Activity: Assembling a Household Circuit
Three bulbs, three switches, a fuse and a power source
Learning Objectives
- Understand the basic components of a household electrical circuit
- Learn the function of switches, fuses, and power sources in a circuit
- Gain practical experience in assembling a working electrical circuit
- Understand series and parallel circuit configurations
- Learn about electrical safety and the importance of fuses
- Apply Ohm's Law and power calculations to real circuit components
Required Materials
Components
- 3 light bulbs (recommended: 12V, 0.2A)
- 3 on/off switches (SPST - Single Pole Single Throw)
- 1 fuse with holder (appropriate rating for your circuit)
- 1 power source (12V DC battery pack or AC/DC adapter)
- Wire connectors/terminals
- Insulated copper wire
- Bulb holders/sockets
Tools
- Wire strippers
- Wire cutters
- Screwdriver set
- Multimeter
- Electrical tape
- Safety gloves
- Circuit board or mounting base
Safety Precautions
Always disconnect the power source before making any changes to the circuit.
Ensure all connections are properly insulated to prevent short circuits.
Use a fuse with the appropriate rating for your circuit to prevent overloading.
Use wire strippers and cutters safely to avoid injury.
Keep your circuit and electrical components away from water or moisture.
Understanding Circuit Components
Circuit Elements and Symbols
Light Bulbs
Light bulbs convert electrical energy into light and heat. They act as resistive loads in the circuit.
A light bulb's filament has resistance $R$ that follows Ohm's Law:
$V = I \times R$
The power consumption (in watts) is calculated by:
$P = V \times I = I^2 \times R = \frac{V^2}{R}$
For example, a 12V, 0.2A bulb:
$P = 12V \times 0.2A = 2.4W$
$R = \frac{V}{I} = \frac{12V}{0.2A} = 60\Omega$
Switches
Switches control the flow of current by making or breaking the circuit path.
Common switch types include:
- SPST (Single Pole Single Throw): Basic on/off switch
- SPDT (Single Pole Double Throw): Selects between two circuits
- DPST (Double Pole Single Throw): Controls two separate circuits simultaneously
- DPDT (Double Pole Double Throw): Controls two separate circuits with two positions each
In our activity, we'll use SPST switches, which have two terminals and function as simple on/off controls.
Fuses
Fuses are safety devices that break the circuit when current exceeds a safe level.
A fuse contains a metal wire or strip that melts when too much current flows through it, breaking the circuit. The fuse rating should be:
$I_{fuse} \geq 1.5 \times I_{normal}$
But also:
$I_{fuse} < I_{dangerous}$
Where:
- $I_{normal}$ is the normal operating current of your circuit
- $I_{dangerous}$ is the current that could damage components or wiring
For our circuit with three 0.2A bulbs, the total current could be up to 0.6A, so a 1A fuse would be appropriate.
Power Source
Provides the electrical energy needed to power the circuit.
Power sources provide a voltage differential that drives current through the circuit. The relationship between voltage, current, and resistance is described by Ohm's Law:
$I = \frac{V}{R}$
For safety in this exercise, we're using a low-voltage DC source (12V) which reduces shock hazards while still providing enough power to light the bulbs.
The total power capacity needed for our circuit is:
$P_{total} = \sum P_{components} = 3 \times (12V \times 0.2A) = 7.2W$
Ensure your power source can provide at least this much power.
Circuit Design
Circuit Configuration
For this activity, we'll create a parallel circuit with three separate branches, each containing a switch and a bulb. A fuse will protect the entire circuit.
Circuit Diagram
Series vs. Parallel Circuits
In a series circuit, components are connected end-to-end, forming a single path for current. In a parallel circuit, components are connected across the same voltage source, providing multiple paths for current.
Characteristics of Our Parallel Circuit:
1. Each bulb receives the full voltage of the power source:
$V_{bulb1} = V_{bulb2} = V_{bulb3} = V_{source}$
2. The total current is the sum of the individual branch currents:
$I_{total} = I_{bulb1} + I_{bulb2} + I_{bulb3}$
3. The equivalent resistance of the parallel circuit is:
$\frac{1}{R_{eq}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}$
For identical bulbs with resistance $R$:
$R_{eq} = \frac{R}{n} = \frac{R}{3}$
4. Advantages of our parallel design:
- Each bulb can be controlled independently
- If one bulb fails, the others continue to work
- Each bulb receives full voltage (brighter light)
Assembly Instructions
Organize your components and tools on a clean, dry surface. Ensure you have enough space to work comfortably.
Arrange and secure the bulb holders, switches, and fuse holder on your mounting base or circuit board.
Cut lengths of wire as needed for your layout. Strip about 1cm of insulation from each end of the wires.
Connect one terminal of the fuse holder to the positive terminal of your power source. This forms the main protection for your circuit.
Connect a wire from the other terminal of the fuse holder to create a common positive line. This will supply power to all three switches.
Connect one terminal of each switch to the positive distribution line.
Connect one terminal of each bulb holder to the free terminal of its corresponding switch.
Connect the other terminal of all three bulb holders to a common negative (return) line.
Connect the negative distribution line to the negative terminal of your power source.
Insert bulbs into their holders and install the appropriate fuse in the fuse holder.
Double-check all connections to ensure they are secure and properly insulated.
Connect the power source and test each switch to ensure all bulbs work properly.
Testing and Troubleshooting
Basic Circuit Tests
| Test | Procedure | Expected Result | Actual Result |
|---|---|---|---|
| Switch 1 Operation | Toggle Switch 1 on and off | Bulb 1 turns on and off | |
| Switch 2 Operation | Toggle Switch 2 on and off | Bulb 2 turns on and off | |
| Switch 3 Operation | Toggle Switch 3 on and off | Bulb 3 turns on and off | |
| Independent Operation | Turn on only one switch at a time | Only the corresponding bulb lights up | |
| All Switches On | Turn on all three switches | All three bulbs light up |
Common Issues and Solutions
| Problem | Possible Cause | Solution |
|---|---|---|
| No bulbs light up |
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| One bulb doesn't light up |
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| Bulbs are dim |
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| Fuse keeps blowing |
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Using a Multimeter for Troubleshooting
Continuity Testing: Use the continuity or resistance setting on your multimeter to check for breaks in wires or connections.
A continuous circuit will show:
- Near 0Ω resistance or
- An audible beep (if your meter has a continuity buzzer)
Voltage Testing: Use the DC voltage setting to verify:
- Power source is delivering the correct voltage
- Voltage is present at both sides of a closed switch
- Voltage is present at the bulb terminals when the switch is closed
Current Testing: Use the DC current setting (with the meter in series) to check:
- Current draw of individual bulbs
- Total circuit current
For our circuit with three 0.2A bulbs, the expected readings would be:
- Individual branch current: approximately 0.2A
- Total current with one bulb on: approximately 0.2A
- Total current with two bulbs on: approximately 0.4A
- Total current with all bulbs on: approximately 0.6A
The total current can be calculated as:
$I_{total} = n \times I_{bulb}$
Where $n$ is the number of active bulbs, and $I_{bulb}$ is the current of one bulb.
Measurements and Calculations
Record Your Measurements
| Measurement | Theoretical Value | Measured Value | Difference (%) |
|---|---|---|---|
| Power Source Voltage | 12V | ||
| Current with Bulb 1 only | 0.2A | ||
| Current with Bulb 2 only | 0.2A | ||
| Current with Bulb 3 only | 0.2A | ||
| Current with all bulbs on | 0.6A | ||
| Resistance of one bulb |