An Electronic Thermal Transducer

Transcription

1 An Electronic Thermal Transducer PRJ NO: 16 By Okore Daniel F17/969/ Supervisor : Dr. Mwema Examiner: Dr. Abungu

2 INTRODUCTION The aim of this project was to design and implement a temperature sensor in miniature form that will alert the user to the possibility that he may have a fever or hypothermia. The sensor to be implemented will be coupled to an LED that will turn on when the temperature exceeds a certain threshold. There are number of different technologies that could be used to implement a temperature sensor; Thermocouples make use of the Seebeck effect. Resistance temperature detectors are sensors whose resistance changes with temperature. Thermistors also rely on changes in material resistance to measure temperature. Transistor based sensors make use of the base-emitter voltage and its collector current.

3 Body Temperature LITERATURE REIEW Normal body temperature is considered to be 7 c (98.6 F). If the body temperature is too high, the functions of the cells may become impaired or damaged, if too low, then rate at which food is metabolized decreases. A fine balance between heat production and loss is maintained imperceptibly in the normal individual. Production of body heat is primarily the result of conversion of chemical energy in foods to heat by metabolic and mechanical mechanism. Heat produced is conserved by vasoconstriction and diversion of blood flow away from the skin. Heat is lost at the skin surfaces by the mechanisms of ; Convection Radiation Evaporation.

4 ariations from the ideal body temperature could be due to: Physical Activity Fever Hypothermia Types of temperature sensors Thermocouple It consists of two dissimilar metals, jointed together at one end. One of the junctions is held at a reference temperature (usually c) and the other junction is held at the temperature to be measured. When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature. Principle of Operation. When a temperature differential is maintained across a metal, the vibration of atoms and motion in electrons is affected so that a difference in potential exists across the material. This potential difference is related to the fact that the electrons in the hotter end of the material have more thermal energy than those in the cooler end, and thus tend to drift towards the cooler end. This dislocation of electrons produces a voltage difference, which forces electrons to flow in the opposite direction. In a state of dynamic equilibrium, both processes are in balance. If the circuit is closed by connecting the two ends through another conductor, a current is found to flow in the closed loop.

5 Thermocouple Characteristics Sensitivity Range Signal conditioning Reference Compensation Noise Resistance Temperature Detector An RTD is basically a temperature sensitive resistor. It is a positive temperature coefficient device, which means that the resistance increases with temperature. Principle of Operation As electrons move through the metal, they collide with the stationary atoms, increasing thermal energy of the metal. The conduction electrons tend to collide even more with the vibrating atoms. This impedes the movement of the electrons and absorbs some of their energy, that is, the material exhibits an increased resistance to the flow of electric current. Thus, metallic resistance is a function of the vibration of atoms and hence of temperature. RTDs act somewhat like an electrical transducer, converting changes in temperatures to voltage signals by the measurement of resistance.

6 RTD Characteristics Accuracy Stability Response Time Self-heating Thermistors In contrast to metals, electrons in semiconductor materials are bound to each molecule with sufficient strength that no conduction electrons are contributed from the valence band to the conduction band. Thus at low temperatures, semiconductors behave like insulators because there are no conduction electrons to carry current through the material. When the temperature of the material is increased the molecules vibrate more. Such vibration provides additional energy to the valence electrons. When such energy equals to or exceeds the energy gap, the electrons become free of the molecules and more to the conduction band where they are free to carry current through the material. Thus, semiconductors become better conductors of current as their temperature is increased. This is the opposite of what happens in metals.

7 Thermistor Characteristics Sensitivity Range Response time Active Semiconductor Sensors. These are sensor whose operating principle relies on the potential barrier between the conducting layer and the valence layer in semiconductors such as transistors and diodes. Transistor based sensors Transistor based temperature sensors make use of the relationship between a bipolar junction transistors base-emitter voltage and its collector current. The base emitter voltage of a transistor is universally proportional to its temperature. It exhibits a negative temperatures coefficient of approximately - m/ c. As the temperature increases, the reserve saturation current increases exponentially. Based on the Eber-Moll s equation, in order to maintain the same value of collector current, the base-emitter voltage decreases.

8 The band-gap ( BE ) oltage reference R1 Ω R Ω 1 7 U LM71H Q1 Q BJ T _NPN_IR T UAL BJ T _NPN_IRT UAL R Ω 6 R Ω

9 This involves generating a voltage with a positive temperatures coefficient (tempco) which is the same as the negative BE tempco. This when added to BE, the resultant voltage will have a zero tempco. The ratio, r of the emitter current densities of the two transistors is typically 1:1 and using Elbersmoll diode current equations, the collector current I c ri c1 can be shown to have a positive tempco. J E J I Ι E o I O qbe ( exp 1 kt exp exp q BE kt q kt BE In J Ι E Q kt BE BE kt q In J I E

10 BE BE BE 1 kt q In J Ι E x kt q In J Ι E 1 kt q In J J E E 1 kt q In r J E is then converted to a voltage using the resistor R which thus sets the amount of positive tempco voltage required to get an overall zero tempco reference voltage. This occurs when the voltage is equal to the silicon band gap voltage at K which is approximately 1..

11 System design and Simulations The choice of what type of temperature sensor to use was based on the following criteria Temperature range Accuracy required Economic Considerations Based on all these considerations, a transistor based sensor was picked as the most practical to fabricate. Design based on the variation of BE with temperature The circuit is fed from cc of 5. The.9kΩ and 1.kΩ resistors form a voltage divider that feeds a voltage of 1.5 volts to the bases of Q1 and Q. This is approximately equal to the band-gap voltage of silicon (1.). Q1 and Q are forced to operate at current ratios of 1:1(56KΩ & 5.6KΩ) by feedback from the collector voltages. Op amp 1 acts as an error amplifier that ensures that c1 and c are equal. As the current flows to the ground through the 1 kω and kω resistors, a voltage is developed across the kω resistor that is directly proportional to the difference in BE and hence to the temperature.

12 Temperature Sensor Circuit CC 5 R1.9kΩ R 1.kΩ R 1kΩ R 1kΩ Q1 BC18BP R5 5.6kΩ R6 56kΩ Q BC18BP Q BC18BP R7 1kΩ 9 7 R8 kω 6 5 UA LM9N 5 1 UA LM9N 5 1 D1 1N D 1N 1 R9 1kΩ CC 5 CC CC 5 CC CC 5 CC 5 U1 LM71H LED1 R1.7kΩ R11.kΩ 1 CC R1 16kΩ R1 1.8kΩ CC CC 8 1 1

13 ariation of oltage with temperature at the temperature tap TEMPERATURE OLTAGE (v) 1 $1, Temperature m $1, Temperature m $1, Temperature m $1, Temperature m 5 $1, Temperature m 6 $1, Temperature m 7 $1, Temperature m 8 $1, Temperature m 9 $1, Temperature m 1 $1, Temperature m 11 $1, Temperature 7.75 m

14 Comparator Design A comparator is a circuit that switches output states when its input exceeds a certain level called the trip or set point or reference voltage. In Out The idea is that as long as in is less than the trip level t the output is low. When in exceeds t, the output instantly switches high mathematically, it can be written as; max min for for in in > < t t ` Window Comparator A window comparator is used to indicate whether or not a voltage is within a range of values that is determined by two reference voltages. Divider networks R 1 -R and R -R are used to establish the upper and lower limits of the window.

15 Discussion The supply voltage that is used to drive the voltage dividers must be relatively stable for good performance. In some cases a very stable and accurate reference voltage, separate from the normal supply voltage, will be employed to drive the trigger voltage levels and for transducer excitation. Resistor R 1 and R would most likely be implemented with multiple turn potentiometers, which would be adjusted to set the upper and lower trip levels to the desired levels. The aim of having a comparator in this case was that we can have a voltage that toggles between 5v and Ov. Since the voltage at 9 c is 71.7mv, this was picked as the upper trigger point and the voltage at 5 c is m, this was picked as the lower trigger point. The design is to be such that that the comparator output is Ov at voltages below the upper and lower trigger levels. oltage dividers comprising of 1.8kΩ and.kω, 16kΩ and.7kωwere used to scale down the 5v source to obtain the upper and lower trip levels.

16 CC 5 CC R1 16kΩ CC 5 CC u 1 5 U 1A 6 D 1 R.7kΩ 1 1N LM9N 5 1 p k khz CC 5 CC R 1.8k Ω CC 5 CC 5 U A 7 D L R.kΩ 1 1N LM9N R5 1kΩ o

17 Results The circuit was successfully implemented on a bread board. It was, however, not possible to test it and get accurate results due to the difficulty in getting a temperature of 9 c or 5 c given the equipment in the lab. Based on the results from the simulations, the author is fairly confident that the circuit will work as designed. Conclusion The main objective of designing and implementing a temperature sensor in miniature form was successfully met. The design was based on the temperature dependence of the base-emitter voltage of a transistor. A number of compromises such as in the sensitivity of the sensor had to be made to keep its cost low and to make sure the sensor was implemented in time. It was not possible to test the implemented sensor but simulations of the circuit were successfully done using the temperature sweep function in Electronic Workbench.