EE40, Spring 2015, Pre-Lab 3

LED Circuit and Voltage Regulator Circuit

Logistics

You should submit your prelab assignments on Gradescope before your lab section. This will be different

than the Gradescope page of the course where you submit your homework and you should already be

able to see the page of your lab in addition to the main course in your Gradescope account. If not, you

can email your lab GSI to do that for you.

You will complete the lab assignments during the lab sessions. You should ask your GSI to verify it and

check you off the list at the end of each lab session before you leave.

If you complete your lab at home before beginning of the session, please show up to your lab session and

make sure your GSI verifies that you have completed all the required tasks correctly and checks you off.

Objectives of Lab 3

For prelab 3 you will simulate the LED circuit and the voltage regulator circuit using Multisim. You will also

build these circuits in lab 3. Below are the tasks that you should complete for lab 3.

Prelab assignment (to be completed before your lab session)

o Simulations: LED Circuit and Voltage Regulator Circuit

o Questions: Q.1 to Q.4

Lab assignment (to be completed during your lab session)

o Build the LED and Voltage Regulator Circuit

Outline

1. Prelab assignment

2. Lab guideline

1. Prelab Assignment

You should complete your prelab assignment and submit it to the Gradescope account of your lab

section before beginning of your lab session.

Your prelab assignment consists of two parts. In the first part you will do a simulation using Multisim

and in the second part you should answer a few intuitive questions about the simulation.

1.1. LED Circuit Simulation

Consider the circuit shown in the following figure.

Figure 1. LED Circuit

The basic theory for this circuit is explained in Edge-edX course webpage modules 1.0-1.3. Please

review those modules carefully and make sure you understand how this circuit works before

starting the simulations.

Start Multisim and make two schematics of the circuit shown in Figure 1, one with the 300

resistor and one with a wire in its place. Measure the voltage across the LED for each case.

Measure the current across the LED for each case.

The Multisim schematic of the circuit and component directory are provided in Figure 2 and Table

1, accordingly. Refer to the user guide of Multisim or ask your GSI in case you have questions.

Figure 2. SPICE Schematic

Component MultiSim Directory

Voltage Source Place/Component/Group:Sources/POWER_SOURCES/DC_POWER

Resistor Place/Component/Group:Basic/RESISTOR

LED Place/Component/Group:Diodes/LED/LED_red

Table 1. Component Directory

1.2. Voltage Regulator Circuit Simulation

A voltage regulator is designed to maintain the output voltage of a power source within desired

limits. In this assignment, you are going to simulate MAX15006A voltage regulator which

maintains the output voltage between 3.25V and 3.35V with a typical value of 3.3V. Here you

can find the datasheet of MAX15006A. Please review edX edge course webpage module 1.4 for

a detailed explanation on the voltage regulator circuit that you are going to simulate in this

assignment and build in the lab.

Consider the circuit shown in the following figure.

Figure 3. Voltage Regulator Circuit

Start Multisim and make the schematic shown in Figure 3. Measure the voltage between the VDD

and GND terminals. The Multisim schematic of the circuit and component directory are provided

in Figure 4 and Table 2, accordingly. Refer to the user guide of Multisim or ask your GSI in case

you have questions.

Figure 4. SPICE Schematic

Component MultiSim Directory

Voltage Source Place/Component/Group:Sources/POWER_SOURCES/DC_POWER

Capacitor Place/Component/Group:Basic/CAPACITOR

Voltage Regulator Place/Component/Group:Power/VOLTAGE_REGULATOR/MAX15006AASA+

Table 2. Component Directory

Important Note: In lab 3, we are going to build the voltage regulator using LM1086-3.3 instead of

MAX15006A. The nominal output voltage of LM1086-3.3 is also 3.3V like that of MAX15006A. The

reason we usedMAX15006A in the simulation is because the SPICE model for LM1086 is not available

in Multisim. The basic theory still holds and the output voltage you measure will also be the same.

1.3. Questions

Q.1: In the first simulation, you measured the current across the LED with and without a resistor

in series. What did you observe? What would happen to a real LED if you connected its terminals

directly to the terminals of a 9V battery? Use the measurements you made as evidence to support

your answer and explain the purpose of the series resistor in the circuit.

Q.2: In the second simulation, you measured the voltage between the VDD and GND terminal.

What does VDD signify? What does GND signify? (Hint: you can find these on Wikipedia) Using

the measurement you made, describe the purpose of this circuit and explain what a Voltage

Regulator does.

Q.3: Imagine that you put a 100k ohm resistor and an LED in series between the VDD and GND

terminals of this circuit. If you replace the 100k resistor with a 1k resistor, what would you

observe of the LED?

Q.4: If you put a 100k resistor and an LED in series between the VDD and GND terminals of this

circuit, would it be brighter or dimmer than if you put a 100k resistor and an LED in series

between the positive and negative terminals of a 9V battery?

2. Lab Guidelines

Build the LED circuit shown in Figure 1 on your breadboard. Try out different resistors and

observe what happens to the light.

Build the voltage regulator circuit shown in Figure 3 on your breadboard. Detailed instructions

are available in the video on edge.edx.org. Remember that you will use LM1086 to build this

circuit instead of MAX15006A that was used in the simulation.

Going forward, it is a good practice to connect voltage sources LAST after you and your

partner have both independently checked the circuit that you have built. This will save you

the headache of burning out chips or controllers and having to order new ones.