# Voltage Instrument:

## Introduction

This experiment introduces the Analog Discovery's™ voltage instrument. The voltage instrument provides fixed +5V and -5V voltages.

In this exercise, we will apply voltage to a diode to produce light—the type of diode we will use is called (reasonably enough) a light emitting diode, or LED. When the voltage difference across the LED exceeds the threshold voltage of the LED, current flows through the LED and light is emitted. The physical appearance of LEDs is as shown in Fig. 1. The anode and cathode on a physical LED can be distinguished by identifying some specific characteristics; the anode pin is longer than the cathode pin and the cathode side of the plastic diffusion lens is typically slightly flattened. You will need to download Digilent's® WaveForms™ software for the Analog Discovery projects if you do not have it already.

We will also use a resistor in our circuit. Resistors are commonly used to limit the current in a circuit. In our case, the resistor is used to limit the current to levels which do not exceed the allowable current delivered by the Analog Discovery. Alternate use of current-limiting resistors is to protect componentsâ€”if the voltage across a diode is too large, excessive current can flow through the diode and the diode can burn out.

Our overall circuit will be constructed using a solderless breadboard. Breadboards provide a convenient way for us to create and modify electrical circuits.

##### Before you begin, you should:
• Be able to identify ground, V+, and V- terminals on the Analog Discovery.
• Be able to identify the anode and cathode of an LED.
• Be able to state how the holes in a solderless breadboard are connected.
##### After you're done, you should:
• Be able to use the voltage instrument on the Analog Discovery to apply Â± 5V relative to the Analog Discovery's ground.

## Inventory:

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

## 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.

#### B. Create Circuit

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

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

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

## Step 2: Set up Instruments

#### A. Open Voltage Instrument

1. Return to the WaveForms main window.

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

#### B. Turn on Power

1. The LED should light up!

## Test Your Knowledge!

1. We don't need to use V+ to turn on the LED. As long as the LED's anode is at a higher voltage than the anode, the LED will light up. We can just as easily use V- to light up the LED.
• The V- power supply provides negative 5V relative to ground so that ground is 5V above the V- voltage level.
• To use V- to light up your LED, connect ground to the point where V+ was previously connected, and V- to the point where we connected ground above.
• Turn on power to V- to light up the diode!
2. Two uses for resistors in circuits like the above are to:
1. Limit the current to a component (such as the diode) to keep it from burning out, or
2. Keep the current within ranges allowed by the source (V+ in our circuit).
• The resistor in our circuit is used for reason 2 above.
• To see what happens without the resistor, simply remove the resistor from the above circuit—so that V+ is connected directly to the diode's anode—and turn on power. You should get the message shown below:
• Without the resistor, the diode attempts to draw too much power from the Analog Discovery. To protect the USB port on your computer, the Analog Discovery shuts itself down!

• Other product and company names mentioned herein are trademarks or trade names of their respective companies. © 2014 Digilent Inc. All rights reserved.
• Circuit and breadboard images were created using Fritzing.