EE40, Spring 2015, Pre-Lab 7 Transistors & Motors

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 7

For prelab you will simulate various amplifier circuits using Multisim. You will also build these circuits in

lab 7. Below are the tasks you should complete for lab 7.

Prelab assignment (to be completed before your lab session)

o Simulations: S.1, S.2, S.3, S.4, S.5

o Questions: Q.1, Q.2, Q.3, Q.4, Q5

Lab assignment (to be completed during your lab session)

o Build two motor driver circuits

o Demonstrates robot bouncing

o Write MSP430 code to drive the motors

Outline

1. Prelab Assignment

2. Lab guidelines

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. BJT Transistors

The Bipolar Transistor basic construction consists of two PN-junctions producing three

connecting terminals with each terminal being given a name to identify it from the other two.

These three terminals are known and labelled as the Emitter ( E ), the Base ( B ) and the

Collector ( C ) respectively. Bipolar Transistors are current regulating devices that control the

amount of current flowing through them in proportion to the amount of biasing current applied

to their base terminal ( B ), acting like a current-controlled switch. The principle of operation of

the two transistor types PNP and NPN, is exactly the same, with the only difference being in

their biasing and the polarity of the power supply for each type. The following figure shows a

NPN BJT transistor circuit symbol and its simplified cross section.

Please refer to Edge-edx module 6.3 to learn more on how they work.

Please build the following circuit that includes 3 NPN BJT transistors. The NPN transistors used

in this simulation is to represent the TO-92 packaged NPN transistors in your kit and can be

found in Multisim directory: Place/Component/Group: Transistors/BJT_NPN/BC547BG

Now were going to do a DC sweep simulation on this circuit. In our case, were interested in

measuring the collector current (IC) and base current (IB) of each transistor while sweeping the

collector voltage VCE. To run a DC sweep, you can go to Simulate -> Analyses -> DC Sweep. Fill

out the information as is applicable to your circuit:

S.1 Please sweep VCE from 0V to 9V with a linear step of 0.1V and include the resulting base emitter

voltage (VBE) of all 3 transistors in the same plot in your prelab write-up. You should plot VBE on

the Y axis and the VCE value on the X axis.

S.2 Please sweep VCE from 0V to 9V with a linear step of 0.1V and include the resulting collector

current (IC) of all 3 transistors in the same plot in your prelab write-up. You should plot IC on the

Y axis and the VCE value on the X axis.

S.3 Please build the following different circuit with the same NPN BJT transistors. This time sweep

Vin from 0V to 3.3V with a linear step of 0.1V and include the resulting collector current ( IC) of all

3 transistors in the same plot in your prelab write-up. You should plot IC on the Y axis and the Vin

value on the X axis.

1.2. Motor Driver Circuit

A motor will often need a higher voltage as well as higher current than can be supplied directly

from a microcomputer chip so an external power supply is normally used to provide this. The

simplest way of driving a motor is directly through a transistor, as shown below:

An output pin from the MSP430 Microcomputer chip is connected through a 1k resistor to the

base of a transistor. The motor is in parallel connection with a diode and sits between the

collector and the positive 9V external power. A motor is basically an electromagnet or coil; in

electronic terms this is an inductor with very small resistance value. So a very simplified motor

model can be represented by an inductor in series with a resistor, as shown in the following

simplified motor driver circuit with no diode (This simplified model may not be accurate, but at

least it can help students intuitively understand the motor driver circuit):

S.4 Please build the above simplified motor driver circuit without the diode. Use a pulse voltage

between 0V and 3.3V for Vin with 1s rise & fall time, 1ms period and 0.5ms pulse width. Run a

transient analysis (between 0s and 2ms) on the transistor collector voltage Vc, the base current

(IB) and the collector current (Ic).

The pulse voltage source used in this simulation can be found in Multisim directory:

Place/Component/Group:Sources/SIGNAL_VOLTAGE_SOURCES/PULSE_VOLTAGE.

S.5 Now lets add the snubber diode into this motor driver circuit (as shown in the following circuit).

Again use a pulse voltage between 0V and 3.3V for Vin with 1s rise & fall time, 1ms period and

0.5ms pulse width. Please do a transient analysis (between 0s and 2ms) on the transistor

collector voltage Vc and the collector current (Ic) again. Compare the difference with the

previous simplified motor driver circuit with no diode.

1.3. Questions

In this part you will answer a few questions about the simulation that you did in the previous

part, hence, it is important for you to finish all the simulations before starting with the

questions.

Q.1: In simulation (S.1 & S.2), what is the approximate value of VBE and IC of each transistor at

VCE = 9V? For transistor Q2 and Q3, is the VBE value proportional to the collector current IC?

Q.2: In simulation (S.2), explain why a 1k resistor is needed in series with the BJT transistors

base terminal.

Q.3: According to simulation (S.3), what is the minimum Vin for BJT transistors to be out of the

cut-off region (i.e., the device is turned on and VBE is roughly constant)?

Q.4: Why do we need a motor driver circuit to drive a motor? What if we directly drive the

motor with the output pin of MSP430 microcomputer chip?

Q.5: In simulation (S.4), what is the current ratio IC/IB when the BJT transistor is turned on?

Compare simulation (S.4) and (S.5), why do we need a diode in shunt with the motor in the

motor driver circuit? What could be the issues for motor driver circuit with no diode?

2. Lab Guidelines

In this lab, you will complete the very last circuit for your robot the motor driver! To achieve

MSP430-driven control of a DC motor, you will need to build an electronic switching circuit as

the following. You have two bipolar junction transistors (BJTs) in your kit that can be used as

electronically-controlled switches.

You will attach a steel weight (a large nut) off-axis to each of the DC motors. This adds

eccentricity to the motors rotation, causing its rotation to be more elliptical than circular. This

motion will result in the desired robot bounce. Eccentric weight motors are a common way to

introduce vibration to a device (like in game controllers or cell phone ringers).

Lab Deliverables

1. Build two motor driver circuits and demonstrate they switch the motors.

2. Mount eccentric weight motors to robot frame and demonstrate robot bouncing.

3. Program MSP430 to drive the motors.