Lab 2: Series and Parallel Resistance
Resistor Circuit, Series, Parallel
In this experiment you will examine different resistor circuits. Using an ammeter, you will measure the current in the circuits to determine their overall resistances. You will also determine values for unknown resistances in several pre-wired circuits.
Background material for this experiment can be found in Chapter 2 of this manual
Parts List:
Protoboard
Digital multimeter
Various resistors and wires
Black tape to cover “unknown” resistors
Part 1 – Circuits with known elements
Series Circuit and KVL
In this part of the experiment, you will examine, wire, and test the series circuit shown below. The schematic for this circuit is shown in Figure 2.1. Do not be concerned with setting the source voltage to 5.0 V exactly but measure the voltage precisely. All calculations and Multisim simulations are to be performed with the measured values of VB and the resistances, not the nominal values shown in the circuit.
Figure 2.1: Series Circuit to be tested.
After you have wired up and powered up this circuit, connect the leads of the ammeter to measure the current flowing through the entire circuit. Record the current reading from the ammeter on the laboratory worksheet, and then use Ohm’s Law to calculate the overall resistance of this circuit. In addition, record the voltages across R1 and R2, and note that their sum is equal to the voltage of the battery, verifying KVL.
Parallel Circuit and KCL
In this part of the experiment, you will examine and only simulate the parallel circuit shown below. The schematic for this circuit is shown in Figure 2.2.
Figure 2.2: Parallel Circuit to be tested.
Record the currents through R1 and R2 and show that their sum is equal to the overall current (read through XMM1), hence verifying KCL. In addition, divide VB by the overall current, and show that the ratio is equal to the equivalent resistance of R1 and R2 in parallel. You are reminded again to use the measured values of VB and the resistances in your calculations. Remember that a voltmeter is placed across (in parallel) the component whose voltage you are trying to measure, and an ammeter is to be inserted (in series) in the path through which the current is flowing.
Part 2 – Circuits with an unknown element
Part II is to run under Multisim only. The circuits will be like the ones shown in figures L2.1 and L2.2, with the difference that R2 is supposed to be unknown. Under Multisim, hide the value of the resistance R2, and answer the questions asked in the next paragraphs, and use the relevant measurements proposed in those paragraphs to calculate the value of the unknown resistance.
Unknown element in a series circuit
Build the circuit shown in L2.1, with the same values except that R2 is assumed to be unknown
(choose values between 470Ω and 5kΩ, so that whatever measurement you take, the values will be substantial enough to lead to more accurate results) and measure only voltages across each resistor. You are not allowed to measure the current. Show how by using those two voltages, you can evaluate the resistance of the unknown resistor value.
Unknown element in a parallel circuit
Draw the circuit shown in figure 2.2 with R2 unknown, and measure only the current through each resistor, then evaluate the resistance of the unknown resistor value using those measured currents only.
Show the record of the experiment in the laboratory notebook at the end of the class.
In the report, use measured values of currents and voltages (Part I) to confirm KVL and KCL. How well your measurements agree with these laws (express any discrepancies in %)?
For Part 2 show calculations of the unknown resistor in the series and parallel circuits. Compare the two results; how closely (%) do they agree?
Part 3 – Matlab
There are three resistors whose resistance are x, y, and z. The sum of their resistances is 10kΩ. When we add 1000Ω to x and 1000Ω to y, the new value of x becomes double the value of the new resistance of y. When we add 2000Ω to x and 2000Ω to z, the new value of x becomes double the value of the new resistance of z. Use Matlab to find the current values of x, y, and z.