Lab 4.2.3 Voltage Measurement

Estimated time: 30 min.

Objectives:

> - Demonstrate your ability to measure voltage SAFELY with the multimeter

Background:

> The digital multimeter is a versatile testing and troubleshooting device. In this lab, you will learn how to perform both Direct Current (DC) and Alternating Current (AC) voltage measurements. Voltage is measured in either AC or DC volts (indicated by a V). Voltage is the pressure that moves electrons through a circuit from one place to another. Voltage differential is essential to the flow of electricity. The voltage differential between a cloud in the sky and the earth is what causes lightning to strike. NOTE: It is very important to be careful when taking voltage measurements since it is possible to receive an electrical shock.

> Direct Current (DC): DC voltage rises to a set level and then stays at that level and flows in one direction (positive or negative). Batteries produce DC voltage and are commonly rated at 1.5v or 9v (flashlight batteries) and 6v (lantern and vehicle batteries). Typically the battery in your car or truck is a 12v battery. When an electrical "load" such as a light bulb or motor, is placed between the positive (+) and negative (-) terminals of a battery, electricity flows.

> Alternating Current (AC):

> AC voltage rises above zero (positive) and then falls below zero (negative) and actually changes direction very rapidly. The most common example of AC voltage is the wall outlet in you house or business. These outlets provide approximately 120 volts AC directly to any electrical appliance that is plugged in such as a computer, toaster or television. Some devices such as small printers and laptop computers have a transformer (small black box) that plugs into a 120V AC wall outlet and then converts the AC voltage to DC voltage for use by the device. Some AC outlets can provide a higher voltage of 220V for use by devices and equipment with heavier requirements such as clothes dryers and arc welders.

Tools / Preparation:

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

> Required voltage measurement items:

> - Fluke 12B multimeter (or equivalent)
> - An assortment of batteries: A cell, C cell, D cell, 9 Volts, 6 V lantern
> - Duplex wall outlet (typical 120v)
> - power supply (for laptop, or other networking electrical device)

> Optional voltage measurement items:

> - A lemon, with a galvanized nail stuck in one side, and a piece of uninsulated copper wire stuck in the opposite side.
> - Solar cell with leads attached.
> - home-made generator (wire wound around a pencil 50 times and a magnet).

Step 1 – Move the rotary selector to the V symbol for voltage (black V) in order to be able to measure.

Press the button that has the VDC and VAC symbol to select between Direct Current (DC) or Alternating Current (AC) measurements.

Direct Current Measurements:

The screen will show a V (voltage) with a series of dots and a line over the top. There are several scales available depending on the voltage to be measured. They start from millivolts (abbreviated mV = 1,000/th of a volt ) to voltages up to hundreds of volts. Use the Range button to change the range of DC voltage to be measured based on what voltage you expect to measure. Batteries (less than 15 volts) can typically be measured accurately with the VDC scale and 0.0 range. DC voltage measurements can be used to determine if batteries are good or if there is voltage coming out of an AC adapter (transformer or converter) which are very common and used with hubs, modems, laptops, printers and other peripherals. These adapters can take wall outlet AC voltage and step it down to lower AC voltages for the device attached or can convert the AC voltage to DC and step it down. Check the back of the adapter to see what the input (AC) and output voltages (AC or DC) should be.

Alternating Current Measurements:

The screen will show a V (voltage) with a tilde or (~) after it. This represents alternating current. There are several scales available depending on the voltage to be measured. They start from millivolts (abbreviated mV = 1,000/th of a volt ) to voltage up to hundreds of volts. Use the Range button to change the range of AC voltage to be measured based on what voltage you expect to measure. Voltage from power outlets (120v or greater) can typically be measured accurately with the VAC scale and 0.0 range. AC voltage measurement are useful in determining if there is adequate voltage coming from an AC outlet to power the equipment plugged in.

Use a Fluke 12B multimeter (or equivalent) to measure the voltage of each of the following:

Step

2 – Check the following voltages. Be sure to turn the meter off when finished.

> Item to Measure the Voltage of: | Set Selector and range scale to: | Voltage reading:
> Batteries: A cell (AA, AAA) , C cell, D cell, 9 Volts, 6 V lantern |  |
> Duplex wall outlet (typical 120v) |  |
> Power supply (converts AC to lower AC or DC) for laptop, mobile phone or other networking electrical device |  |
> (Optional) A lemon, with a galvanized nail stuck in one side, and a piece of uninsulated copper wire stuck in the opposite side |  |

Reflection Question:

> Why might you want to measure voltage when troubleshooting a network?