Lab 7 – Switching Transistors and Optical DevicesByStudent.docx

Lab 7 – Switching Transistors and Optical Devices
By
Student’s Name
ECT246 Electronic Systems III with Lab
Professor’s Name
DeVry University Online
Date
Part A-Switching Devices
Week seven activities introduce the concept of electronic
switching. Transistors can be used to turn on and off devices
such as LEDs. The Silicon Controlled Rectifies (SCR) is
another semiconductor that can be used as a switch. Controlling
small dc motors is a common requirements and a special IC
called an H-bridge is used.
TCO #8:
Given a DC switching application such as driving a motor in

one and two directions use a SCR and a transistor and an H-
bridge driver to successfully perform the operation, calculate
and measure the operating voltages and currents.
A. Briefly describe how a transistor is used as an electronic
switch including conditions of cutoff and saturation.
a. Write one to two paragraphs describing how a BJT is used as
a switch.
b. Write one to two paragraphs describing how a JFET is used
as a switch.

B. Given a transistor switch, calculate the saturation voltages
and currents and simulate the circuit to verify proper
operations.
a.
Using the specification sheet, determine the maximum collector
current for a 2N3904. = ________
b.
Given the circuit, calculate if.
= __________

c.
Calculate = __________
d.
Determine the value of required to drive the transistor into
saturation if. = _________.
e. Download the ECT246_Week_7_BJTsw1.ms file from Doc
Sharing, week 7. If you have a different version of Multisim,
use your version to create the circuit below. Verify the
simulated results against the calculated values.

= _________
= _________
= _________ (diode current)
Notice the series LED and a resistor in parallel with.
Determine why the LED is not on. Use the Multisim
specifications for the LED (Left double click). Explain why the
LED will not turn on. Correct the problem and verify the
circuit’s operation.
f. Prototype the simulated circuit. Verify the operation of the
LED. Measure the following values
= _________

= _________
= _________
Compare them to the calculated and simulated values.
C. Describe an SCR’s structure and operation.
a. Draw the pn structure of a SCR.
b. Draw the schematic symbol and label each terminal.
c. Briefly describe the SCR’s operation.

D. Given the data sheet of a typical SCR such as a 2N5060,
determine the electrical characteristics and ratings and
determine its on-off conditions.
a. Use the specification sheet to determine the Gate, Cathode
and Anode terminals. Draw the diagram.
b. Record the following values:
= _________
= _________

= _________
= _________
= _________
= _________
E. Simulate a circuit using an SCR to control a motor or LED.
a.
The Multisim circuit below uses a 2N5060 SCR to control an
LED with, and. Use the specification sheet for additional
information and determine the value of needed to trigger the
SCR. Calculate the expected anode current.

= __________
= _________
b. Download the Multisim file ECT246_Week_7_SCR1.ms from
Doc Sharing, week 7. If you have a different version of
Multisim, create the circuit below. Start the simulation.
Closing SW1 should activate the SCR and the LED. The circuit
does not work properly. Beginning with the SCR circuit, use
the DMMs to troubleshoot the SCR and determine why it will
not trigger. Explain why the circuit is not working and what
changes are needed.

c. Make the changes. Use the DMMs to verify that the problem
has been corrected.
d. Now troubleshoot the LED circuit. Use Multisim to
determine the properties of the LED, double click on the diode.
= _________ LED Current
= __________
e. Use the DMMs to troubleshoot the LED circuit and determine
why it will not illuminate. Explain why the circuit is not
working and what changes are needed.
F. Calculate the gate and anode current, prototype and test the
circuit and measure the operating values of a SCR used to
control a motor or LED. Compare the results with the simulated
values.
a. Prototype the working circuit used in the simulation.
b. Verify that the circuit will turn the LED on and off.
c. Measure the following values:
= __________

= __________
= __________ LED Current
= _________
How do these values compare to the calculated and simulated
results.
G. Given a data sheet of an H driver integrated circuit such as a
L293D determine the operating parameters and use drawing
software such as Visio to create a schematic of a reversible
motor control circuit.
a. Use the specification sheet for an H-Bridge IC driver such as
the L293D or L293D and draw the package layout. Label the
pins and their functions. For each pin, specify its function.

b. Record the maximum ratings for
= _________
= _________
= __________
c. Draw a schematic of a bi-directional dc motor circuit
controlled by mechanical switches and explain how the circuit
allows bi-directional operations.

H. Prototype a reversible motor control circuit then measure and
record the operating parameters and troubleshoot any faults.
a. Given a dc motor determine the following.
Operating voltage = _________
Operating current = _________
Static dc winding resistance = _________
b. Prototype a motor control circuit using an H-bridge IC. The
schematic below contains the LS7404 and the L293D motor
driver. Will the L293D support the motor’s current
requirements? Yes/No.
c. Confirm the motors bi-directional operations
I. Discuss the need for back EMF protection when switching
motor circuits.
a. Draw the schematic of a transistor switch used to control a
small dc motor. Include a protection circuit for reverse voltage
spikes.

b. Explain the need for the protection circuit and how it works.
Part B-Optical Devices
TCO#9:
Given an application that requires electrical isolation choose an
appropriate optical isolation device to provide the necessary
isolation for that particular application.
A. Explain the operation of a 4N25 optical coupler.
a. Draw the internal layout for an optical coupler such as a
4N25. Label the pin numbers and functions.
b. Describe briefly the operation of an optical coupler.

B. Use the specification sheet for typical optical couplers such
as the 4N25 and determine suitable operating parameters.
a. Use the specification sheet to determine the following values
Diode forward voltage = __________
Diode forward current = __________ Max Value
Collector output current = __________
Isolation voltage = __________
Collector to emitter voltage = __________
C. Simulate a circuit that will provide the isolation between two
different supply voltages, such as a 5V TTL control circuit and
a 12V motor circuit.

a.
Using the circuit below, calculate the value of that allows the
switch (Sw) to operate the motor. , , , , , . Use I(LED) of 2mA
= __________

b. Download the Multisim files Opto1.ms from Doc Sharing,
week 7. Simulate the circuit. If you have a different version of
Multisim, use your version to create the circuit.
c. Record the following values from the simulation.
= __________
= __________
D. Prototype a circuit that will provide the isolation between
two different supply voltages, such as a 5V TTL control circuit
and a 6V motor circuit and compare the simulated values with
the measured ones.

a. Prototype the circuit above.
b. Record the following values.
= __________
= __________
c. Compare the measured values to the simulated and calculated
results.

Part C-Internet Device Searches
1. Use the Internet to locate information on the SN754410
Quadruple Half-H Driver. Download the specification sheet.
Specify its package layout. Write a brief description of the
device. Is it compatible with the L293D? Why or why not.

2. Use the Internet to locate information on the 4N33 optically
coupled isolator. Download the data sheet. Give a brief
description of the device. What is its package configuration?
How does it differ from a 4N25? Can it directly substitute for a
4N25? Why or why not.
2
23904
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(max)
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CW
CCW
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=
6
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=
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M
ImA
=

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