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1 Objective We have to design a temperature meter using instrumentation amplifier. Which can measure the temperature of any substance in specific range of temperature. The instrumentation system is consists of three stage. 1. Input Stage. 2. Intermediate stage. 3. Output stage. Instrumentation Amplifier The most useful amplifier for measurement, instrumentation, or control is the instrumentation amplifier. It is designed with several op-amps and precision resistors, which make the circuit extremely stable and useful for accuracy is important. There are now many integrated circuits available in single packages. Although this packages are more expensive than a single op-amp, when performance and precision are required, the instrumentation amplifier is well worth the price, because it’s performance cannot be matched by the average op-amp. Generally a transducer is used at the measuring site to obtain the required information easily and safely. The transducer is a device that converts one form of energy into another form. An instrumentation system is used to measure the output signal produced by a transducer and often to control the physical signal producing it. Figure-1 shows block diagram of an instrumentation system. Transmission lines Physical quantity To be measured Input stage Intermediate stage Transducer + preamplifier Figure:1 Instrumentation amplifier Block diagram of an instrumentation system Output stage Indicator & automatic process controller 2 The input stage is composed of a pre-amplifier and some sorts of transducers depending on the physical quantity to be measured. The output stage may devices such as meters, oscilloscopes, charts, or magnetic recoders. In the block diagram transmission lines are used especially when the transducer is at a remote test site monitoring hazardous conditions such as high temperatures. The transmission lines permit signal transfer from unit to unit. The signal source of the instrumentation amplifier is the output of the transducer. To amplify the low-level output signal of the transducer it can drive the indicator or display is the major function of the instrumentation amplifier. The instrumentation amplifier is intended for precise , low-level signal amplification where low noise, low thermal and time drift, high output resistance , and accurate close-loop gain required . There are many instrumentation amplifier , such as the µA725, ICL7605, and LH0036, that make a circuit extremely stable and accurate . The requirements for instrumentation op-amps are more rigid than those for general-purpose amplification. Such amplifier is called differential amplifier . such most instrumentation system use a transducer in a bridge circuit. For our project we will consider a simplified instrumentation amplifier system using a transducer bridge circuit. Instrumentation Amplifier Using Transducer Bridge Figure-2 shows s simplified differential instrumentation amplifier using a transducer bridge. A resistive transducer whose resistance changes as a function of some physical energy is connected in one arm of the bridge with a small circle around it and is denoted by (Rt±∆R), where Rt is the resistance of the transducer and ∆R the change in resistance Rt. The bridge in the circuit of figure-2 is dc excited but ac excited as well. For the balance bridge at some reference condition, 𝑉𝑏 = 𝑉𝑎 or 𝑅𝑏(𝑉𝑑𝑐) 𝑅𝑏+𝑅𝑐 = 𝑅𝑎(𝑉𝑑𝑐) 𝑅𝑎+𝑅𝑡 3 That is, 𝑅𝑐 𝑅𝑏 = 𝑅𝑡 𝑅𝑎 Circuit Diagram RT±∆R Resistive transducer Va Vb Vo Vab Vab Rm R3=Rf Indicating meter Figure:2 Differential instrumentation amplifier using a transducer bridge Generally, resistors Ra, Rb and Rc are selected to that they are equal in value to the transducer resistance Rt at some reference condition. The reference condition is the specific value of the physical quantity under measurement at which the bridge is balanced. The bridge is balanced initially at a desired reference condition. As the physical quantity to be measured changes , the resistance of the transducer is also changes, which causes the bridge is unbalanced (Va≠Vb). The output voltage of 4 the bridge can be expressed as a function of the change in resistance of the transducer Let the change in resistance of the transducer be ∆R. Since Rb and Rc are fixed resistor, the voltage Vb is constant. Voltage Vo varies as a function of the change transducer resistance. According to the voltage-divider rule we get. 𝑉𝑎 = 𝑉𝑏 = 𝑅𝑎 (𝑉𝑐𝑐 ) 𝑅𝑎 +𝑅𝑇 +∆𝑅 𝑅𝑏 (𝑉𝑐𝑐 ) 𝑅𝑏 +𝑅𝑐 The voltage Vab across the output terminal 𝑉𝑎𝑏 = 𝑉𝑎 -𝑉𝑏 = 𝑅𝑎 𝑉𝑐𝑐 𝑅𝑎 +𝑅𝑡+∆𝑅 – 𝑅𝑏 𝑉𝑐𝑐 𝑅𝑏 +𝑅𝑐 If 𝑅𝑎 =𝑅𝑏 =𝑅𝑐 =𝑅𝑇 =R , Then 𝑉𝑎𝑏 = −∆𝑅(𝑉𝑐𝑐 ) 2(2𝑅+∆𝑅) The negative (-) sign in the equation indicates that 𝑉𝑎 < 𝑉𝑏 because of the increase in the value of ∆R. The gain of the basic amplifier is (𝑅𝑓 /𝑅1 ); Therefore the output voltage Vo is 𝑉𝑜 = 𝑉𝑎𝑏 (- 𝑅𝑓 𝑅1 )= (∆𝑅)𝑣𝑐𝑐 𝑅𝑓 2(2𝑅+∆𝑅) 𝑅1 The change in resistance of transducer ∆R is very small. we can approximate (2R+∆R) ≅ 2𝑅. Thus the output voltage is 𝑅𝑓 ∆𝑅 𝑉𝑜 = 𝑉𝑐𝑐 𝑅 1 4𝑅 The equation indicates that Vo is directly propotional to the change in resistance ∆R of the transducer. Since the change in resistance is caused by a changed in terms of the units of the physical energy. In our instrumentation amplifier we have used “Thermistor” as a transducer. Thermistors are essentially semiconductor that behave as resistor. Usually with a negative temperature coefficient of resistance. That is as the temperature of the thermistor increases, the resistance decrease. The temperature coefficient of the resistance is expressed ohms per unit change in degrees Celsius (˚C). 5 Thermistor with a high temperature coefficient of resistance are more sensitive to temperature change and therefore suited to temperature measurement or control . Temperature Meter Temperature meter is an electronic device which can measure can measure the temperature of anysubstance in a specific range of temperature. The temperature meter can measure the temperature inCelsius or Kelvin or Fahrenheit scale. The internal circuit which is designed by using instrumentationamplifier converts temperature changes to a voltage and the indicting meter converts the output voltage into temperature, such as in Celsius or Kelvin or Fahrenheit scale. Then the indicating meter can display the temperature of the substance. Design Procedure of a Temperature Meter The instrumentation amplifier shown in Fig-3 is the circuit diagram of a temperature meter because the transducer in the bridge circuit is a thermistor and the output meter is calibrated in degree Celsius or Fahrenheit. The bridge can be balanced at a desired reference condition, for instance 25˚C. As the temperature varies from its reference value, the resistance of the thermistor changes and the bridge become unbalanced. This unbalanced bridge in turn produce the meter movement. The meter can be calibrated to read a desired temperature range by selecting an operating gain for the differential instrumentation amplifier. The meter movement is dependent on the amount of imbalance in the bridge, that is the change of ∆R in the value of thermistor resistance. The ∆R of the thermistor can be determined as follow ; ∆R= (Temperature coefficient of resistance ) (final temperature –reference temperature). 6 Circuit Diagram + Resistive transducer RT±∆R Va Vb Vo Vab Vab °C R3=Rf Indicating meter Figure:3 temperature meter circuit using instrumentation amplifier Calculation of Temperature Measurement I figure-2 if we put the value 𝑅1 =1kΩ, 𝑅𝑓 =4.7kΩ, 𝑅𝑎 =𝑅𝑏 =𝑅𝑐 =100 kΩ , 𝑉𝑐𝑐 =+5 V, op-amp supply voltage= ± 15V, T=100 kΩ, reference temperature is 25˚C, temperature coefficient of resistance = 1kΩ/˚C or1%/˚C . Now we can calculate the value of the output voltage at 0˚C and at 100˚C. ∆R = −1𝑘Ω ˚𝐶 (0˚C-25˚C) = 25 kΩ Therefore , at 0C˚ we get 𝑉𝑜 = 4.7(103 ) 25(103 ) 1( 103 ) 400(103 ) (5) 7 𝑉𝑜 = 1.47 V Similarly at 100˚C, ∆R = 𝑉𝑜 −1𝑘Ω ˚𝐶 = (100˚C-25˚C) = -75 kΩ 4.7(103 ) (−75)(103 ) 1( 103 ) 400(103 ) (5) 𝑉𝑜 = -4.41 V From this calculation we get the result that when Vo = 1.47 V the meter face can be marked as 0˚C , and when 𝑉𝑜 = -4.41 V , it can be marked as 100˚C . here at 25˚C , 𝑉𝑜 = 0 V; therefore a center-zero meter is required . thus , using the resistance-temperature characteristics of the thermistor the meter can be calibrated from 0˚C to 100˚C . Thermistor with relatively higher resistance (Rt ≥ 1 𝑀Ω) and sensitivity (temperature coefficient of resistance ≥ 3%/˚C) are best suited for remote measurement because the effect of transmission-line resistance is negligible. 8 Beside instrumentation amplifier we can produce an electronic temperature meter using practical bridge amplifier. Circuit Diagram 𝑅𝑡𝑟𝑎𝑛𝑠 = 𝑅𝑟𝑒𝑓 +∆R I= Bridge excitation voltage current I 𝑉𝑜 =- ∆𝑅 𝑅1 +𝑅𝑟𝑒𝑓 =I(∆R) E Figure: 4 Practical bridge amplifier with transducer 𝐸 𝑅1 + 𝑅𝑟𝑒𝑓 9 Advantages 1. By temperature meter we can measure the temperature of any substance easily. 2. We can get the temperature in different scale such as Celsius or Kelvin or Fahrenheit scale. 3. In temperature meter the resistive transducer is very sensitive with temperature therefore it converts very little change in temperature into resistance change. 4. The operational amplifiers used in instrumentation system are precise special-purpose circuits in which the electrical parameters such as offset, drifts, power consumption are minimized, whereas input resistance, CMRR, and supply range are optimized. So it make the circuit stable and accurate. Limitations 1. This Temperature Meter using instrumentation amplifier cannot measure the temperature of high range. 2. The operational amplifier are used are quite expensive. 3. Output voltage does not depends on 𝑉𝑐𝑐 . It can not exit the saturation voltage. So it can not measure high range of temperature. 10 Conclusion The bridge circuit converts the resistance change of a transducer into a voltage change. An input temperature change of 25°C to 100°C gives an output voltage change of 0 V to -4.41V. The sensitivity of the temperature to voltage converter can be increase easily by increasing 𝑉𝑐𝑐 . The maximum value of 𝑉𝑐𝑐 . Is set the maximum thermistors current to avoid the self heat. Therefore 𝑉𝑐𝑐 . Has a maximum value of 𝑉𝑐𝑐 . = I(𝑅𝑇 +𝑅1 ) Reference Books 1. Op-Amp Linear Integrated circuits (Ramakant A. Gayakward) 2. Operational amplifier and Linear Integrated circuits (Robert F. Coughlin &Frederick F. Driscoll)