Lab 6 Series-Parallel DC Circuits

Objective: Students successfully completing this lab will accomplish the following objectives: 1. Gain experience analyzing and verifying, by measurements, the characteristics of series-parallel resistive circuits. 2. Increase understanding of the relationship of voltage, current and resistance in a series-parallel circuit 3. Learn to compute currents through the use of voltmeter readings. A formal lab report on lab exercises 5, and 6 will be required. Keep your results from this lab exercise. They will be used as part of the formal report. The formal lab report will be due one week after lab 6 is performed. Digital Multimeter (DMM), connecting leads, alligator clips, breadboard, jumper wires, LEDs, resistors (100 Ω, 1 kΩ).

Lab Report:

Equipment:

Procedure: In a circuit in which the components are soldered to a circuit board, direct measurement of current can be a nuisance. Doing so requires desoldering a connection, measuring the current in the circuit gap, and re-soldering the connection. If the circuit contains a sufficient number of resistors, it may be possible to use a voltmeter and some simple calculations using Ohm’s law to determine circuit currents. In this latter method, no desoldering is required. We demonstrate this technique in the following steps. 1. Select three resistors: R1 = R2 = R3 = 100 Ω, ½ W. Measure the resistors and record the values in Table 1 below. Table 1: Measured Values of Resistors Resistor R1 R2 R3 Measured Value

2.

Construct the circuit shown in Figure 1 below. Turn the power supply off and then connect it to the breadboard. R1

A

a

R2

V

R3

ES

LED1

LED2

b

Figure 1: Series-parallel circuit containing LEDs

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3.

Adjust the power supply to 9 V. Both LEDs should illuminate. Measure and record the resistor and LED voltages in Table 2 below. Table 2: Measured Resistor and LED Voltages Quantity VR1 VR2 VR3 VLED1 VLED2 Measured Value

4.

Calculate the voltage Vab by adding your measured values of VR2 and VLED1. Then, calculate Vab by adding your measured values of VR3 and VLED2. Record your calculations in Table 3 below. Table 3: Calculated Voltages Across Parallel Branches Quantity Vab Vab Calculation Method VR2 + VLED1 VR3 + VLED2 Calculated Value

Are the two calculations approximately the same? ____________ 5. Add the voltages Vab + VR1. Vab + VR1 = ____________ Based on Kirchhoff’s Voltage Law (KVL), what must Vab + VR1 be equal to? ____________ Does this calculation satisfy KVL? ____________ 6. With the resistance measurements in step 1 and the voltage measurements in step 3, use Ohm’s Law to calculate the various circuit currents. Table 4: Circuit Currents Calculated from Measured Values Quantity IR1 Calculated Value

IR2

IR3

IT

7.

Apply Kirchhoff’s Current Law (KCL) at node a. Write the KCL equation below.

Do the values calculated above for currents IR1, IR2 and IR3 satisfy KCL? ____________

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8.

With the ammeter inserted into the circuit, measure the source current, IT. Record the measured value below. IT = ____________ Are your calculated value for IT in step 6 and your measured value for IT in step 8 approximately the same? ____________

9.

Select three resistors: R1 = 100 Ω, R2 = R3 = 1 kΩ. Measure the resistances and record your results in Table 5 below. Table 5: Measured Resistor Voltages Resistor R1 R2 R3 Measured Value

10.

Connect R1 (100 Ω) in series with R2 (1 kΩ). Attach a 5 V power supply to the series circuit. Measure the resistor voltages. Record your results in Table 6 below. Table 6: Measured Resistor Voltages Quantity VR1 VR2 Measured Value

Apply KVL around the circuit loop. Write the KVL equation below.

Do the measured values above satisfy KVL? ____________ 11. Using the nominal resistor values, show the calculations for the resistor...