Signal Conditioning of Thermistor

Introduction Signal condition of thermal resistor Signal conditioning way manipulating an analogue show in such(prenominal) a way that it meets the requirements of the succeeding(a) stage for further processing. Ope proportionalitynal amplifiers(op-amps) atomic number 18 grossly employed to widelyn out the amplification of the signboard in the signal conditioning stage. The signal conditioning equipment may be essential to do linear processes give c ar amplification, attenuation, integ proportionalityn, differentiation, addition and subtraction.They are in addition require to do non-linear processes like modulation, demodulation, sampling, filtering, carving and clamping squaring, linealizing or multiplification by an separate prevail etc. the signal conditioning or data acquisition equipment in galore(postnominal) a inter fire site be an excitation and amplification system for unresisting transducer. It may be an amplification system for active transducer. In both the applications, the transducer issue is brought up to a sufficient direct to make it useful for conversion, processing, indicating and recording.Excitation is needed for passive transducers because these transducers do not generate their own voltage or current. accordingly passive transducers like crinkle gauges, potentiometers, immunity thermometers, inductive and capacitive transducers required excitation from external sources. The active transducers like techno generators, thermocouples, inductive pick ups and piezo-electric crystals. The thermal resistor constitute one girdle or more than one arm of a wheatstone noseband which is excited by an isolated DC source. The duet lavatory be balanced by a potentiometer and fanny also be calibrated for derangementd conditions.Thermistor is a concent proportionalityn of the marge caloric Resistor. It is essentially a semiconductor, which be take a shits as a impedance with a racy damaging temperature coefficient of imped ance. That is, as the temperature of the thermistor accessions, its subway system decreases. The temperature co-efficient is verbalized in ohms per unit diverseness in degree Celsius ( C). thermistors with mettlesome temperature co-efficient of resistance are more natural to temperature change and are therefore swell up suited to temperature eyeshadement and verify.CONTENTS OBJECTIVE EXPLAINATION 1. WHEATSTONE BRIDGE 2. THERMISTOR 3. OPERATIONAL AMPLIFIER 4. instrumentality AMPLIFIER methodological analysis SCOPE 1. Wheatstone bridge Whetstone bridge is the most immaculate rule available for measuring resistances and is popular for laboratory use. The circuit diagram of representative Wheatstone bridge is given in figure Rx is the un cognize resistance to be measured R, R2 and R are resistors of known resistance and the resistance of R2 is adjustable.If the ratio of the twain resistances in the known leg (R2 / R) is equal to the ratio of the ii in the unknown leg (Rx / R3), then the voltage surrounded by the two midpoints (B and D) will be zero and no current will fly the coop finished the galvanometer Vg. R2 is varied until this condition is reached. The direction of the current indicates whether R2 is too noble or too low. Detecting zero current can be make to extremely high accuracy (see galvanometer). Therefore, if R, R2 and R3 are known to high precision, then Rx can be measured to high precision.Very small changes in Rx disrupt the balance and are readily detected. At the point of balance, the ratio of R2 / R = Rx / R3 Therefore, Alternatively, if R, R2, and R3 are known, but R2 is not adjustable, the voltage difference crossways or current flow through the meter can be utilise to calculate the value of Rx, using Kirchoff s Circuit laws (also known as Kirchhoff s rules). This setup is frequently utilise in strain gauge and resistance thermometer measurements, as it is usually faster to read a voltage level off a meter than to adju st a resistance to zero the voltage.In practical Wheatstone bridge, at least one of the resistance is made adjustable, to permit balancing. When the bridge is balanced, the unknown resistance (normally connected at Rx) may be primed(p) from the setting of the adjustable resistor, which is called a measure resistor because it is a precision guile having very small tolerance. Rx= (R2/R1). R3 screening OF WHEATSTONE BRIDGE A Wheatstone bridge may be used to measure the DC resistance of various personas of wire, either for the purpose of quality hold in of the wire it self or of some assembly in which it is used.For example, the resistance of beat back winding, transformers, solenoids, relay coils and resistance of thermistor, RTD also can be measured. PRINCIPLE shelter of thermistor changes with change in temperature. subway system of thermistor decreases with increase in temperature while resistance of thermistor increases with decrease in temperature. It is a thermally sensit ive resistor. FORMULA- Rti = Rt2Exp p (1 / T1 1 / T2) Where,RT= resistance of thermistor at unfathomable temp. Tlk RT2= resistance of thermistor at absolute temp. T2k P= a constant depending upon the material of the thermistor (Typically amid 3 vitamin Dk to 4500k)CONSTRUCTION- Thermistors are composed of sintered mixture of metallic oxides such as Manganese, Nickel, Cobalt, Copper, Iron and Uranium. They are available in a variation of sizes and shapes. They may be in the form of beads, roads and discs. WORKING- A thermistor change in electrical resistance due to a corresponding temperature change is evident whether the thermistors body temperature is changes as result of conduction or radiation from the surrounding environment or due to self heating brought intimately by power dissipation within the device. conjecture- .Thermistor is a concentration of the term Thermal Resistor. It is essentially a semiconductor which be move overs as a resistor with a high negative temp erature coefficient of resistance. That is, as the temperature of the thermistor increases, its resistance decreases. The temperature co-efficient is expressed in ohms per unit change in degree celcius ( C). thermistors with high temperature co-efficient of resistance are more sensitive to temperature change and are therefore well suited to temperature measurement and control. Thermistors are available in a wide variety of shapes and sizes.However, thermistor beads sealed in the tips of glass rods are most commonly used because they are relatively easy to mount TYPES OF THERMISTOR- 1PTC 2NTC NTC (Negative Temperature Coefficient) It implies that the resistance of thermistor decreases with increases in its temperature. These thermistors can detect changes in temperature, which could not be sight with RTDs or Thermocouple circuits. NTC type thermistors mostly used in industry. Resistance of thermistor is used in industry is 2. 2 k Q. for temperature 30C. Its cost near about Rs. 10-15 SPECIFICATION- *Range(? ) 50-300 *Accuracy 1 * blockage 0. 01 ? C *Scale Non-Linear *Thermister Constant 4000? K OPERATIONAL AMPLIFIRES One type of amplifier, which finds its way into almost all points of measurement and orchestration system, is the operative amplifier. The book of account operational indicates that the amplifier can perform mathematical operations like anastrophe addition, subtraction, multiplication, division, integration and differentiation etc. Properties of ideal operational amplifier are 1. It should withstand an infinite gossip impedance. 2. It should have zero outfit impedance. 3.It should have an infinite agnize ( stool of the order of 105 to 109) 4. It should have flat response oer a wide frequency station. The operational amplifier consists of a very high fix amplifier with a negative > feedback. The gain of operational amplifier with negative feedback is determined by feedback components and not by the internal amplifier circuitry. APPLICAT IONS OF OPERATIONAL AMPLIFIER Some of the all-important(a) applications of an op-amp are 1. Amplifiers 2. mobile filters 3. Arithmetic circuits 4. Log and antilog amplifiers 5. Voltage comparators 6. wave shape generators 7.Precision rectifiers 8. Multipliers 9. Timers 10. Multivibrators 11. Regulated power supplies Operational amplifier characteristics 1. stimulant starting signal voltage The scuttlebutt offset voltage is defined as the voltage that must be use to the excitant terminals to drive the rig to zero. This is about 2mV for 741 amplifier. It should be visit thet the offset voltage changes with temperature. 2. Input offset current just as a voltage offset may be required crosswise the input to make the output voltage zero, so a net current may be required between the inputs to zero the output voltage.This current is called input offset current. This is equal to the difference between the two input currents. 3. Input preconception current It is defined as the p issed of the two input currents required to make the output voltage zero. 4. err rate it is the highest rate at which the output can change, it is expressed in terms of v/jiS. 5. Unity gain frequency in many cases, specifications include the frequency response including unity gain frequency. This is the frequency at which the open loop gain of the amplifier becomes unity. The low frequency gain is about 20,000 and falls to unity at about 1MHz. he amplifier is said to have a 1 MHz gain bandwidth produt. 6. Common dash rejection ratio (CMMR) it is defined as the ratio of derived function gain to common manner gain. CMRR is infinite for ideal op-amp. Thus the output voltage corresponding to the common mode noise is zero. IDEAL CHARACTERISTICS OF OP-AMP 1. Gain is level best. 2. Input impedence should be infinite. 3. product impedence should be zero. 4. CMRR should be infinite. 5. Bandwidth should be infinite. TYPICAL VALUES OF OP- AMP 1. Input offset voltage 6mV. 2. Input offset current 200nAmp 3. Input bias current 7nAmp 4.Differential input resistance/impedence 2m CI 5. Input impedence for 741 (FET op-amp) 1000 G Q 6. Output impedence 75 Q. 7. Gain 2 lakhs 8. Output voltage rock 26Vpp 9. Supply current 2. 8 mAmp APPLICATIONS- 1. It is used in lab and medical purpose. 2. PTC type thermistors are used to protect the motor from overheating. 3. It gives very accurate reading at high temperature. 4. For measurement of level pressure, flow of the liquid, account of gases, thermal conductivity and vaccum measurement. UA741 General-purpose mavin operational amplifier UA741 General-purpose single operational amplifierFeatures * Large input voltage range * No latch-up * lofty gain * Short-circuit protection * No frequency earnings required * Same pin configuration as the UA709 Applications * Summing amplifiers * Voltage followers * Integrators * Active filters * Function generators Description The UA741 is a high consummation monolithic operational amplifier constructed on a single silicon chip. It is intended for a wide range of analog applications. N DIP8 (plastic package) D SO-8 (plasticmicropackage) Pin connections (top view) 2 Inverting input 3 Non-inverting input 4 VCC- 5 Offset null 2 6 Output 7 -Vcc+ N. C. The high gain and wide range of operating voltages provide superior performances in integrator, summing amplifier and general feedback applications. The internal compensation network (6 dB/octave) ensures stability in closed- loop circuits. 1/11 www. st. com schematic drawing diagram 1 Schematic diagram UA741 Figure 1. Schematic diagram Absolute maximum ratings and operating conditions 2 Absolute maximum ratings and operating conditions Table 1. Absolute maximum ratings Symbol Parameter treasure Unit Vcc Supply voltage 22 V Vid Differential input voltage 30 V Vi Input voltage 15 V Output short-circuit duration Infinite Rthja Thermal resistance junction to ambient SO-8 DIP8 125 85 C/W Rthjc Thermal resistance junction to case SO-8 DIP8 40 41 C/W ESD HBM human body model(1) DIP package SO package 500 400 V MM machine model(2) 100 V CDM super supercharged device model(3) 1. 5 kV Tstg Storage temperature range -65 to +150 C 1. Human body model a 100 pF capacitor is charged to the specified voltage, then discharged through a 1. 5kfl resistor between two pins of the device. This is done for all couples of connected pin combinations while the other pins are floating. . Machine model a 200 pF capacitor is charged to the specified voltage, then discharged directly between two pins of the device with no external series resistor (internal resistor < 5 n). This is done for all couples of connected pin combinations while the other pins are floating. UA741 3. charged device model all pins and the package are charged unneurotic to the specified voltage and then discharged directly to the ground through only one pin. This is done for all pins. 4. 4. INSTRUMENTATION AMPLIFIER The low level signal output of electrical transducers often need to be mplified out front further processing. This is done by the use of instrumentation amplifier. The important features of instrumentation amplifier are as follows. 1. Selectable gain with high gain accuracy and gain linearity. 2. Differential input capability with high gain common mode rejection. 3. High stability of gain with low temperature co-efficient. 4. low DC offset and ramble errors referred to input. 5. low output impedance. The input amplifiers A and A2 act as input buffers with unity gain for common mode signals ecm and with a gain of (1+2R2/Ri) for differential signals.A high input impedance is ensured by the non-inverting configuration in which they operate. The common mode (cm) rejection is achieved by the following stage which is connected as a differential amplifier. The optimum common mode rejection can be obtained by adjusting R6 or R7 ensuring that Ei Ei R4 R6 The amplifier A3 can also be made to have some nominal gain f or the whole amplifier by an appropriate plectrum or R4, R5, R6 and R7. The drift errors of the encourage stage add to the product of the drift errors of the kickoff amplifier and first stage gain.Hence, it is necessary that the gain in the first stage be enough to prevent the overall drift performance from being significantly affected by the drift in the second stage. The drift problem of instrumentation amplifier can be improved if amplifiers Ai and A2 have offset voltages, which tends to track the temperature. The gain of an instrumentation amplifier can be varied by changing R alone. A high gain accuracy can be obtained by using precision metal film resistors for all the resistance. Figure shows a simplified differential instrumentation amplifier using a transducer bridge.A resistive transducer, thermistor, whose resistance changes as a function of some physical quantity such as temperature is connected in one arm of the bridge and is denoted by (Rr A R), where RT is the res istance of the thermistor and delta R is the change in the Generally, resistors Ra, Rb and Rc are selected so that they are equal in value to the transducer resistance RT at some summon book book condition. The bridge is balanced initially at a desired reference condition. However, as the temperature changes, the resistance of the thermistor also changes causing the bridge to unbalance (Va 4- Vb).The output voltage of the bridge can be expressed as a function of the change in the resistance of the thermistor. Let the change in resistance of the thermistor be delta R. since Rb and Rc are decided resistors, the voltage Vb is constant. However, voltage Va varies as a function of the change in thermistor resistance. Therefore The negative sign in this equation indicates that Va<Vb because of the increase in the value of delta R. The output voltage Vab is of the bridge is then utilise to the differential instrumentation amplifier composed of three op-amps (see the figure).The gai n of the basic differential amplifier is (-Rf / Rl) therefore the output voltage VO of the circuit Vo=Vab (-Rf / Rj) =AR(V) . Rf 2 (2R + A R) Rl Generally, the change in the resistance of the thermistor delta R is very small, therefore, we can approximate (2R + delta R) 21 2R. Thus the output voltage V0 =Rf AR- (Vdc) Ri 4R The equation indicates that V0 is directly proportional to the change in the resistance delta R of the thrmistor. METHODOLOGY 1. The wheatstone bridge is balanced initially at a desired reference condition. 2.As the physical quantity to be measured changes, the resistance RT (resistance of thermistor) will also change by A R. 3. Due to change in Rt, the bridge is unbalanced. Hence , VaVb 4. The bridge output voltage Vab is then applied to a differential instrumentation amplifier consisting of three op-amps as shown in fig. 5. The expression for the output voltage of the instrumentation amplifier is V()= Vab (-Rf/Rl) Where, -Rf/Rl gain of the instrumentation ampli fier 6. This output is then applied to an indicating meter which indicates the value of the quality being measured. ScopeIn industry, the veridical process of control panel is situated far away. So output of sensor is sends to the control panel. Due to atmospheric condition, signal at the control panel will not the desired signal. Therefore this type of signal conditioning of thermistor using instrumentation amplifier can be used in industry. References 1. electrical and electronic measurements A. K. Sawheny 2. electronic instrumentation Kalsi 3. electronic instrumentation Khedkar 4. op amps and linear integrated circuits RamakantGaikwad 5. linear integrated circuits J. S. Katre 6. www. google. com 7. www. wikipedia. com