ECE 248
Lab 6
Spring 2016
Lab 6: BJT Switches (May 17/18)
GOAL
The overall goal of this lab is to better understand how to use BJT switches.
OBJECTIVES
To build, test, simulate, and understand the following circuits:
1) Relay controller
2) DC motor driver using pulse width modulation
GENERAL GUIDELINES
You should know the guidelines by this point in the course …
PARTS AND MATERIALS

Lab kit (breadboard, wire stripper, wire)

Digital oscilloscope, scope probes, function generator, coaxial cable (with alligator clips), benchtop power supply

Miniature relay

DC motor

Npn transistor:
2N3904
(1)
2N2222A
(1)

Resistors:
560 ohm (green/blue/brown)
(2)

Diode
1N4002
(2)
1 ECE 248
Lab 6
Spring 2016
PART 1: RELAY CONTROL
In this section you will see how a BJT switch is used to control an electromechanical relay for a DC fan. The fan requires
700 mA or more of current. This is quite a bit of current, but it is easily handled by a relay. Nice! Meanwhile, the relay coil
only needs about 80 mA, which is no problem for a small-signal transistor like the 2N3904.
Fig. 1: Relay control circuit using a BJT switch. The relay coil is powered by +5V, while the fan is powered by +12V.

Step 1a: Build the relay circuit in Fig. 1.
o
Spread out your circuit, since you’ll need to measure base and collector currents.
o
Wire the benchtop power supply to provide TWO outputs:
o
o

+5V is for the relay coil. Set the max current of the power supply to 0.2A.

+12V is for the fan. Set the max current to 3A.
The colored terminals on your breadboard should be the following:

RED terminal = +12V.

YELLOW terminal = +5V.

GND terminal = GND.
Insert the relay across the breadboard “gap” (i.e. like an op amp). The relay connections are the following:

Pin 1 = empty
Pin 12 = FAN+

Pin 5 = Transistor collector
Pin 8 = +5V

Pin 6 = +12V
Pin 7 = GND
2 ECE 248

Lab 6
o
The 2N3904 pin diagram is on the course website.
o
Remember the “gray band” of the 1N4002 diode is the cathode.
Step 1b: Test the relay circuit.
o
o
The control signal is the Agilent function generator with the following settings:

HighZ output

Waveform = SQUARE WAVE,
Frequency = 0.1Hz

Amplitude: Low Level = 0V,
High Level = 5V
Your circuit works properly when the relay is clicking and the fan turns on and off.


Spring 2016
The fan is quite powerful … and loud.
Step 1c: DC measurements when the fan is ON.
o
Make the necessary multimeter measurements to complete VCE, IC, and IB in Table 1.

You should get VCE ≈ 0.15V, IC ≈ 60 mA, and IB ≈ 8 mA.

Is the BJT switch in hard saturation?

o
Remember that hard saturation requires IC/IB ≈ 10.
The benchtop power supply displays both the voltage AND current output. What is the fan current?

You should get IFAN ≈ 0.7 A.
Table 1: DC measurements when the fan is on. VCE
IC
IB
DO NOT DISASSEMBLE THIS CIRCUIT!
(see next page for Part 2)
3 IFAN
ECE 248
Lab 6
Spring 2016
PART 2: PWM CONTROL OF MOTOR SPEED
Pulse width modulation (PWM) is a very common technique to control the
speed of a motor. The basic idea is to power the motor with a series of
PULSES rather than a DC voltage.
Low Duty
Cycle
Med Duty
How does this affect speed? The duty cycle (% of on-time) affects the
Cycle
average voltage seen by the motor. For example, a DC voltage of +12V is
basically a 100% duty cycle waveform. Therefore, the motor sees an average
of +12 V (full speed). A 0-to-12 V square wave with a 50% duty cycle results
High Duty
in an average voltage of only +6V (about half speed). The pulse frequency
Cycle
must be fairly high (> kHz) to ensure the motor does not respond to
individual pulses – remember that it is the AVERAGE value that we want! Fig. 2: Schematic of voltage pulses with different duty cycles. 


Step 2a: Build the motor control circuit shown in Fig. 3.
+12V
o
Notice the 1N4002 flyback diode in parallel with the motor.
o
The PWM signal comes from the Agilent function generator.
o
See the course website for the 2N2222A pin diagram (data sheet).
MOTOR
1N4002G
Step 2b: Configure the Agilent function generator.
o
Choose the “Pulse” waveform with a 25 kHz frequency.
o
Make sure the LOW amplitude= 0V and HIGH amplitude = 5V.
o
Use alligator clips to attach the function generator output to the
“PWM” input of the circuit.
Step 2c: Test the circuit.
From
Function
Generator
560Ω
Q1
2N2222A
Fig. 3: PWM‐driven transistor switch to control the speed of a DC motor.
o
Your circuit works if the motor speed can be controlled by the
PWM duty cycle (vary from 10 to 90%).
o
Use the multimeter to measure the DC motor voltage when the PWM duty cycle is 20%, 40%, 60%, and 80%.
Complete Table 2.
Table 2: DC measurements of motor voltage at various PWM duty cycles. 20%
o
40%
60%
When the Agilent duty cycle is 80%, adjust the scope to display a few cycles for each trace.

Record scope traces of the PWM input (from function generator) and the transistor’s collector voltage.
4 80%
ECE 248
Lab 6
PART 3: CIRCUIT DEMOS
As always, Buma will reset the scope and function generator.
1) Circuit #1: Relay
o
Show your completed Table 1.
o
Set the Agilent function generator to the following settings:

HighZ output

Waveform = SQUARE WAVE, Frequency = 0.1Hz

Amplitude: Low Level = 0V,
High Level = 5V
o
The relay should be clicking, the motor should be turning on and off.
o
Use the multimeter to show VCE.
2) Circuit #2: PWM driver for DC motor.
o
Show your completed Table 2 and recorded scope snapshots.
o
Set the function generator to:
o

Waveform = PULSE

Low Level = 0V,
High Level = 5V

Frequency = 25 kHz,
Duty cycle = 80%
Adjust the scope to show a few cycles of the transistor’s collector voltage.

NOTE: BUMA FORBIDS ANY STUDENT FROM USING THE AUTOSCALE BUTTON.
Buma will deduct 4000 pts from your lab grade if he catches you using Autoscale!
o
Adjusting the PWM duty cycle should vary the motor speed.
(End of Lab 6)
5 Spring 2016