Electrical/Electronic Systems Project

Objective— Working as part of a team, the learner will diagnose problems in a circuit and recommend corrections to help him or her understand and explain the relationship between the length and gauge of wiring.

PROJECT DESCRIPTION

Scenario— A customer’s car is in the repair shop. The customer has received a warning citation because the brake lights were barely visible when the brakes were applied. A visual inspection reveals that other lights in the rear of the car are also dim. The customer reports that the factory- installed wiring harness to the rear of the car was damaged by rodents chewing on the wires. He replaced the wiring using a smaller wire he had in his shop.

Divide the class into teams that work in the repair shop. They will use Ohm’s law and the wire- resistance formula to help them understand the relationship between length and gauge of wiring and resistance in an electrical circuit. They will evaluate improperly designed circuits and make recommendations for improving those circuits. Finally, they will communicate to the customer an explanation of the circuit problem in his car and recommend a solution.

Have the teams use the formula for resistance in a wire and the area formula to show the relationship of length and size of wire to resistance in a circuit. Show how this relationship affects the current flow in the circuit.

Physics— Students will use Ohm’s law and the formulas for wire resistance and area to show the relationships.  After completion of the activity, use the assessment instrument of ICS G20 Appreciation of Diversity.

Have the teams determine the gauge of the wire needed in circuits that specify power source, wire length, amps, and maximum volt drop.

Physics— Students will use Ohm’s law and the formulas for wire resistance and area to show the relationships.

Have the teams diagnose a circuit created on a project board; the circuit should simulate the customer’s complaint using only a power supply, wire, and bulb. Students will (1) measure the voltage across the bulb in the defective circuit and compare this to the design voltage of the bulb, (2) calculate the design current in the bulb, (3) calculate the maximum acceptable voltage drop in the wire, (4) calculate the maximum acceptable resistance in the wire, (5) calculate the minimum diameter wire that should be used in a circuit that has four of the bulbs in parallel, (6) use the standard American wire gauge table to determine the smallest acceptable wire gauge to use in this circuit, and (7) replace the wire in the circuit using one of the same length but of the acceptable gauge. The students will also use four bulbs in parallel in the circuit. Students will evaluate the change in illumination of the bulbs, compared to the original circuit.

Create circuits on project boards prior to class. Use 30 feet of 28-gauge wire and a Sylvania 3156 single-filament light or equivalent bulb. (1) Measuring the voltage across the bulb in the original circuit should give an indication that the wire is too small. (2) Use the formula P = IE, where P is the wattage of the bulb, I is the designed current of the bulb, and E is the designed voltage of the bulb. For the Sylvania bulb specified, P = 26.88 watts and E = 12.8 V. You will need to determine these values for substituted bulbs. (3) The maximum voltage drop in the wire is equal to the power supply voltage minus the designed voltage of the bulb. (4) Use Ohm’s law, E = IR, where E is the voltage drop calculated in step 3, I is the designed current calculated in step 2, and R is the wire resistance. (5) Use the formula for wire resistance,

R = 4p  x  l   , to find d.
         p          d²

Use 7 × 10^–-7 ohms in. for r and 3.1416 for p. (6) Find the minimum wire gauge from the table. (7) Create the circuit as directed, and evaluate.

The circuit with the replaced component can be used as an assessment.

Create project board models of the circuit shown below.

 

Students will create four circuits using test wires— 10 feet of 10-gauge wire, 10 feet of 20-gauge wire, 50 feet of 10-gauge wire, and 50 feet of 20-gauge wire. They will measure the current and voltage across the test wire in each circuit. Have students calculate the wire resistance for each test wire using Ohm’s law.

The students should build the circuits as directed. They will measure the current in the line and the voltage across the test wire. They can then use Ohm’s law to calculate the resistance in the test wire. The students should compare the calculated resistance of each wire to determine the effect of (1) wire length remaining constant and wire gauge increasing or decreasing and (2) wire gauge remaining constant and wire length increasing or decreasing. Have the students compare these effects to their earlier observations that wire resistance is directly related to the length of the wire and inversely related to the diameter of the wire. The calculated wire resistance of each wire and students’ observations of the effect of wire length and/ or wire gauge can be used as an assessment.

Each team will write a report to the customer explaining the cause of the problem and recommending corrective action.

Source: AYES