ECE5320 - Mechatronics Assignment 1: Literature Survey on Sensors and Actuators Topic: Thermistors (Sensors) Prepared by: SIDDHARTH P. RAO Dept. of Electrical and Computer Engineering Utah State University Tel: 435-753-4306(Home) Email: email@example.com Tel: 435-797-5237(Work) Email: firstname.lastname@example.org Overview “Sensor is a device that when exposed to a physical phenomenon (temperature, displacement, force, etc.) produces a proportional output signal (electrical, mechanical, magnetic, etc.)”. The „Thermistor‟ uses resistance to detect temperature. Thermistors can measure temperatures across the range of -40 ~ 150 ±0.35 °C (-40 ~ 302 ±0.63 °F). Typical operation resistances are in the kW range, although the actual resistance may range from few W to several MW. Typical Thermistor Types The adjoining figure shows typical types of thermistors. The shape of the thermistor probe can take the form of a bead, washer, disk, or rod. Basically, thermistors are broadly classified as Ceramic, PTC (positive temperature coefficient) and NTC (negative temperature coefficient) thermistors. Basic Working Principle The electrical resistance of metals depends on temperature. By measuring the changing resistance, the temperature can be determined. The change in resistance can easily be converted to an electrical signal transmittable. A thermistor is made of semiconductor, a mixture of metal oxide. Basic Working Principle Metals usually have a positive resistance coefficient with respect to temperature. Unlike metals, the semiconductors have a negative resistance coefficient. This is the main difference between a thermometer and a thermistor. Thus, it can be said that a PTC Thermistor is similar to an Resistance Temperature Detectors (RTD). Basic Working Principle Thus, thermistors are based on the principle of when the temperature of the resistors changes, the electrical resistance of the resistors will change correspondingly. In Negative Temperature Coefficient (NTC) thermistors, when the temperature of the resistors increases, the resistance of the resistors will be decreased. In Positive Temperature Coefficient (PTC) thermistors, when the temperature of the resistors increases, the resistance of the resistors will also be increased. The PTC Working Principle The PTC (Positive Temperature Coefficient) is a temperature sensitive semiconductor, which is made of doped polycrystalline ceramic on the basis of barium titanate. The resistance of these thermistors increases sharply when a defined temperature is reached. This property is the reason for the self-regulation characteristic, which the PTC heating elements make use of. The PTC Working Principle Due to the special Resistance- Temperature-characteristic, there is no additional temperature regulation or safety device necessary while reaching high heat-power level when using the low resistance area. The PTC-heating element regulates the power sensitively according to the required temperature. The power input depends on the requested heat output. The NTC Working Principle The NTC thermistors which are discussed herein are composed of metal oxides. The most commonly used oxides are those of manganese, nickel, cobalt, iron, copper and titanium. As seen from the adjoining figures, the resistance of these thermistors decreases with the increase in temperature. The NTC Working Principle In the basic process of fabrication, a mixture of two or more metal oxide powders are combined with suitable binders, formed to a desired geometry, dried, and sintered at an elevated temperature. By varying the types of oxides used, their relative proportions, the sintering atmosphere, and the sintering temperature, a wide range of resistivities and temperature coefficient characteristics can be obtained. Sample Configuration in Application (PTC Thermistor) As to their possibilities of application, PTC thermistors can be divided on the basis of their „function‟ and their „application‟. Out of the so many possible applications, I would like to like to show the use of „PTC thermistors for over-current protection‟. It‟s one of the simplest configurations and is very easy to understand. Here, PTC thermistor is used in the form of a fuse which is connected in series with the load in the circuit. Sample Configuration in Application (PTC Thermistor) Ceramic PTC thermistors are used instead of conventional fuses to protect loads such as motors, transformers, etc. or electronic circuits against over-current. They not only respond to inadmissibly high currents but also if a preset temperature limit is exceeded. Sample Configuration in Application (PTC Thermistor) Thermistor fuses limit the power dissipation of the overall circuit by increasing their resistance and thus reducing the current to a harmless residual value. In contrast to conventional fuses, they do not have to be replaced after elimination of the fault but resume their protective function immediately after a short cooling-down time. Sample Configuration in Application (PTC Thermistor) The adjoining figure illustrates the two operating states of a PTC fuse. In rated operation of the load, the PTC resistance remains low (operating point A1). Upon overloading or shorting the load, however, the power consumption in the PTC thermistor increases. Sample Configuration in Application (PTC Thermistor) It increases so much that it heats up and reduces the current flow to the load to an admissible low level (operating point A2). Most of the voltage then lies across the PTC thermistor. The remaining current is sufficient to keep the PTC in high-resistance mode ensuring protection until the cause of the over-current has been eliminated. Sample Configuration in Application (NTC Thermistor) There are a variety of instrumentation / telemetry circuits in which an NTC thermistor may be used for temperature measurements. In most cases, a major criterion is that the circuit provides an output that is linear with temperature. When the use of a constant-current source is desired, the circuit used should be a two-terminal network that exhibits a linear resistance-temperature characteristic. The output of this network is a linear voltage-temperature function. Sample Configuration in Application (NTC Thermistor) Under these conditions, a digital voltmeter connected across the network can display temperature directly when the proper combination of current and resistance level are selected. Consequently, the design of NTC thermistor networks for most instrumentation / telemetry applications is focused on creating linear resistance-temperature or linear conductance- temperature circuits. The simplest NTC thermistor network used in many applications is the “voltage divider circuit”. Here, with the increase in temperature, the resistance decreases, thus increasing the output voltage across the divider network. Sample Configuration in Application (NTC Thermistor) In this circuit, the output voltage is taken across the fixed resistor. This has the advantages of providing an increasing output voltage for increasing temperatures and allows the loading effect of any external measurement circuitry to be included into the computations for the resistor, R , and thus the loading will not affect the output voltage as temperature varies. Sample Configuration in Application (NTC Thermistor) The output voltage as a function of temperature can be expressed as follows: From the plot of the output voltage, we can observe that a range of temperatures exists where the circuit is reasonably linear with good sensitivity. Sample Configuration in Application (NTC Thermistor) Therefore, the objective will be to solve for a fixed resistor value, R , that provides optimum linearity for a given resistance-temperature characteristic and a given temperature range. A very useful approach to the solution of a linear voltage divider circuit is to normalize the output voltage with respect to the input voltage. Major Specifications in Thermistors To the design engineer attempting to specify, or, to the purchasing agent attempting to procure, the task of choosing the correct NTC thermistor may sometimes seems to be an impossible task. While the process can be difficult at times because of subtleties in the use of each product type, it is not nearly impossible if one has a good understanding of the basics. This, knowing and understanding the major specifications of a Thermistor is important. Following are the major specifications of a Thermistor. Major Specifications in Thermistors Resistance-Temperature Curves : Usually varies and is provided by the manufacturer. Nominal Resistance Value : Usually varies and is provided by the manufacturer. Resistance Tolerance : The standard tolerances available for each thermistor type are given on the specific product data sheet. Beta Tolerance : The beta of a thermistor is determined by the composition and structure of the various metal oxides being used in the device. Application of Thermistors The thermistor is a versatile component that can be used in a wide variety of applications where the measurand is temperature dependent. Depending on the type of application and the specific out put requirements, the PTC or the NTC Thermistor is used. Thus, the application have to be broadly divided as PTC Thermistor applications and NTC Thermistor applications respectively. Following are the various applications. Application of PTC Thermistors PTC thermistors are used for over-current protection. PTC thermistors are used for telecommunication applications. PTC thermistors are used for picture tube degaussing. PTC thermistors are used for time delay and switching applications. PTC thermistors are used for motor starting. PTC thermistors are used as heating elements. Application of PTC Thermistors Apart from these, Power PTC thermistors are used as a „Fuse‟ for Short-circuit and over-current protection. They are used as a „switch‟ for Motor start Degaussing. They are used as a „temperature sensor‟ in measurement and control & over temperature protection circuits. They are used to limit temperature for motor protection and over temperature protection circuits. They are also used as „level sensors‟ and „limit indicators‟. Application of NTC Thermistors NTC thermistors are used in General Industrial Applications such as Industrial process controls, Photographic processing, Copy machines, Soldering irons (controlled), Solar energy equipment, etc. They are used in Consumer / Household Appliances like Thermostats, Burglar alarm detectors, Refrigeration and air conditioning, Fire detection, etc. They are used in Medical Applications like Fever thermometers, Dialysis equipment, Rectal temperature monitoring, Respiration rate measurement, Blood analysis equipment, Respirators, etc. Application of NTC Thermistors They are used in Instrumentation Applications like Motor winding compensation, Infrared sensing compensation, Instrument winding compensation, etc. They are used in Automotive and Transportation Applications for Emission controls, Differential temperature controls, Engine temperatures, Aircraft temperatures, Rotor/bearing temperatures, etc. They are used in Food Handling Applications like Fast food processing, Perishable shipping, Oven temperature control, Coffee makers, Freezing point studies. Application of NTC Thermistors They are used in High Reliability Applications for monitoring Missiles & spacecraft temperatures, Aircraft temperature, Submarines & underwater monitoring and as a Fire control equipment. They are used in Communications Applications for Transistor temperature compensation, Gain stabilization, Piezoelectric temperature compensation. Apart from all these, they are also used in RF / Microwave power measurement, Voltage regulation circuits, Time delay devices, Sequential switching, Surge suppression, Inrush current limiting, etc. Advantages of Thermistors High accuracy, ~±0.02 °C (±0.36°F), better than RTDs, much better than thermocouples. High sensitivity, ~10 times better than RTDs, much better than thermocouples. As a result, lead wire and self-heating errors are negligible. Small in size compared to thermocouples. Response time shorter than RTDs, about the same as thermocouples. Reasonable long term stability and repeatability. Limitations of Thermistors Limited temperature range, typically -100 ~ 150 °C (-148 ~ 302 °F). Nonlinear resistance-temperature relationship, unlike RTDs which have a very linear relationship. They can be affected by self-heating errors that result from excitation current being dissipated in the thermistor. Thermistors are also relatively fragile, so they must be handled and mounted carefully to avoid damage. Exposure to higher temperatures can de-calibrate a thermistor permanently, producing measurement inaccuracies. Selection, Cost & Buying Info Selection of thermistors completely depends on the type of applications in which it is being used. It can be a PTC, an NTC or a Ceramic thermistor with respective temperature range, etc. Based on the type of application, thermistors range from as low as $0.5 to as high as over $500 per piece. There are many online stores from where thermistors can be purchased depending on the type of application. Some of the good e-stores are as follows: www.omega.com , www.ussensor.com , www.sensorsci.com , www.jameco.com , www.component.com , etc. References eFunda: Introduction to Thermistors Thermistors : Vishay Module 1.4: Sensors and Transducers Sensors : September 2000 – Temperature Measurement EPCOS AG : PTC Thermistors – Application Notes Thermo metrics : NTC Thermistors – Notes Thank You…!