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THERMISTOR CHARACTERISTICS TRAINER
Model No : (ITB - 06ACE)
User Manual
Version 1.0
Technical Clarification /Suggestion :
N/F
Technical Support Division,
Vi Microsystems Pvt. Ltd.,
Plot No :75, Electronics Estate,
Perungudi, Chennai - 600 096, INDIA.
Ph: 91- 44-2496 1842, 91-44-2496 1852
Mail : [email protected],
Web : www.vimicrosystem.com
THERMISTOR CHARACTERISTICS TRAINER
ITB - 06A CE
INTRODUCTION
Thermistor is a contraction of a term "thermal resistors". Thermistors are generally composed
of semi-conductor materials. Although positive temperature co-efficient of units (which exhibit
an increase in the value of resistance with increase in temperature) are available, most thermistors
have a negative coefficient of temperature resistance, i.e., their resistance decreases with increase
of temperature.
The negative temperature coefficient of resistance can be as large as several percent per degree
Celsius. This allows the thermistor circuit to detect very small change in temperature which could
not be observed with a RTD or a thermocouple. In some cases the resistance of thermistor at
room temperature may decrease as much as 5 percent for each 1°C rise in temperature. This high
sensitivity of temperature change makes thermistor extremely useful for precision temperature
measurements control and compensation.
Thermistors are widely used in applications which involve measurements in the range of -60°C
to 150°C. The resistance of thermistors ranges from 0.5 to 0.75M. Thermistor is a highly
sensitive device. The price to be paid for the high sensitivity is in terms of linearity. The
thermistor exhibits as highly non linear characteristic of resistance versus temperature.
TECHNICAL SPECIFICATION
Thermistor
 Type
-
NTC
 Probe Material
-
S.S
 Diameter
-
10mm
 Lead Pitch
-
5mm
 Bead colour
-
Blue
 Resistance at 25°C
-
5k
 Temperature Range
-
-80°C to 150 °C
 Tolerance (0 - 70°C)
-
±0.2°C
 Dissipation constants
-
1mw
 Time constants
-
10sec
Vi Microsystems Pvt. Ltd.,
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THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
ITB-06A CE UNIT
 Output Voltage
-
5 - 0 V DC
 Body Material
-
MS with Powder coating
 Size
-
370 × 280 × 90 mm
 Operating Temperature
-
10 - 55°C
 Voltage
-
230 V AC / 50Hz
 Power
-
1000 Watts
 Size
-
200 × 120 × 140 mm
 Size
-
50 × 20mm
 Type
-
Common anode
 Display
-
3.5 Digit
 Segment
-
7 Segment
 Colour
-
Green
 Input
-
230V AC / 50 Hz
 Output
-
+5V / 1A
- 5V / 500mA
+12V / 500mA
-12V / 500mA
Sterilizer
LED DISPLAY
POWER SUPPLY
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Vi Microsystems Pvt. Ltd.,
ON /OFF
POWER
T2
T1
SW1
T4
T3
R1
SW2
+VCC
+
-
INT
EXT
R3
R2
R4
R5
-
+
R6
R7
+
-
R8
T6
T5
Vi Microsystems Pvt.Ltd., Chennai-96
VOLTAGE
THERMISTER CHARACTERISTICS TRAINER ( ITB-06ACE )
THERMISTOR CHARACTERISTICS TRAINER
3. FRONT PANEL DIAGRAM
ITB-06ACE
[3]
THERMISTOR CHARACTERISTICS TRAINER
FRONT PANEL DESCRIPTION
ITB-06ACE
Power ON/OFF
:
Switch ON / OFF the unit.
T1 & T2
:
To insert the thermistor terminals.
SW1
:
To select resistance mode keep this switch towards left.
T3 & T4
:
To measure the resistance of thermistor.
SW2
:
To select either internal or external mode.
External Mode
-
For zero calibration place the switch towards
EXT.
Internal Mode
-
For connecting the thermistor output in circuit.
Keep the switch towards INT.
T5
:
Measure the signal conditioner output voltage.
Zero
:
Adjust this potentiometer to set 5 Volt at 0° C in EXT mode.
T6
:
The common DC GND of this unit.
Seven segment display:
For displaying the output voltage.
Vi Microsystems Pvt. Ltd.,
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THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
THEORY
There are four types of sensors based on the following physical properties, which are temperature
dependent:
1. Expansion of a substance with temperature, which produces a change in length, volume or
pressure. In it's simplest form this is the common mercury-in-glass or alcohol-in-glass
thermometer.
2. Changes in contact potential between dissimilar metals with temperature; thermocouple.
3. Changes in radiated energy with temperature; optical and radiation pyrometers.
4. Changes in electrical resistance with temperature, used in resistance thermometers and
thermistors.
The fourth property is used in our design to create a sensor. Resistance thermometry requires a
resistor properly mounted to create a sensor and a means of measuring the resistance of the
sensor.
Construction of Thermistors
Thermistors are composed of sintered mixture of metallic oxides such as manganese, nickel,
cobalt, copper, iron and uranium. They are available in variety of sizes and shapes. The
thermistors may be in the form of beads, rods and discs. Some of the commercial forms are
shown in Fig. 2
Fig - 2
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THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
A thermistor is in the form of a bead is smallest in size and the bead may have a diameter of 0.015
mm to 1.25 mm. Beads may be sealed in the tips of solid glass rods to form probes which may
be easier to mount than the beads. Glass probes have a diameter of about 2.5mm and a length
which varies from 6 mm to 50mm. Discs are made by pressing material under high pressure into
cylindrical flat shapes with diameters ranging from 2.5mm to 25mm.
CHARACTERISTICS OF THERMISTOR
i. Resistance ii. Voltage
iii. Current
Temperature Characteristics of Thermistors
- Current Characteristics of Thermistors
- Time Characteristics of Thermistors
i. Resistance-Temperature Characteristics of Thermistors
The mathematical expression for the relationship between the resistance of a thermistor and
absolute temperature of thermistor is:
where
and
RT1
RT2

= Resistance of the thermistor at absolute temperature T1 ; °k
= Resistance of the thermistor at absolute temperature T2 ; °k
= a constant depending upon the material of thermistor, typically 3500 to 4500
°k
The resistance temperature characteristics of a typical thermistor are given in Fig - 3. The
resistance temperature characteristics of Fig.2 show that a thermistor has a very high negative
temperature co-efficient of resistance, making it an ideal temperature transducer.
The resistance-temperature characteristics of platinum which is a commonly used material for
resistance thermometers. Let us compare the characteristics of the two materials. Between 100°C and 400°C, the thermistor changes its resistivity from 105 to 10-2 m, a factor of 107, while
platinum changes its resistivity by a factor of about 10 within the same temperature range. This
explains the high sensitivity of thermistors for measurement of temperature.
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THERMISTOR CHARACTERISTICS TRAINER
10
10
Resistivity m 
10
ITB-06ACE
6
4
Thermistor
2
10
10
10
10
10
-2
-4
-6
Platinum
-8
-100
0
100
200
300
400
500
Temperature C
Fig - 3
The characteristics of thermistors are no doubt non-linear but a linear approximation of the
resistance-temperature curve can be obtained over a small range of temperatures. Thus, for a
limited range of temperature, the resistance of a thermistor varies as given by Equation.
A thermistor exhibits a negative resistance temperature co-efficient which is typically about
0.05/°C. An individual thermistor curve can be closely approximated through the Steinhart-Hart
Equation:
where
T
R
A, B, C
= Temperature; °k,
= Resistance of thermistor ; 
= Curve fitting constants
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THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
A, B and C are found by selecting three data points on the published data curve and solving the
three simultaneous equations. When the data points are chosen to span no more than 100°C
within the nominal centre of thermistors temperature range, this equation approaches a remarkable
±0.2°C curve fit.
A simpler equation is:
where A, B and C are found by selecting three (R, T) data points and solving three resultant
simultaneous equations. Equation must be applied over a narrower temperature range in order
to approach the accuracy achieved by Steinhart-Hart Equation. Another, relationship that can be
conveniently used for resistance-temperature curve of thermistors is:
where RTR0 = resistance of thermistor at temperature T°k and ice point respectively.
ii. Voltage-Current Characteristics of Thermistors
These characteristics are shown in Fig - 4 which shows that the voltage drop across a thermistor
increases with increasing current until it reaches a peak value beyond which the voltage drop
decreases as the current increases. In this portion of the curve, the thermistor exhibits a negative
resistance characteristic. If a very small voltage is applied to the thermistor, the resulting small
current does not produce sufficient heat to raise the temperature of the thermistor above ambient.
Under this condition, Ohm's law is followed and the current is proportional to the applied voltage.
Larger currents, at larger applied voltages, produce enough heat to raise the thermistor
temperature above the ambient temperature and its resistance then decreases. As a result, more
current is then drawn and the resistance decreases further. The current continues to increase until
the heat dissipation of the thermistor equals the power supplied to it. Therefore, under any fixed
ambient conditions, the resistance of a thermistor is largely a function of the power being
dissipated within itself, provided that there is enough power available to raise its temperature
above ambient. Under such operating conditions, the temperature of the thermistor may rise
100°C or 200°C and its resistance may drop to one-thousandth of its value at low current.
Vi Microsystems Pvt. Ltd.,
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THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
Fig - 4
This characteristic of self-heat provides an entirely new field of users for the thermistor. In the
self-heat state, the thermistor is sensitive to anything that changes the rate at which heat is
conducted away from it. It can so be used to measure flow, pressure, liquid level, composition
of gases, etc. If, on the other hand, the rate of heat removal is fixed, then the thermistor is
sensitive to power input and can be used for voltage or power-level control.
iii. Current Time Characteristics:
The current - time characteristics shown in Fig - 5 indicate the time delay to reach maximum
current as a function of the applied voltage. When the heating effect just described occurs in a
thermistor network, a certain finite time is required for the thermistor to heat and the current to
build up to a maximum steady-state value. This time, although fixed for a given set of circuit
parameters, may easily be varied by changing the applied voltage or the series resistance of the
circuit. This time-current effect provides a simple and accurate means of achieving time delays
from milliseconds to many minutes.
Fig - 5
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THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
Applications of Thermistor
The other applications of thermistors include:
I)
ii)
iii)
iv)
v)
Measurement of power at high frequencies
Measurement of thermal conductivity
Measurement of level, flow and pressure of liquids
Measurement of composition of gases
Vacuum measurements.
SALIENT FEATURES OF THERMISTORS
1. Thermistors are compact, rugged and inexpensive
2. Thermistors when properly aged, have good stability
3. The response time of thermistors can vary from a fraction of a second to minutes, depending
on the size of the detecting mass and thermal capacity of the thermistor. It varies inversely
with the dissipation factor. The power dissipation factor varies with the degree of thermal
isolation of the thermistor element.
4. The upper operating limit of temperature for thermistors is dependent on physical changes in
the material or solder used in attaching the electrical connections and is usually 400°C or less.
The lower temperature limit of temperature is normally determined by the resistance reaching
such a high value that it cannot be measured by standard methods.
5. The measuring current should be maintained to as low a value as possible so that self heating
of thermistors is avoided otherwise errors are introduced on account of change of resistance
caused by self heating. Where it is not possible to avoid self heating, thermistor stability can
be maintained at given temperature by using an auxiliary heating element. The average power
dissipation can be effectively reduced and the highest sensitivity retained by energizing the
thermistor with pulses of measuring power.
6. Thermistors can be installed at a distance from their associated measuring circuits if elements
of high resistance are used such that the resistance of leads (even though the leads may be
very long) is negligible. This way the resistance of leads does not affect the readings and
hence errors on this count are negligible.
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THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
CIRCUIT DESCRIPTION
The thermistor which senses the temperature from the water bath as resistance This circuit
consists of three amplifier two gain amplifiers and one inverting amplifier. The thermistor is
connected at the feedback of the first gain-amplifier which gives constant voltage at initial stage.
During the time of heating the thermistor, the resistance of thermistor will be reduced. It
converts the resistance into milli volts. The output obtained from non inverting amplifier voltage
is given as input to the signal conditioner for further amplification where the output is tuned with
the range of -5 to -0 V using the trimpot TP1 Zero and TP2 gain. This output is applied to
inverting amplifier to convert the negative input into positive output of range (0-5) VDC. This
signal conditioner voltage can be displayed in the display (voltage).
SAFETY PRECAUTION
i.
Check the following two things before applying power to the heater (230V AC).
ii. Water level in the water bath should above the heating filament. Otherwise sterilizer will be
spoiled .
iii. Thermistor and thermometer should not touch the body of the sterilizer (Any temperature
source)
Vi Microsystems Pvt. Ltd.,
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THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
RESISTANCE /TEMPERATURE CHARACTERISTICS
RS Stock No.151 - 221
Vi Microsystems Pvt. Ltd.,
TEMP °C
RES
-80
3684000
-70
1558800
-60
702450
-50
335050
-40
168250
-30
88500
-20
48535
-10
27665
0
16325
+10
9950
+20
6245
+25
5000
+30
4028.5
+40
2663.3
+50
1801.5
+60
1244
+70
876
+80
627.5
+90
457.65
+100
339.15
+110
255.15
+120
194.65
+130
150.47
+140
117.64
+150
92.985
[ 12 ]
THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
EXPERIMENT - 1
AIM
To study the temperature - resistance characteristics of the thermistor.
APPARATUS REQUIRED
1. ITB-06A CE Unit.
2. Thermistor
3. PC Power Chord
4. Water bath
5. Thermometer
PROCEDURE
*
Interface the thermistor across T1 and T2 & switch ON the ITB-06A unit.
*
For resistance measurement ,SW1 should be in resistance mode.
*
Connect the multimeter (in resistance mode) across T3 & T4.
*
During zero calibration, SW2 should be in EXT mode.
*
The OFFSET POT is adjusted to 5V because thermistor is NTC. type
*
Before conducting the experiment, SW2 should be in INT mode.
*
Insert the thermometer and thermistor into the water bath.
*
Switch ON the water bath.
*
Note down the temperature in thermometer and corresponding resistance output of the
thermistor.
*
Plot the graph between temperature and resistance along X and Y axis respectively.
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THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
TABULATION
Temperature °C
Resistance ()
MODEL GRAPH
RESULT:
Thus the temperature - resistance characteristics thermistor was studied and the graph has been
plotted.
Vi Microsystems Pvt. Ltd.,
[ 14 ]
THERMISTOR CHARACTERISTICS TRAINER
ITB-06ACE
EXPERIMENT - 2
AIM
To study the temperature - voltage characteristics of the thermistor.
APPARATUS REQUIRED
1. ITB-06A CE Unit
2. Thermistor
3. PC Power chord
4. Waterbath
5. Thermometer
PROCEDURE
*
Interface the thermistor across T1 and T2 & switch ON the ITB-06A unit.
*
For resistance measurement SW1 should be in Thermistor mode.
*
Connect the multimeter (in DC -Volt mode) across T5 & T6.
*
During zero calibration, SW2 should be in EXT mode.
*
The OFFSET POT is adjusted to 5V because thermistor is NTC type
*
Before conducting the experiment, SW2 should be in INT mode.
*
Insert the thermometer and thermistor into the water bath.
*
Switch ON the water bath.
*
Now note down the temperature of the thermometer and corresponding voltage output.
*
Plot the graph between temperature and resistance along X and Y axis respectively.
Vi Microsystems Pvt. Ltd.,
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T
THERMISTOR CHARACTERISTICS TRAINER
TABULATION
Actual Temperature °C
MODEL GRAPH
RESULT
ITB-06ACE
Output Voltage (V)
Thus the temperature Vs Voltage characteristics of thermistor was studied and the graph has been
plotted.
NOTE
The type of thermistor sensor is NTC, so the output will be in reverse condition.
i.e. 0°C
- 5V
100°C - 0V
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