Vol. DIY Electronics Projects
Chapter 2 Basic Projects and Test Equipment

Intro Lab - How to Use a Voltmeter to Measure Voltage

In this hands-on introduction to electronics, you will experiment with a multimeter and learn how to measure voltage using the voltmeter function.

Project Overview

In this project, you will learn how to use a voltmeter to measure voltage. Typically, the voltmeter is one of the functions of a multimeter, which is an electrical instrument capable of measuring voltage, current, and resistance (Figure 1).

Parts and Materials

One thing to note is that analog multimeters tend to be less expensive than digital multimeters and may be more beneficial as learning tools for the first-time student of electricity.

Below, in Figure 2, you'll find a representation of the different parts and materials for this project.

Learning Objectives

• How to measure voltage
• Characteristics of voltage: existing between two points
• Selection of proper meter range

Introduction to Measuring Voltage

In all of the experiments in this book, you will be using some test equipment to measure aspects of electricity you cannot directly see, feel, hear, taste, or smell. Electricity—at least in small, safe quantities—is insensible by our human bodies.

Your most fundamental “eyes” in the world of electricity and electronics will be a device called a multimeter. Multimeters indicate the presence of and measure the quantity of electrical properties such as voltage, current, and resistance. In this experiment, you will familiarize yourself with the measurement of voltage.

Voltage is the measure of electrical “push” ready to motivate charges to move through a conductor. In scientific terms, it is the specific energy per unit charge, mathematically defined as joules per coulomb. It is analogous to pressure in a fluid system: the force that moves fluid through a pipe and is measured in the unit of the Volt (V).

Digital Versus Analog Multimeters

As illustrated in Figure 3, there are two types of multimeters: digital and analog.

Figure 3. Digital and analog multimeters for measuring voltage, current, and resistance

Digital multimeters have numerical displays, like digital clocks, for indicating the quantity of voltage, current, or resistance. Analog multimeters indicate these quantities by means of a moving pointer over a printed scale. Your multimeter should come with some basic instructions. Read them well!

If your multimeter is digital, it will require a small battery to operate. If it is analog, it does not need a battery to measure voltage.

Manual Versus Autoranging Multimeters

Manual-ranging meters have several different selector positions for each basic quantity: several for voltage, several for current, and several for resistance. For voltage, these ranges could be μV, mV, and V.

Some digital multimeters are autoranging. An autoranging meter has only a few selector switch (dial) positions. Autoranging is to manual ranging as an automatic transmission is to a manual transmission in a car. Basically, an autoranging meter “shifts gears” automatically to find the best measurement range to display the particular quantity being measured.

Instructions

Step 1: Insert your test probes into the voltmeter. By convention, black probes are used with the negative, or common, input of the voltmeter, while red probes are used with the positive input (see Figure 3).

Step 2: Set your multimeter’s selector switch to the highest-value DC volt position available. Autoranging multimeters may only have a single position for DC voltage, in which case you need to set the switch to that one position.

Step 3: Touch the red test probe to the positive (+) side of a battery and the black test probe to the negative (-) side of the same battery. The voltmeter should now provide you with an indication of the battery's voltage. If your meter is a manual-range type, and the selector switch has been set to a high-range position, the indication will be small.

Step 4: Reverse the test probe connections to the battery if the meter’s indication is negative (on an analog meter, a negative value is indicated by the pointer deflecting left instead of right).

Step 5: If you have a manual-ranging voltmeter, move the selector switch to the next lower DC voltage range setting and reconnect to the battery. The indication should be stronger now, as indicated by a greater deflection of the analog meter pointer (needle) or more active digits on the digital meter display.

For best results, move the selector switch to the lowest-range setting that does not over-range the meter. An over-ranged analog meter is said to be “pegged,” as the needle will be forced all the way to the right-hand side of the scale, past the full-range scale value. Additionally, an over-ranged digital meter sometimes displays the letters “OL” or a series of dashed lines. This indication is manufacturer-specific.

Step 6: What happens if you only touch one of the voltmeter test probes to one end of a battery? How does the meter have to connect to the battery to provide an indication? What does this tell us about voltmeter use and the nature of voltage? Is there such a thing as voltage “at” a single point?

Step 7: Be sure to measure more than one size of the battery, and learn how to select the best voltage range on the multimeter to give you the maximum indication without over-ranging.

Measuring the Voltage Generated by Light on an LED

Step 8: Now switch your multimeter to the lowest DC voltage range available, and touch the meter’s test probes to the terminals (wire leads) of the light-emitting diode (LED) as illustrated in Figure 4.  A LED is designed to produce light when powered by a small amount of electricity, but LEDs also happen to generate DC voltage when exposed to light, somewhat like a solar cell. Point the LED towards a bright source of light with your multimeter connected to it, and note the meter’s indication:

Figure 4. Measuring the voltage induced in a LED when exposed to light.

Batteries develop electrical voltage through chemical reactions. When a battery dies, it has exhausted its original store of chemical fuel. The LED, however, does not rely on an internal fuel to generate voltage; rather, it converts optical energy into electrical energy. So long as there is light to illuminate the LED, it will produce voltage.

Measuring the Voltage of a Motor Functioning as an Electric Generator

Step 9: Another source of voltage through energy conversion is an electric generator. Connect your voltmeter (your multimeter, set to the volt function) to the motor’s terminals just as you connected it to the LED’s terminals, as illustrated in Figure 5.

Figure 5. Measuring the voltage created by a motor functioning as an electric generator.

Step 10: Spin the shaft with your fingers. The meter should indicate voltage by means of needle deflection (analog) or numerical readout (digital). The small electric motor specified in the “Parts and Materials” list functions as an electrical generator if its shaft is turned by a mechanical force.

It may be difficult to maintain both meter test probes in connection with the motor’s terminals while simultaneously spinning the shaft with your fingers. That is why Figure 3 uses alligator clip jumper wires.

Step 11: Next, determine the relationship between voltage and generator shaft speed.

Step 12: Reverse the generator’s direction of rotation and note the change in meter indication. When you reverse shaft rotation, you change the polarity of the voltage created by the generator. The voltmeter indicates polarity by direction of needle direction (analog) or sign of numerical indication (digital).

When the red test lead is positive (+) and the black test lead negative (-), the meter will register voltage in the normal direction. If the test probes are connected with reverse polarity (negative on red and positive on black), the meter will indicate “backwards.”