User Manual. Version 1.0 THERMISTOR CHARACTERISTICS TRAINER. Model No : (ITB - 06ACE)

Size: px
Start display at page:

Download "User Manual. Version 1.0 THERMISTOR CHARACTERISTICS TRAINER. Model No : (ITB - 06ACE)"

Transcription

1 THERMISTOR CHARACTERISTICS TRAINER Model No : (ITB - 06ACE) User Manual Version 1.0 Technical Clarification /Suggestion : / Technical Support Division, Vi Microsystems Pvt. Ltd., Plot No :75, Electronics Estate, Perungudi, Chennai , INDIA. Ph: , Mail : sales@vimicrosystems.com, Web :

2 THERMISTOR CHARACTERISTICS TRAINER INTRODUCTION ITB - 06A CE 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 C to 150 C Tolerance (0-70 C) - ±0.2 C Dissipation constants - 1mw Time constants - 10sec Vi Microsystems Pvt. Ltd., [ 1 ]

3 ITB-06A CE UNIT Output Voltage V DC Body Material - MS with Powder coating Size mm Operating Temperature C Sterilizer Voltage V AC / 50Hz Power Watts Size mm LED DISPLAY Size mm Type - Common anode Display Digit Segment - 7 Segment Colour - Green POWER SUPPLY Input - 230V AC / 50 Hz Output - +5V / 1A - 5V / 500mA +12V / 500mA -12V / 500mA Vi Microsystems Pvt. Ltd., [ 2 ]

4 3. FRONT PANEL DIAGRAM T1 T2 POWER ON/OFF SW1 EXT T3 SW2 INT THERMISTER CHARACTERISTICS TRAINER ( ITB-06ACE ) R8 - R2 + + T4 R1 T5 - R6 + - VOLTAGE +VCC R7 T6 R3 R4 R5 Vi Microsystems Pvt.Ltd., Chennai-96 Vi Microsystems Pvt. Ltd., [ 3 ]

5 FRONT PANEL DESCRIPTION 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., [ 4 ]

6 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 Vi Microsystems Pvt. Ltd., [ 5 ]

7 A thermistor is in the form of a bead is smallest in size and the bead may have a diameter of 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 - Temperature Characteristics of Thermistors ii. Voltage - Current Characteristics of Thermistors iii. Current - 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 R T1 = Resistance of the thermistor at absolute temperature T 1 ; k R T2 = Resistance of the thermistor at absolute temperature T 2 ; k and = 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 C and 400 C, the thermistor changes its resistivity from 10 5 to 10-2 m, a factor of 10 7, 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. Vi Microsystems Pvt. Ltd., [ 6 ]

8 Thermistor Resistivity m Platinum 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 = Temperature; k, R = Resistance of thermistor ; A, B, C = Curve fitting constants Vi Microsystems Pvt. Ltd., [ 7 ]

9 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 R T R 0 = 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., [ 8 ]

10 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 Vi Microsystems Pvt. Ltd., [ 9 ]

11 Applications of Thermistor The other applications of thermistors include: I) Measurement of power at high frequencies ii) Measurement of thermal conductivity iii) Measurement of level, flow and pressure of liquids iv) Measurement of composition of gases v) 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. Vi Microsystems Pvt. Ltd., [ 10 ]

12 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., [ 11 ]

13 RESISTANCE /TEMPERATURE CHARACTERISTICS RS Stock No TEMP C RES Vi Microsystems Pvt. Ltd., [ 12 ]

14 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 T 1 and T 2 & switch ON the ITB-06A unit. * For resistance measurement,sw1 should be in resistance mode. * Connect the multimeter (in resistance mode) across T 3 & T 4. * 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. Vi Microsystems Pvt. Ltd., [ 13 ]

15 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 ]

16 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 T 1 and T 2 & switch ON the ITB-06A unit. * For resistance measurement SW1 should be in Thermistor mode. * Connect the multimeter (in DC -Volt mode) across T 5 & T 6. * 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., [ 15 ]

17 ]T THERMISTOR CHARACTERISTICS TRAINER ITB-06ACE TABULATION Actual Temperature C Output Voltage (V) MODEL GRAPH RESULT 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 Vi Microsystems Pvt. Ltd., [ 16