Lab 4.2.2 Resistance Measurements

Estimated time: 30 min.

Objectives:

> - Demonstrate your ability to measure resistance and continuity with the multimeter.

Background:

> The digital multimeter is a versatile testing-and-troubleshooting device. In this lab you will learn how to perform resistance measurements, and related measurements called continuity. Resistance is measured in Ohms (indicated by the Greek letter Omega or

> Ω). Copper wires (conductors) such as those commonly used in network cabling (UTP and coax) normally have very low resistance or "good" continuity (the wire is continuous) if you check them from end to end. If there is a break in the wire it is called an "open" which creates very high resistance (air has nearly infinite resistance indicated by the infinity symbol or ∞, a sideways eight). The multimeter has a battery in it which it uses to test the resistance of a conductor (wire) or insulator (wire sheathing). When the probes are applied to the ends of a conductor, the battery current flows and the meter measures the resistance it encounters. If the battery in the multimeter is low or dead you must replace it or you will not be able to take resistance measurements.

> With this lab you will test common networking materials so that you can become familiar with them and their resistance characteristics. You will first learn to use the resistance setting on the multimeter. As you measure small resistances, you should also note the continuity feature. The instructions provided are for the Fluke 12B. Other meters will function in a similar way.

Tools / Preparation:

> Prior to starting the lab, the teacher or lab assistant should have several multimeters available (one for each team of two students) and various networking related items for testing resistance. Work in teams of two. You should review semester 1 On-line Lesson 4. The following resources will be required:

> - Fluke 12B multimeter or equivalent
> - 1000 Ohm resistor
> - 10,000 Ohm resistor
> - Pencil for creating graphite paths on paper
> - Cat 5 jack
> - Small (0.2m or appx 6 to 8 inches) section of Cat 5 UTP solid cable
> - BNC terminated coaxial cable
> - Unconnected DB9 to RJ-45 adapter
> - Terminated Cat 5 UTP patch cable

Step 1 - Move the rotary selector to the Omega symbol for Ohms (red Ω) in order to measure resistance.

Press the button that has the Ohms symbol (red Ω) on it to select between resistance measurements and continuity.

Resistance Measurements:

The screen will show Ω (ohms), KΩ (kilohms = thousands of Ohms) or MΩ (megohms = Millions of Ohms). Use the Range button to change the range of resistance to be measured based on what resistance you expect to get. If you expect low resistance (less than 10 ohms), select a low scale (like Ω). If you expect a high reading (over 10,000 ohms), select a high scale (like KΩ). If the resistance reading is over the range selected, the OL or Over Limit indicator will be displayed on the screen. The resistance setting is for measuring exact amounts of resistance

Continuity Measurements:

The screen will show a diode symbol which is a small black triangle pointing to a vertical bar. A diode is an electronic device that either passes or blocks electrical current. You may see a small sound symbol next to it which means that when there is good continuity (no resistance) the beep will sound. The continuity setting is used when you just want to know if there is a good path for electricity or not and don't care about the exact amount of resistance.

Step

2 - Check the following resistances. Turn the meter off when finished or battery will drain.

> Item to Measure the resistance of: | Set Selector and range scale to: | Resistance reading:
> 1000 Ω Resistor |  |
> 10 kΩ Resistor |  |
> Graphite marking from a pencil on a piece of paper |  |
> Cat 5 jack |  |
> 0.2 m section of Cat 5 UTP solid cable |  |
> Touch red and black probe contacts together |  |
> Your own body (touch the tips of the probes with you fingers) |  |
> BNC terminated coaxial cable |  |
> Unconnected DB9 to RJ-45 adapter |  |
> terminated Cat 5 UTP patch cable |  |

Reflection Question:

> What purpose might the multimeter serve in maintaining and troubleshooting a computer network?