Download Lab3 Thermistor

EET 425/4 – Industrial Electronic Control
Laboratory Module
EXPERIMENT 3
THERMISTOR
1. OBJECTIVE:
1.1
1.2
1.3
To balance a bridge network
Determine which type of temperature coefficient a thermistor exhibits
To activate an over-temperature alarm using a Thermistor.
2. INTRODUCTION
Temperature sensing circuits are widely used in industry as system control elements, or to
detect unwanted operating conditions and activate an alarm. These circuits use sensors that
detect temperatures to which they are exposed. The thermistor is one type of sensor used for
temperature detection applications.
The thermistor is classified as a temperature sensing resistor. It is constructed of a nickel,
manganese, and cobalt oxide mixture formed into a semiconductor material in the shape of a
disk. Changes of temperature to which it is exposed cause the resistance of the thermistor to
change. Thermistor have a negative temperature coefficient, which means that an increase in
temperature cause a decrease in resistance.
Thermistor has an operating range from –230 degrees to 650 degrees centigrade. When used
in a bridge circuit with high amplification, it can detect temperature changes as low as 0.001
degrees centigrade.
In this laboratory exercise, the thermistor will be used in a bridge circuit. The output of the
bridge is applied to the input of an operational amplifier that produces a high gain to provide
increased sensitivity. The output of the amplifier is connected to the gate of an SCR.
The thermistor is placed in the bridge network to cause an imbalance that produces a
negative-going signal at the inverting input of the op amp when the temperature to which if is
exposed increases. The result is that the operational amplifier’s output produces a positive
voltage and causes the SCR to turn on. As current flows through the SCR, it turns on a lamp
that indicates a certain temperature has been reached. The SCR turns off when the reset
switch is opened. It will remain off only if the temperature being measured drops to a point
below the level that caused the SCR to turn on.
A circuit of this type can be used for an alarm circuit to indicate that temperature has risen to
an undesirable level.
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EET 425/4 – Industrial Electronic Control
Laboratory Module
3. COMPONENTS AND EQUIPMENTS :
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
3.15
Dual DC power supply – 1 unit
Voltmeter – 1 unit
Ammeter - 1 unit
Breadboard – 1 unit
Resistor 47Ω – 1 unit
Resistor 1kΩ – 1 unit
Resistor 2.7kΩ – 1 unit
Resistor 10 kΩ - 2 unit
Resistor 220kΩ - 1 unit
Potentiometer 100kΩ – 1 unit
Silicon Control Rectifier(SCR) – 1 unit
Thermistor – 1 unit
741 Op- Amp – 1 unit
SPST switch – 1 unit
Wires
4. PROCEDURE:
Step 1
Measure the resistance of the thermistor. Avoid touching the glass part of the thermistor
when attaching the test leads. With the ohmmeter connected, wait a short time for the
resistance to stabilize. Record the stabilized resistance:
__________ ohms
While observing the ohmmeter, grasp the glass bead between your index finger and
thumb. The resistance of the thermistor __________ as the temperature to which it is
exposed increases.
A. increases
B. decreases
This observation shows that the thermistor has a ___________ temperature
coefficient.
A. positive
B. negative
Step 2
Using an ohmmeter, set the decade resistance box to the thermistor resistance value
recorded in Step 1.
Step 3
Assemble the circuit in Figure 18-1. Adjust the dual voltage DC power supply for –9 volts
and +9 volts. Place the reset switch in the open position.
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EET 425/4 – Industrial Electronic Control
Laboratory Module
Step 4
Turn on the dual voltage power supply to energize the circuit. Balance the bridge by
adjusting the decade resistance box to produce a zero indication on the voltmeter connected
to the output of the op amp.
Note: A positive voltage produced by the op amp is required to fire the SCR. To
insure that the SCR does not fire at ambient temperature, set the decade
resistance box so that a slight negative voltage is produced at lead 6 of the
op amp.
Step 5
To test the sensitivity of the circuit, place your finger on the glass bead of the thermistor
while observing the output of the op amp with a voltmeter. If the circuit is operating properly,
the voltmeter should indicate a positive potential when heat is applied by your finger, and a
negative potential when the thermistor is re-exposed to ambient temperature.
Step 6
Observe the voltmeter while trying other forms of heat, such as blowing on it or placing other
heat sources near the thermistor’s body. Avoid temperatures over 150 degrees centigrade.
Note: The op amp produces a positive potential when the voltage at its noninverting input is greater than the voltage applied to its inverting input lead.
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EET 425/4 – Industrial Electronic Control
Laboratory Module
Step 7
Observe and record the voltages at the inputs and output of the op amp before and after
heat is applied to the thermistor.
Before
Inverting input
Non-inverting input
Output
_________ volts
_________ volts
_________ volts
After
Inverting input
Non-inverting input
Output
_________ volts
_________ volts
_________ volts
Step 8
Close the reset switch. Place your finger near the thermistor while a voltmeter is connected
to the gate of the SCR. Observe the voltage as the SCR is fired into conduction. After the
SCR is turned on, remove your finger and wait several seconds.
Observe the voltage at the gate of the SCR. Does it remain at a positive voltage?
__________ (yes, no)
Does the SCR turn off?
__________ (yes, no)
Once the SCR is on, it continues to conduct even when the voltage at its gate is no longer a
positive potential. The only way it can turn off is to reduce the SCR’s holding current.
To prove this, try disconnecting the connection to the gate of the SCR labeled X in the
circuit. Observe what happen to the current? Does the current remain the same(the SCR still
turned ON)?Why?
Step 9
Turn the SCR off by opening the reset switch and reducing its holding current to zero.
Step 10
Disassemble the circuit and return the parts and equipment to their proper storage locations.
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EET 425/4 – Industrial Electronic Control
Laboratory Module
Name
:
______________________________
Matrix No
:
______________________________
Date : ______________
5. RESULTS:
Step 1
__________ ohms
While observing the ohmmeter, grasp the glass bead between your index finger and
thumb. The resistance of the thermistor __________ as the temperature to which it is
exposed increases.
A. increases
B. decreases
This observation shows that the thermistor has a ___________ temperature
coefficient.
A. positive
B. negative
Step 7
Before
Inverting input
Non-inverting input
Output
_________ volts
_________ volts
_________ volts
After
Inverting input
Non-inverting input
Output
_________ volts
_________ volts
_________ volts
Step 8
Observe the voltage at the gate of the SCR. Does it remain at a positive voltage?
__________ (yes, no)
Does the SCR turn off?
__________ (yes, no)
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EET 425/4 – Industrial Electronic Control
Laboratory Module
Name
:
______________________________
Matrix No
:
______________________________
Date : ______________
What happen to the current when the connection to the gate of the SCR labeled X in the
circuit is disconnected? Does the current remain the same(the SCR still turned ON)?
_________________________________________________________________________
Why?
_________________________________________________________________________
_________________________________________________________________________
QUESTIONS:
EXPERIMENT QUESTIONS:
1. A thermistor has a __________ temperature coefficient.
A. positive
B. negative
2. When the temperature to which the thermistor in this bridge circuit rises, the voltage
at the non-inverting input of the op-amp _________, and the voltage at the inverting
input _________.
A. increases
B. decreases
C. stays the same
3. The op amp output goes positive when the voltage applied to its _________ input is
greater than its _________ input.
A. inverting
B. non-inverting
4. The SCR will remain on after the positive gate voltage is removed because it has a
_____________ current.
5. If the resistance of the thermistor is __________ than the resistance of the decade
box, the voltage at the SCR’s gate will be negative.
A. less
B. greater
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