# Measuring DC Voltage

## Introduction

This experiment introduces the use of digital multimeters for voltage measurement. Digital multimeters, or DMMs, perform a variety of functions. When measuring voltage, the DMM is operating as a voltmeter. More information relative to DMMs and their use in voltage measurements are provided in the links to the right.

Voltages in a circuit are a difference in electrical potential energy between two different points in the circuit. DMMs generally have two terminals, or leads. When the DMM is operating as a voltmeter and its leads are connected to two different points in a circuit, the DMM will display the voltage difference between the two points.

##### Before you begin, you should:
• Be able to use the voltage instrument on the Analog Discovery™ to apply ±5V relative to the Analog Discovery's ground.
• Be able to use the Analog Discovery voltmeter to measure constant voltages.
• Be able to state how the reference voltage polarity and connection of voltmeter terminals are related to the sign of the measured voltage.
##### After you're done, you should:
• Be able to use a digital multimeter to measure constant voltage.

## Inventory:

Qty Description Typical Image Schematic Symbol Breadboard Image
1 LED
2 100Ω resistor
1 Digital multimeter

## Procedures

### Shortcut!

• Parts A and B of this project are also done in the Voltmeter Instrument project. If you've already completed that project and your circuit is still intact, simply disconnect the Analog Discovery voltmeter connections, add the DMM connections (as shown in Parts B and C of Step 1 in this exercise), and read the voltage difference using the DMM.

### Step 1: Understanding the Circuit

#### A. Circuit Schematic

1. Use V+ to apply 5V across the LED to light it up.

2. The resistor limits the current.

3. To measure the voltage V, connect the V-Ω terminal of your DMM to the diode's anode and the COM terminal to the diode's cathode.

#### B. Create Basic Circuit

1. Connect V+ (red wire) to the 100Ω resistor.

2. Connect the 100Ω resistor to the LED anode.

3. Connect ground (, the black wire) to the cathode of the LED.

#### C. DMM Setup and Connections

1. Connect the V-Ω terminal of the DMM to the diode's anode.

2. Connect the COM terminal of the DMM to the diode's cathode.

3. Turn the function selection knob on the front of the DMM to the position.

### Step 2: Set up Instruments

#### A. Open Voltage Instrument

1. Open WaveForms™ to view the main window.

2. Click on the Voltage instrument icon to open the Power Supplies window.

#### B. Turn on Power and Determine Diode Voltage

• The LED should light up and the DMM should display the voltage difference across the diode.

## Test your Knowledge!

1. Try reversing the polarity on your voltage measurement (just interchange the V-Ω and COM terminals). This should only change the sign for the displayed number.

2. Use the DMM to measure the voltage across the resistor, with the assumed polarity as shown below. (Hint: you should get a negative value.)

• Reverse the polarity on the resistor measurement and verify that the sign on the measured voltage changes.

3. Use your DMM to measure the voltage difference across a battery. (Any battery will do, as long as you can determine the expected voltage across the battery.)
• Compare the measured voltage with the expected battery voltage and the expected polarity. (Both the expected voltage and the polarity should be printed on the battery; the polarity is generally indicated by a “+” sign near the appropriate terminal—the opposite terminal, of course, is the negative terminal.)
• Reverse the leads of the DMM and verify that the sign (but not the value) of the displayed voltage changes.

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• Circuit and breadboard images were created using Fritzing.