ELEC 101
Prof. Rich Kozick

## Laboratory 2 Resistors: Measurement, Color Code, and Design

### Objectives

In this lab, we will learn how to measure the electrical resistance of a device. The color code on resistors will be explained, and a design exercise is included in which you choose resistance values in a network of switches and series/parallel resistors to achieve specified values of current flow.

### Introduction

Resistors dissipate power in the form of heat or light when there is an electrical current through them. The current through a resistor, I, is related to the voltage across the resistor, V, by Ohm's Law, V=IR. The electrical resistance, R, of a device is a measure of the difficulty with which charges flow through the device when a voltage is applied across the device. The resistance, given in units of ohms (), depends on the geometry and the resistivity, , of the device. For example, for a material between two parallel electrodes (Figure 1), the resistance is:

where d and A are the thickness and cross-sectional area of the material, respectively. The resistivity, an intrinsic property of the material of which the resistor is composed, is large for good insulators and small for good conductors (Table 1).

 Table 1: Resistivity of Conductors and Insulators Material Resistivity at 0 oC () Resistivity at 22 oC () Classification Silver 1.47 1.61 Conductor Copper 1.55 1.70 Conductor Gold 2.05 2.20 Conductor Aluminum 2.50 2.74 Conductor Ceramic (Porcelain) - 1018-1020 Insulator Plastic (Acrylic) - >1021 Insulator

Combinations of Resistors:

When n resistors are connected in series (end to end), the equivalent resistance, Req, of the series combination of resistors is the sum of the individual resistances such that where Ri is the resistance of the ith resistor. (Note: Elements in series have the same current through them.)

When n resistors are connected in parallel, the equivalent resistance, Req, of the parallel combination of resistors is the sum of the individual resistances such that where Ri is the resistance of the ith resistor. (Note: Elements in parallel have the same voltage across them.)

### Lab Procedure

1. Complete the E-lesson on measuring resistance as follows. Record the steps for measuring resistance with the digital multimeter (DMM) in your lab notebook for future reference.

2. For information on resistor color codes and an interactive program to determine a resistor's value, see the following links. (These are linked to the Laboratories page on the course web page.) You may want to print the color code description and paste it into your lab notebook for future reference.
3. Resistor Color Code Description:
http://www.eg.bucknell.edu/~bsprunt/classes/references/resistor_color_codes/resistor_color_codes.htm

Resistor Code Calculator:
http://www.eg.bucknell.edu/~bsprunt/classes/references/resistor_values/resistor.htm

4. Design a network of resistors and switches according to the following description.

Study the circuit below. It has five resistors: R1, R2, R3, R4, R5 and four switches S1, S2, S3, S4. These switches can be opened and closed to control the current i. In a more advanced system, a computer could be used to turn on and off various switches in sequence to control the current in the circuit.

** WE WILL NOT USE ACTUAL SWITCHES; JUST USE WIRES TO CLOSE SWITCHES. **

Design one set of resistor values that can be used with various switch settings such that the following set of currents can be selected. Only use resistors that have values of 1 K ohm or greater.
• 0.25 mA
• 0.33 mA
• 0.50 mA
• 0.67 mA
• 1.00 mA
• 2.00 mA
• 2.50 mA

Record your calculations and circuits in your lab notebook. Also, record the resistor values and the switch settings required to obtain each of the desired currents in your lab notebook.

Measure the resistance of the circuit that you designed for each of the switch settings using the ohmmeter setting on the DMM.

You do not have to connect the 5 V source or measure current. Please measure the equivalent resistance of your network for various switch settings, and use Ohm's Law to determine the current that would result with a 5 V source.

Thank you.