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*Chap 09 (28 plates) � Diagram Visual Information Ltd.
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Electronic Switching
Topic
The transistor
Introduction
Transistors are important components of electronic circuits because they use
relatively small input signals to control circuits carrying large currents.
Transistors are used as switches and amplifiers. The type of transistor used in
this experiment (see diagram 1 below) is made of three layers of semiconductor
material two layers of n-type, containing an excess of negative charge carriers
(electrons) on either side of a thin layer of p-type material containing an excess
of positive charge carriers. The transistor has three terminals (base, emitter, and
collector) one to each semiconductor layer. In the first part of this experiment,
you will examine current flows through a transistor
when no input signal is applied. You will do this by
testing the resistance between the terminals of the
transistor. In the second part of the experiment, you
will apply a small voltage to the base terminal of
the transistor and show how this affects current
flowing through the transistor. (You will do this by
adjusting a variable resistor connected between the
terminals of the transistor; this divides the supply
voltage between the terminals of the transistor.)
Structure of the transistor
Time required
40 minutes
Materials
Part A
2N3053 transistor
multimeter
suitable connection system (e.g., a breadboard see Experiment 9.01)
Part B
2N3053 transistor
10 ohm resistor (0.25 watts)
100 ohm resistor (0.25 watts)
560 ohm resistor (0.25 watts)
1 kilohm variable resistor (0.25 watts)
5 mm (0.2 in.) LED (any color)
1
base
collector
emitter
n
p
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9 volt battery and snap connector
multimeter
clip leads or suitable connection system
(e.g. a breadboard see Experiment 9.01)
If you are using a breadboard, you will also need three short lengths of
connecting wire (e.g., approximately no. 20 gauge, insulated, tinned copper wire
with about 0.5 cm of insulation stripped away at each end).
Electronic components are available from suppliers such as RadioShack
(http://www.radioshack.com). The appearance of the components may vary
among suppliers. Simple wiring diagrams and circuit diagrams are given in this
experiment to show the arrangement of the components. Diagram 2 below
shows the symbols used in the circuit diagrams in this experiment.
Symbols used in circuit diagrams
Procedure
Part A: Current flow with no voltage applied
Terminals of a transistor
1. Use diagram 3 above to help you identify the terminals of the transistor.
2. Insert the terminals of the transistor in the breadboard (make sure they are not
in holes that are connected beneath the surface).
3. Adjust the multimeter to read resistance.
4. Touch the red and black probes of the multimeter to the terminals of the
transistor in sequence, being careful not to touch the body of the transistor
with the probes. Most of the readings will be very high and may not register
3
transistor body
base (B)
collector (C)
tab
emitter (E)
Safety note
Do not use an electrical outlet.
2
resistor
variable resistor
transistor
LED � Diagram Visual Information Ltd.
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on the multimeter; some of the results will be lower and only register briefly.
Record your results in data table A below as either HIGH or LOW.
Part B: Current flow with voltage applied
Wiring diagram (A) and circuit diagram for Part B
1. Connect the equipment as shown in diagram 4 above with the adjustor in the
center of the variable resistor turned as far as possible in a counterclockwise
direction (i.e. R
2
is zero).
2 Adjust the multimeter to read voltage and carefully touch one probe to the
base wire of the transistor and the other probe to the emitter wire. Enter the
multimeter reading in data table B on the next page for the initial value
(i.e., transistor is OFF).
3. Turn the variable resistor in a clockwise direction (this increases R
2
) until the
LED just lights. Carefully touch one of the probes of the multimeter to the
base wire of the transistor and the other probe to the emitter wire. Enter the
multimeter reading in data table B for the baseemitter voltage when the
transistor is ON.
D
ATA TABLE
A
Red probe
Black probe
Result (HIGH or LOW)
E
C
E
B
C
E
C
B
B
E
B
C
0 V
+9 V
R
1
R
2
4A
4B
breadboard
connecting wire
battery
connecting wire
tab
base terminal
1 kilohm
variable resistor
1 kilohm
variable resistor
560 100 10 C
E
B
9 volt
10 ohm resistor
100 ohm resistor
560 ohm resistor
LED
transistor + � Diagram Visual Information Ltd.
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Analysis
Part A: Current flow with no voltage applied
1. Which connections produced a low resistance reading?
2. What do the resistance readings suggest about current flow through the
transistor?
Part B: Current flow with voltage applied
1. What was the value of the baseemitter voltage when the transistor was OFF?
2. What was the baseemitter voltage when the transistor was ON and the LED
was glowing?
Want to know more?
Click here to view our findings.
D
ATA TABLE
B
Multimeter reading (volts)
Baseemitter voltage when transistor is OFF
Baseemitter voltage measured when transistor is ON Part B: Operation of the reed relay
1. When the photocell was illuminated, the motor started working.
2. When the photocell was covered, the motor stopped working.
When the photocell was illuminated, its resistance dropped. Current flowed in
circuit 1, the coil of wire (electromagnet) in the reed relay attracted the reed
switch in circuit 2 (see the diagram below), current flowed in circuit 2, and the
motor started working.
When the photocell was covered, its resistance rose. The current stopped flowing
in circuit 1 and the coil of wire in the reed relay (electromagnet) no longer
attracted the reed switch in circuit 2. Therefore, the current stopped flowing in
circuit 2 and the motor stopped.
Even when illuminated, the photocell retains a high resistance and only allows a
small current to flow in circuit 1. However, the small current flowing in circuit 1
can operate the reed relay and thus control the much larger current needed to
operate the motor in circuit 2.
9.06 Electronic Switching
Part A: Current flow with no voltage applied
1. There is a low resistance reading in two cases:
(i) When the red probe touches the base terminal (B) and the black probe
touches the emitter terminal (E).
(ii) When the red probe touches the base terminal (B) and the black probe
touches the collector terminal (C).
2. A low resistance reading indicates that current will flow. A high value shows
that current will not flow. Current will flow through a transistor from the base
to the emitter or from the base to the collector, but not from the collector to
the emitter.
The collector (C) and emitter (E) are the connections to the n-type layers. The
base (B) is the connection to the p-type layer in the center. If a voltage is applied
as shown in the diagram on the left on the next page, no current flows from C to
E through the transistor. This is because the upper (pn) junction is reverse
biased (i.e., it behaves like a diode, which will not conduct in that direction).
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10.45 OUR FINDINGS
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glass capsule
reeds
circuit 2
circuit 1
Operation of the reed relay Voltage applied to upper pn junction
Voltage applied to lower pn junction
Part B: Current flow with voltage applied
1. The initial value of the baseemitter voltage is zero.
2 Our voltage reading was 0.60.7 volts. This is the usual potential difference
between the base and emitter required to allow the transistor to conduct.
If a voltage is applied between base and emitter to the lower pn junction of a
transistor (as in the diagram above right), electrons are able to flow from the
emitter to the base. As the base is very thin, the electrons pass through to the
collector. This allows the top junction to conduct, and current then passes
through the transistor. The small voltage applied to the base therefore allows a
larger current to flow through the transistor.
The voltage applied between the terminals of a transistor can be varied by
changing the values of resistors connected between its terminals. When designing
circuits using transistors, it is important to know what value of resistors to use to
supply the appropriate voltage to the transistor terminals. The values can be
calculated using Ohms Law:
Baseemitter voltage = Value of supply voltage
�
R
2
R
1
+ R
2
where R
1
is the value of the resistor connected between the base and the
collector, and R
2
is the value of resistor connected between the base and the
emitter.
To switch the transistor on, the baseemitter voltage should be 0.60.7 volts. If
R
1
is much bigger than R
2
, the baseemitter voltage is very small. If R
1
is much
less than R
2
, the base-