AppNote 2 Copy Copy

APPLICATION NOTENo. 2 Selecting and Using Thermistors for Temperature Control The price of the thermistor’s high sensitivity is paid with nonlinearity.5K 5K 10K 25K 50K 100K -20 0 20 40 60 80 100 120 140 160 Temperature (oC) Figure 1. Resistance .Selecting and Using Thermistors for Temperature Control By Lawrence A. By studying Figure 1. At these high resistances. When used with the proper thermoelectric (TE) cooler and support circuitry. thermistors are operated at temperatures where they exhibit resistances of thousands of ohms.Generally. fast response times. Each thermistor is labeled according to its resistance at 25 oC. By way of example. ruggedness. 1 . small size. Johnson Thermistor temperature sensors are used widely in laser diode and detector cooling applications because of their high sensitivity. a factor which makes their selection and use a bit more challenging than might be otherwise expected. That means its resistance decreases with increasing temperature. Although other types are available. Resistance-temperature response curves for nine common thermistors.001 oC. and low cost. a common thermistor costing less than a dollar has enough sensitivity to stabilize the temperature of a laser diode to better than 0. the R-T characteristics of nine common NTC thermistors are shown in Figure 1. many of the practical aspects of selecting and working with thermistors become apparent. simple two-wire resistance For Varying Room Temperature Resistances Thermistor R/T Curves 40 Resistance (thousands) 30 20 500 10 250 0 -60 -40 1K 2. most thermistors have a negative temperature coefficient (NTC). Thermistor Characteristics Thermistors are generally two-terminal semiconductor devices that have an electrical resistance that varies non-linearly with 50 temperature. such as the LDT-5910. the Steinhart-Hart equation introduces errors of less than 0. the differences between thermistors of the same nominal resistance are relatively small. and C is discussed in detail in ILX Lightwave’s Application Note #4. microprocessors have greatly simplified this task with their ability to quickly calculate complex expressions. The R-T characteristic of most thermistors can be well characterized by an equation known as the Steinhart-Hart equation: 1/T = A + B*(Ln R) + C*(Ln R)3 In this relationship. However. B.1 oC over a temperature range of -30 oC to +125 oC. as shown in Figure 2. coarse resistance control is accomplished through the use of different metal oxides to form the semiconductor junction. Fortunately. For the 10 K thermistor shown. T is the absolute temperature (in degrees Kelvin) and A. and each combination leads to a slightly different R-T characteristic. the sensitivity varies as follows: Temperature -20 oC 25 oC 50 oC Sensitivity 5. design engineers often resorted to resistor-based linearizing networks which were effective only over narrow temperature ranges. Thermistor Families During the manufacture of thermistors. B. Often.measurements work well using conventional DMMs. In the past. This sensitivity drops rapidly in going from low to high temperatures. several different material combinations may be used to arrive at the same nominal 25 oC resistance. This variety of available R-T characteristics often seems to complicate thermistor selection. Assuming good calibration data is available**. Resistance (thousands) 10 9 8 7 6 5 4 3 2 1 0 -40 -20 0 20 40 60 80 Sensitivity . **These constants are supplied to the user with the purchase of any ILX Lightwave product which uses thermistors for temperature control. in most practical applications the range of any one thermistor is limited.Although the available thermistors span a wide temperature range. Calibration The non-linearity of thermistors complicates their calibration. Temperature (oC) Figure 2.600 ohms/oC 439 ohms/oC 137 ohms/oC Range . 2 .Thermistors achieve their highest sensitivity at low temperatures. and *Calculation of the constants A. and C are constants which can be determined from measured values of resistance and temperature*. R-T slope changes for differing thermistor materials. errors of less than 0. typical high temperature limits would be: Sensing Current 10 uA 100 uA 10 K Thermistor Temp. Thermistor Selection To understand practical thermistor selection trade-offs. There the Steinhart-Hart equation is used to calculate temperature. The lower part of the graph shows the system measurement resolution in degrees C per A/D converter step. The voltage is digitized and then input to the microprocessor. Thermistor resistance is sensed by forcing a known. increasing temperature. This figure shows a temperature sensing and readout system like the one used in the ILX Lightwave model LDT-5910. For example. limitations arise at both high and low temperatures. This part of the graph 3 . using a typical 10 K thermistor. As the thermistor temperature decreases. The upper part of the graph shows the voltage input to the A/D converter. In the LDT-5910 a 12-bit A/D converter is used. Sensing Current 10 K Thermistor Temp. are shown graphically in Figure 4. 15 oC 72 oC The low and high temperature limits of the measurement system. The practical high temperature limit of the system occurs when the temperature measurement resolution drops below the minimum desired level.5 volts. 10 uA -51 oC 100 uA -10 oC As the thermistor temperature increases. and the maximum A/D input voltage is about 4. resulting in an A/D input voltage resolution of about 1 mV. Since the system A/D converter has a fixed resolution. In the LDT-5910 you can select a sensing current of either 10 or 100 uA. this means that temperature measurement resolution decreases with Figure 3. Although this approach is very flexible. take a look at the system block diagram shown in Figure 3. suppose that at high temperatures you need a measurement resolution of at least 0. These factors lead to the following typical low temperature limits. In that case. and so does its sensitivity to temperature change. This part is used to determine the low temperature limit of the system. constant current through the thermistor and then measuring the voltage drop that develops.01 oC between -20 oC to +50 oC.2 oC. The practical lower temperature limit is reached when the voltage exceeds the maximum input voltage of the A/D converter. so does the voltage across it. its resistance decreases. Microprocessor-based temperature measurement system. its resistance increases and likewise. 1 0 -60 -40 -20 0 20 40 60 80 100 Temperature 10 µA Sense Current (oC) 100 µA Sense Current Figure 4.0 2.0 A/D Over-Range LDT-5910 Temperature Range Voltage Across Thermistor 3.0 0 -60 -40 -20 0 20 10 µA 0. 4 .5 V and 0.0 4.5 40 60 80 100 µA 100 0.4 Degrees/ADC Step 0.5 4. Graphical representation of temperature range for LDT-5910 at 10 uA and 100 uA.2 oC per A/D step as limits.(Using Typical 10KΩ @ 25 C Thermistor) 5.3 0.0 1.2 0. using 4. the above method may be used to determine the type of thermistor and sensing current required. the same approach can be used with other thermistors. The type of thermistor you choose will depend primarily on the required operating temperature range. We will be glad to answer your questions. Although Figure 4 provides data only for a typical 10 K thermistor. For a given temperature range and resolution.is used to determine the upper temperature limit. Thermistor R-T curves. From the figure you can see that 10 K thermistors are generally a good choice for most laser diode cooling applications where high stability is required near room temperature. 10 K thermistors are also often a good choice for detector cooling applications where operation at temperatures from -40 oC to room temperature are required. Similarly. the useful temperature range of a thermistor is extended for a given A/D converter. If you require more information on thermistors or any of our products. 5 . like that in Figure 1. By varying the sensing current. The temperature ranges for the LDT-5910 are shown by the horizontal bars in the center of Figure 4. are usually supplied by the thermistor manufacturer. contact your ILX Lightwave representative (see the back cover). com and quickly get information on all of our products. receive technical support and download Technical Notes. Copyright © 2000 ILX Lightwave Coproration. and Canada Phone: 406-586-1244 Fax: 406-586-9405 Email: [email protected] Log on to our website at www. Our commitment to you is to provide the highest quality instrumentation and technical support available. Application Notes.ILX Lightwave Corporation was founded in 1986 as a privately held. LabVIEW® drivers and other information.S.ilxlightwave. For application assistance or additional information on our products or services you can contact us at: ILX Lightwave Corporation P. Box 6310 Bozeman.O. 05/01 . MT 59771-6310 Toll Free: 1-800-459-9459 in the U. venture financed manufacturer of photonic test and measurement instruments. All Rights Reserved Rev.