 # Resistors:

## Introduction

In this experiment, we will use Digilent's® Analog Discovery™ and a digital multimeter to explore a fundamental equation used in electric circuit analysis and design: Ohm's Law. Ohm's law describes the relationship between the voltage across a resistor and the resulting current passing through the resistor. The links in the related material section provide more detail regarding these topics.

Ohm's law states that the voltage across a resistor and the current through the resistor are proportional to one another. The constant of proportionality is the resistance. There are three variations of Ohm's Law in equation form:

$v = i \cdot R$ $i = \frac{v}{R}$ $R = \frac{v}{i}$
##### Before you begin, you should:
• Be able to use the Analog Discovery's voltage instrument and AWG to provide constant (DC) voltages.
• Be able to measure voltage using a voltmeter.
• Be able to measure current using an ammeter.
##### After you're done, you should:
• Be able to calculate the resistance of a resistor from measured voltage and current.
• Given a voltage difference, be able to calculate the resistance necessary to provide a desired current.
• Given a current, be able to calculate the resistance necessary to provide a desired voltage difference.
• Be able to determine nominal resistance from resistor color codes.
• Be able to state Ohm's law from memory.

## Inventory:

Qty Description Typical Image Schematic Symbol Breadboard Image
1 Digital multimeter   1 A resistor with red as its third color band

(They will be in the 1kΩ to 9.9kΩ range)   2 Analog Discovery pins  ## Step 1: Understanding the Circuit

#### A. Circuit Schematic

1. Use V+ to apply 5V across the resistor.

2. Use the DMM in series with the resistor in the circuit to measure the current.

3. Use channel 1 (1+ and 1-) to measure the voltage difference (VOUT) across the resistor.

#### B. Create Circuit

1. Connect V+ (red wire) to one removable pin.

2. In a different row, connect the other removable pin to the resistor.

3. Connect 1+ (orange wire) and 1- (orange and white striped wire) across the resistor, with 1+ on the side closest to V+.

4. Connect ground ( , black wire) to the resistor.
• Note: The removable pins are where the leads of the DMM will be connected.

## Step 2: Set up Instrument

#### A. Open Voltmeter Instrument

1. Open WaveForms™ to view main window.

2. Click on the More Instruments dropdown menu to open the Voltmeter instrument.

• Note: The Voltmeter instrument will allow us to measure the voltage difference (VOUT) across the resistor.

#### B. Open Voltage Instrument

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

#### C. Turn on Power The above screenshots are of Digilent's WaveForms running on Microsoft Windows 7

• Channel 1 on the Voltmeter instrument will not show a voltage difference across the resistor since the DMM is not connected to the removable pins yet.

## Step 3: Experiment

#### A. Collect Data

1. Set the DMM to measure milliamperes (mA). Connect it to the circuit by pressing the red lead to the removable pin connected to V+. The black lead (COM) should be pressed to the other removable pin.

2. Record the current value measured on the DMM as well as the voltage value measured on Channel 1 of the Voltmeter instrument.

#### B. Analyze Data

1. Use the equations provided in the introduction to this project and the values you measured in Part A to estimate the resistance of the resistor you chose for your experiment. Record this resistance value.

#### C. Compare with Expectations

1. Use the color bands on the resistor to determine its nominal resistance. Calculate a percent difference between the value calculated in Part B and the expected value determined from the color bands. Percent difference is calculated as:

$\% {\rm{ Error = }}\left| {\left. {\frac{{Measured - Expected}}{{Expected}}} \right|} \right. \times 100$

Is the difference within the range you would expect based on the tolerance color band of the resistor?