PNP) 2 each
- Battery or power supply (9V) 1
- Connecting wires As required
- Multimeter 1
- Switch (SPST) 1
- Potentiometer (10kΩ) 1
Circuit Theory
Ohm's Law
The relationship between voltage (V), current (I), and resistance (R) is given by:
Where:
- V = Voltage in Volts (V)
- I = Current in Amperes (A)
- R = Resistance in Ohms (Ω)
Power Calculation
The electrical power (P) consumed in a circuit is:
Where P is measured in Watts (W)
Series and Parallel Resistances
For resistors in series:
For resistors in parallel:
Circuit Diagram
LED Circuit with Current-Limiting Resistor
Fig 1. Basic LED circuit with current-limiting resistor and power source
Assembly Procedure
Verify Components
Identify and verify all the components required for the circuit assembly. Check resistor values using the color code.
Resistor Color Code
The color bands on resistors represent specific values:
- Black: 0
- Brown: 1
- Red: 2
- Orange: 3
- Yellow: 4
- Green: 5
- Blue: 6
- Violet: 7
- Grey: 8
- White: 9
For a 4-band resistor:
- First band: First digit
- Second band: Second digit
- Third band: Multiplier (10^n)
- Fourth band: Tolerance
Formula: Value = (First digit × 10 + Second digit) × 10^Multiplier ± Tolerance%
Example: Red-Violet-Orange-Gold = 27 × 10^3 Ω ± 5% = 27kΩ ± 5%
Set Up the Breadboard
Understand the internal connections of the breadboard before placing components.
Breadboard Internal Connections
A standard breadboard has the following connections:
- Power rails (marked + and -) run horizontally along the top and bottom
- Terminal strips run vertically and are separated by a central channel
- Each 5-hole terminal strip is connected internally
- The central channel separates the board into two electrically isolated sections
Calculate the Current-Limiting Resistor Value
For the LED circuit, calculate the appropriate resistor value to ensure safe operation.
LED Current-Limiting Resistor Calculation
To calculate the resistor value needed to limit current through an LED:
Where:
- $V_{supply}$ = Supply voltage (e.g., 9V)
- $V_{forward}$ = LED forward voltage (typically 1.8V-3.3V depending on color)
- $I_{desired}$ = Desired LED current (typically 15-20mA for standard LEDs)
Example calculation for red LED with 9V supply:
- $V_{supply} = 9V$
- $V_{forward} = 2V$ (typical for red LED)
- $I_{desired} = 20mA = 0.02A$
Use a standard 330Ω or 470Ω resistor for safety.
Connect the Power Supply
Connect the 9V battery or power supply to the breadboard power rails, ensuring correct polarity.
Place the Current-Limiting Resistor
Place the calculated resistor value onto the breadboard.
Connect the LED
Place the LED on the breadboard, ensuring correct orientation (the longer lead is the anode/positive terminal).
LED Polarity Identification
LEDs are polarized components and must be connected correctly:
- Anode (positive): Longer lead
- Cathode (negative): Shorter lead
- The cathode side often has a flat edge on the LED casing
Incorrect polarity will prevent the LED from illuminating and may damage it under certain conditions.
Add the Switch
Connect the SPST switch to control the circuit.
Complete Circuit Connections
Use connecting wires to complete the circuit according to the diagram.
Testing and Measurements
| Measurement Point | Expected Value | Measured Value | Notes |
|---|---|---|---|
| Voltage across power supply | 9V | ||
| Voltage across LED | ~2V (red LED) | ||
| Voltage across resistor | ~7V | ||
| Current through circuit | ~20mA | ||
| Power dissipated by resistor | P = V × I = 7V × 0.02A = 0.14W |
Voltage Division Principle
In a series circuit, voltage divides according to resistance values:
This explains why we see voltage drops across different components in the circuit.
Circuit Variations
Parallel LED Configuration
Connect multiple LEDs in parallel, each with its own current-limiting resistor.
In a parallel LED configuration:
- Each LED must have its own current-limiting resistor
- The total current drawn is the sum of all individual LED currents
- If one LED fails, others continue to operate
Variable Brightness Control
Add a potentiometer to control LED brightness.
A potentiometer can be used as a variable resistor to control current flow:
- Connect the potentiometer in series with the LED
- As resistance increases, current decreases, dimming the LED
- As resistance decreases, current increases, brightening the LED
Where $R_{variable}$ is the resistance value of the potentiometer setting.
Troubleshooting Guide
| Problem | Possible Cause | Solution |
|---|---|---|
| LED doesn't light up | Incorrect LED polarity | Reverse the LED orientation |
| LED doesn't light up | Loose connections | Check and secure all connections |
| LED doesn't light up | Power supply not connected | Verify power supply connections |
| LED too dim | Resistor value too high | Replace with lower value resistor |
| LED burns out quickly | Resistor value too low | Replace with higher value resistor |
Assessment Questions
- Why is a current-limiting resistor necessary when connecting an LED to a power source?
- Calculate the appropriate resistor value for a blue LED (forward voltage 3.3V) connected to a 12V power supply with desired current of 15mA.
- Explain why the voltage across the LED remains relatively constant regardless of the supply voltage.
- If three identical LEDs are connected in parallel, each with its own identical resistor, how does the total current compare to a single LED circuit?
- What would happen if you reversed the polarity of the power supply in this circuit?