Thermistors

�� Linearization of the NTC Thermistor Characteristic Curve �� The change in the resistance of a NTC thermistor is �� remarkably non-linear. If a nearly linear resistance �� curve is required while measuring a wide range of �� temperature, such as in a dial thermostat, a resistor �� in�� will �� connected series in ��parallel provide an�� or �� of �� approximation linearity however the temperature �� range�� exceeds� 50 to�� 100 Kelvin. �� a) Linearization of NTC thermistor by paralleled resistor ��b)�� Signal linearization circuitry �� The �� will �� voltage Ve and power consumption Pv of�� a linearized NTC NTC��thermistor reduce the �� thermistor �� NTC of an accuracy. �� The R/T curve of linearization of a NTC thermistor by means of a �� parallel connection of a�� resistor �� The combination of a NTC thermistor and a resistor connected in parallel will produce an S shaped characteristic curve. The best linearization will be obtained if the inflection is placed in the middle �� of the operating temperature range. Under these conditions, the �� resistance of the connection be approximated by �� parallel � �� can �� 1)�� Sample of a simple �� amplifier circuit applying an exponent: �� voltage �� at the load resistor R(sub L) as a function of �� 2) The output �� the temperature �� HOW TO CHOOSE AND WORK WITH NTC THERMISTORS Advantages of the NTC thermistor �� The NTC thermistor and temperature sensor compared with other �� sensors in temperature measurement and control applications: �� The resistance of which are in parallel connection is: �� RT, ��RP �� 1) Reliable performance; �� 2) High precision, Good tolerances and interchangeability; 3) Large temperature coefficient of resistance, High accuracy 4) Low cost, especially for middle-or-low temperature measure�� ment and control. �� at �� In ��the equation: RTM is�� the NTC�� thermistor resistance average �� 5) High dissipation coefficient: Test current can be greater than of �� temperature TM�� B is�� the B value of NTC�� thermistor��(linear) slope �� of�� traditional sensors, simplified circuitry. �� curve: �� of��the characteristic �� Application Notes: �� of�� the �� 1. Please supply�� all characteristics the application. Include resistance and tolerance, B value, dimensions, length of wire and application temperature range etc. 2. If you are not certain of the characteristics, please provide the following data: 1) Purpose, application details 2) Environmental conditions 3) Range of temperature measurement and control 4) Dimensions 5) Testing power 6) The zero power resistance and errors at two or more temperatures 3. Insulation and housings can be added according to the requirement of users high-dissipation coefficient. Test current can be far larger that that of an other type of sensor which will simplify the circuitry. Special builds are available according to your requirements (characteristics, dimensions and wire) WARNINGS 1. Avoid sudden changes of ambient temperature of the thermistors and temperature sensors, this could cause premature aging. 2. Excess current passing through the thermistor will cause the components to self-heat and result in a variation of the temperature reading. This factor should be considered before selecting. (When the heat of the component is 1/ 10 of dissipation coefficient (mW/°C) the temperature variance will be 0.1°C., when it is 1/100 of diss.coef. the temperature difference will be 0.01°C) 3. The excess current caused by bad insulation, electrostatic induction, poor contacts to circuitry will damage the thermistor. Pay particular attention to the method of connection, and that too much current is not allowed to pass through thermistor. 4. Measurements should be taken only after 5 to 7 seconds. 5. Small size and short time constant should be selected if the application requires fast response and high precision. 6. If water, dust or ionic compounds are between the ends of the lead wire or on the surface of insulation, the resistance will decline and become unstable causing a difference in the temperature reading. Moisture protection and insulation precautions should be taken to insure dryness. 2010/Sep NTC 8415 Mountain Sights Avenue • Montreal (Quebec), H4P 2B8, Canada Tel: (514) 739-3274 • 1-800-561-7207 • Fax: (514) 739-2902 E-mail: sales@cantherm.com Website: www.cantherm.com | Division of Microtherm ovals • Rated Zero Power Resistance R25 • Beta Value • Temperature Coefficient of Zero Power Resistance� • Dissipation Coefficient � • Thermal time constant � • Max. Steady State Current • Resistance-Temperature Characteristic • Static V-I Characteristic B - B value Intro Dissipation Coefficient δ NTC Thermistors are ceramic semi-conductor elements made from • Basic Characteristic and Application of Powerand NTC repeatable R-T curve. metal oxides which have aExample predictable Thermistor The resistance changes are non-linear and exhibit a Negative Temperature Coefficient therefore their resistance, at a determined Application Guide for Temperaturepower, Measurement andas Control measuring declines the temperature of the device • Temperature Measurement and Control increases versa. NTC thermistors can be used when • Linearization and of the vice NTC Thermistor Characteristic • Curve Application Notes and Warnings temperature compensation, temperature measurement or control, or inrush surge current protection are needed. Static V-I Characteristic Where: � T - The temperature coefficient of the zero power resistance at T RT - The zero power Coefficient resistance at T • Dissipation � dissipation coefficient is the ratio of the rate of change of Static V-I Characteristic refers to the relationship between voltage TThe - Temperature B B value the dissipation power consumption thermistor to the change of it’s and current whenofthe The coefficient is of thea ratio of the rate of change of the power consumption of a thermistor to the change it’sNTC thermistor establishes the thermal corresponding temperature, namely: corresponding temperature, namely: • •Static V-I balance state, because the variable range of the relationship Static V-ICharacteristic Characteristic �� Selection Criteria for Power NTC Thermistors 1. The maximum operating current of the resistor > (is greater loop 2. Rated resistance of power NTC Thermistor R is: 1. The maximum operating current of the resistor > (is greater than) the operating curren 2. than) Rated the resistance of power NTC in Thermistor R is: operating current actual power between terminal voltage and current of the thermistor is very �� wide, its�� voltage and�� current curve is often�� represented by double �� logarithm coordinates. �� The dissipation coefficient is the ratio of the rate of change of the power consumption of a thermistor to the change of it’s The � temperature, willwill change for different ambientambient temperatures and transfer mediums and should�� be used for reference �� corresponding namely: Thevalue valueof of change for different temperatures �� The curve of��the relationship between Igu and Igl�� of NTC thermistor �� purposes only. In the equation: In the equation: • Dissipation Coefficient � δ and transfer mediums and should be used for reference E is the loop voltage, Im is the surge current is the loop voltage, Impower, is the surge current ForEconversion power, reversion switch power, UPS power Im = 100 times operating current. purposes only.constant The dissipation constant of a thermistor is the in (mW/°C) required to self-heat it by 1°C The dissipation of a thermistor is the amount of power expressed Zero Power Resistance Rt ForFor filament, heater, etc. add thereversion loop Im = 30power, times operating conversion power, switchcurrent. power, UPS power Im above ambient temperature. amount of power expressed in (mW/°C) required to self-heat it NTC Thermistors are ceramic semi-conductor elements made from metal oxides which have a predictable and repeatable = 100 times operating current. R-T curve. The resistance changes are non-linear and exhibit a Negative Temperature Coefficient therefore theirambient temperature. 3. When the B value is higher, the final resistance and the temperature rise will be les by 1°C above The resistance value measured atpower, the rated temperature using a of the device increases and vice versa. NTC resistance, at a determined measuring declines as the temperature For filament, heater, etc. add the loop Im = 30 times operating The value of � will change for different ambient temperatures and transfer mediums and should be used for reference thermistors be used when atemperature measurement or control,purposes or inrush only. surge current power levelcanwhich causes resistancecompensation, change thattemperature can be ignored 4. Generally, the greater the product of the time constant and the dissipation coefficien current. protection are needed. of the resistor and greater surge current protection. relative to the measurement error as a whole. Since the resistance • Thermal time constant � The dissipation constant of a thermistor is the amount of power expressed in (mW/°C) required to self-heat it by 1°C values are high and the change in R values are generally great, 3. When the B value is higher, the final resistance and the Thermal Time Constant τ above ambient temperature. the errors created by measurement and long lead wires can be The thermal time constant is the time in seconds needed for a thermistor to register a change of 63.2% of the difference temperature rise will be less. between the initial temperature of the thermistor and that of its surroundings when subjected to a stepped change in ignored. �� • Zero Power Resistance Rt 4. Generally, the greater the product of the time constant and temperature under zero power conditions. The thermal time constant is the time in seconds needed for �� the dissipation coefficient result in a larger thermal capacity of a thermistor totime register a change of 63.2% of the difference The resistance Zero value measured at the rated temperature using aR25 power level which causes a resistance change that can • Thermal constant � Rated Power Resistance the resistor and greater surge current protection. be ignored relative to the measurement error as a whole. Since the resistance values are high and change in to R the �between is the in direct thermal capacity (C)thermistor of the thermistor and of in inverse ratio to the dissipation coefficient, namely: �� theratio initial temperature of the and that its values are generally great, the errors created by measurement and long lead wires can be ignored. The NTC thermistor is especially suited for use as a temperature sensor due to its hig surroundings subjected to ina seconds steppedneeded change tempera-to register a change of 63.2% of the The thermal timewhen constant is the time forin a thermistor difference �� operating temperature range of –55°C to +300°C, it is ideally suited for measurement a The rated resistance of thermistor which is the zero power resisbetween the initial temperature of the thermistor and that of its surroundings when subjected to a stepped change in ture under zero power conditions. also relatively easy to monitor and of low cost to purchase. tance measured at 25°C and indicated on the thermistor. This is Application Guide for temperature under zero power conditions. 1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop NTC thermistors should be selected according to the following criteria: the most common value used to describe the resistance value of a Temp. Measurement τ is in direct ratio to the thermal capacity (C) of the thermistor • Rated Zero Power Resistance R25 - The required range of temperature and Control 2. Rated resistance of power NTC Thermistor R is: thermistor. - The required range of resistance �and is inindirect ratioratio to thetothermal capacity (C) of the thermistor and in inverse ratio to the dissipation coefficient, namely: inverse the dissipation coefficient, namely: - The required measuring accuracy The rated resistance of thermistor which is the zero power resistance measured at 25°C and indicated on the thermistor. Temperature Measurement and Control The NTC thermistor is - Environment (medium of heat transfer) Basic Characteristic & Application Example This is the most common value used to describe the resistance value of a thermistor. Basic - The expectedsuited time constant especially for use as a temperature sensor due to its high BasicCharacteristic Characteristicand andApplication ApplicationExample Example • Max. Steady State Current Beta Value of Power NTC Thermistor - The dimensions ofofPower PowerNTC NTCThermistor Thermistor levelgeometrical of accuracy. Within the operating temperature range of –55°C The maximum allowable continuous current allowed to pass through the thermistor at 25 deg. C. +300°C, it to is use ideally suited for measurement and control of A to practical circuit for temperature measurement with a NTC thermistor B or beta value is an indication of the slope of the curve which could be a Wheatstone Bridge in which a NTC thermistor forms one leg ofcost the Power PowerLoad-Temperature Load-TemperatureCharacteristic CharacteristicCurve Curve temperature, and is also relatively easy to monitor and of low • Beta Value bridge. Power Load-Temperature Characteristic Curve represents the relationship between the resistance and the to purchase. In the equation: temperature of a particular thermistor measured under zero power If the sensor temperature changes in the balanced bridge circuit, a measurable E is the loop voltage, Im is the surge current B or beta value is an indication of the slope of the curve which represents the relationship between the resistance • Max. Steadyand State State Current Current Max. Steady For conversion power, reversion power, switch power, UPS power Im = 100 times operating current. current will pass through the ammeter. In some cases a variable resistor R3 is Resistance-Temperature Characteristic conditions. Theof higher the thermistor Beta value the greater in the temperature a particular measured underthe zerochange power conditions. The higher the Beta• value the greater the NTCand thermistors be selected according to the For filament, heater, etc. add the loop Im = 30 times operating current. used, according toshould the resistance value of R3 from which wefollowing can infer to the change in resistance per degree C. can calculate the RT2 using this resistance per degree C. You temperature measured (In the balanced state). The maximum allowable continuous currentcurrent allowed allowed to pass through the thermistor at 25 deg. C. criteria: The maximum allowable continuous to pass The R/T characteristic is the relationship between the zero power resistance of the thermistor and its temperature. Since 3. When the B value is higher, the final resistance and the temperature rise will be less. formula: You can calculate the RT2 using this formula: - The required range of temperature this relationship is non-linear described through the thermistor at it25is deg. C. by the R/T curve. Also, NTC thermistors and sensors which are used in conjunction with relays or magn 4. Generally, the greater the product of the time constant and the dissipation coefficient result in a larger thermal capacity - The required range of resistance appropriate alarm and protection equipment and are used in applications requiring tem of the resistor and greater surge current protection. R-T curve of NTC thermistor The required measuring accuracy • Resistance-Temperature Characteristic - Environment (medium of heat transfer) - The expected time constant The R/T characteristic is the relationship between the zero power resistance of the thermistor and its temperature. Since Here: B=3380 T1=25T1=25 RT1=10Kohm Here: B=3380 RT1=10Kohm RT1 - The zero power resis�� - The geometrical dimensions RT1 - The zero power resistance at T1 this relationship is non-linear it is described by the R/T curve. Resistance-Temp. Characteristic tance atzero T1 power RT2resistance - The zero RT2 - The at T2 power resistance at T2 �� R-T curve of NTC thermistor A practical circuit to use for temperature measurement with a Unless otherwise indicated, the B value is calculated using the zero power resistance at 25 deg. C (298.15K) and at 50 is the relationship between the zero The R/T characteristic Unless otherwiseThe indicated, B value is calculated using the deg. C (323.15K). Beta valuethe is not a rigorous constant and is temperature dependant within a small range of of the thermistor and its temperature. Since �� NTC thermistor could be a Wheatstone Bridge in which a NTC power resistance operating temperatures. The NTC thermistor is especially suited for use as a temperature sensor due to its high level of accuracy. Within the Sketch Map of Surge Current Protection in Circuit zero power resistance at 25 deg. C (298.15K) and at 50 deg. C thermistor one leg of this relationship is non-linear it is described by the R/T curve. operating temperature range of –55°C to +300°C, it is ideally suited for measurement and controlforms of temperature, andthe is bridge. Sketch Map of Surge Current Protection in Circuit of Power NTC Thermistor Sketch Map of Surge Current Protection in Circuit of Power NTC Thermistor of Poweralso NTC Thermistor (323.15K). The Beta value is not a rigorous constant and is temrelatively easy to monitor and of low cost to purchase. perature dependant within a small range of operating temperatures. If the sensor temperature R-T curve of NTC thermistor NTC thermistors should be selected according to the following criteria: - The required range of temperature changes in the balanced bridge • Temperature Coefficient of Zero Power Resistance T � - The required range of resistance circuit, a measurable current will Temp. Coefficient of Zero - The required measuring accuracy The temperature coefficient or alpha (symbol) at a specified temperature is the average percent change of the zero power pass through the ammeter. In - Environment (medium of heat transfer) resistance per degreeResistance C to the rated resistance - The expected time constant Power T(R25). some cases a variable resistor - The geometrical dimensions R3 is used, and according to the The temperature coefficient or alpha (symbol) at a specified temA practical circuit to use for temperature measurement with a NTC thermistor resistance value of R3 from which could be a Wheatstone Bridge in which a NTC thermistor forms one leg of the perature is the average percent change of the zero power resiswe can infer to the temperature bridge. tance per degree C to the rated resistance (R25). measured (In the balanced state). Introduction α If the sensor temperature changes in the balanced bridge circuit, a measurable current will pass through the ammeter. In some cases a variable resistor R3 is used, and according to the resistance value of R3 from which we can infer to the temperature measured (In the balanced state). Namely: Namely: Where: Where: � T - The temperature coefficient of the zero power resistance at T RT - The zero power resistance at T - The temperature coefficient of the zero power resistance T -T Temperature B RT- B- value The zero power resistance at T α at T T - Temperature B - B value • Dissipation Coefficient � • Static V-I Characteristic Also, NTC thermistors and sensors which are used in Typical Application- Power NTC Thermistor Circuit conjunction Also, NTC thermistors and sensors which are used in conjunction with relays or magnetic amplifier loops of the with relays or magnetic amplifier loops �� appropriate alarm and protection equipment and are used in applications requiring temperature control. When the of the appropriate alarm and protection equipment and are used in applications requiring temperature control. When the temperature changes, the resistance of the NTC thermistor will also change, which will cause the bridge circuitry to become unbalanced and a current will pass through the control circuit which sense the current, so the temperature in the controlled area will be adjusted. �� The dissipation coefficient is the ratio of the rate of change of the power consumption of a thermistor to �� the change of it’s �� corresponding temperature, namely: �� The value of � will change for different ambient temperatures and transfer mediums and should be used for reference �� 1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop 1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop 2. Rated resistance of power NTC Thermistor R is: 2. Rated resistance of power NTC Thermistor R is: ovals • Rated Zero Power Resistance R25 • Beta Value • Temperature Coefficient of Zero Power Resistance� • Dissipation Coefficient � • Thermal time constant � • Max. Steady State Current • Resistance-Temperature Characteristic • Static V-I Characteristic B - B value Intro Dissipation Coefficient δ NTC Thermistors are ceramic semi-conductor elements made from • Basic Characteristic and Application of Powerand NTC repeatable R-T curve. metal oxides which have aExample predictable Thermistor The resistance changes are non-linear and exhibit a Negative Temperature Coefficient therefore their resistance, at a determined Application Guide for Temperaturepower, Measurement andas Control measuring declines the temperature of the device • Temperature Measurement and Control increases versa. NTC thermistors can be used when • Linearization and of the vice NTC Thermistor Characteristic • Curve Application Notes and Warnings temperature compensation, temperature measurement or control, or inrush surge current protection are needed. Static V-I Characteristic Where: � T - The temperature coefficient of the zero power resistance at T RT - The zero power Coefficient resistance at T • Dissipation � dissipation coefficient is the ratio of the rate of change of Static V-I Characteristic refers to the relationship between voltage TThe - Temperature B B value the dissipation power consumption thermistor to the change of it’s and current whenofthe The coefficient is of thea ratio of the rate of change of the power consumption of a thermistor to the change it’sNTC thermistor establishes the thermal corresponding temperature, namely: corresponding temperature, namely: • •Static V-I balance state, because the variable range of the relationship Static V-ICharacteristic Characteristic �� Selection Criteria for Power NTC Thermistors 1. The maximum operating current of the resistor > (is greater loop 2. Rated resistance of power NTC Thermistor R is: 1. The maximum operating current of the resistor > (is greater than) the operating curren 2. than) Rated the resistance of power NTC in Thermistor R is: operating current actual power between terminal voltage and current of the thermistor is very �� wide, its�� voltage and�� current curve is often�� represented by double �� logarithm coordinates. �� The dissipation coefficient is the ratio of the rate of change of the power consumption of a thermistor to the change of it’s The � temperature, willwill change for different ambientambient temperatures and transfer mediums and should�� be used for reference �� corresponding namely: Thevalue valueof of change for different temperatures �� The curve of��the relationship between Igu and Igl�� of NTC thermistor �� purposes only. In the equation: In the equation: • Dissipation Coefficient � δ and transfer mediums and should be used for reference E is the loop voltage, Im is the surge current is the loop voltage, Impower, is the surge current ForEconversion power, reversion switch power, UPS power Im = 100 times operating current. purposes only.constant The dissipation constant of a thermistor is the in (mW/°C) required to self-heat it by 1°C The dissipation of a thermistor is the amount of power expressed Zero Power Resistance Rt ForFor filament, heater, etc. add thereversion loop Im = 30power, times operating conversion power, switchcurrent. power, UPS power Im above ambient temperature. amount of power expressed in (mW/°C) required to self-heat it NTC Thermistors are ceramic semi-conductor elements made from metal oxides which have a predictable and repeatable = 100 times operating current. R-T curve. The resistance changes are non-linear and exhibit a Negative Temperature Coefficient therefore theirambient temperature. 3. When the B value is higher, the final resistance and the temperature rise will be les by 1°C above The resistance value measured atpower, the rated temperature using a of the device increases and vice versa. NTC resistance, at a determined measuring declines as the temperature For filament, heater, etc. add the loop Im = 30 times operating The value of � will change for different ambient temperatures and transfer mediums and should be used for reference thermistors be used when atemperature measurement or control,purposes or inrush only. surge current power levelcanwhich causes resistancecompensation, change thattemperature can be ignored 4. Generally, the greater the product of the time constant and the dissipation coefficien current. protection are needed. of the resistor and greater surge current protection. relative to the measurement error as a whole. Since the resistance • Thermal time constant � The dissipation constant of a thermistor is the amount of power expressed in (mW/°C) required to self-heat it by 1°C values are high and the change in R values are generally great, 3. When the B value is higher, the final resistance and the Thermal Time Constant τ above ambient temperature. the errors created by measurement and long lead wires can be The thermal time constant is the time in seconds needed for a thermistor to register a change of 63.2% of the difference temperature rise will be less. between the initial temperature of the thermistor and that of its surroundings when subjected to a stepped change in ignored. �� • Zero Power Resistance Rt 4. Generally, the greater the product of the time constant and temperature under zero power conditions. The thermal time constant is the time in seconds needed for �� the dissipation coefficient result in a larger thermal capacity of a thermistor totime register a change of 63.2% of the difference The resistance Zero value measured at the rated temperature using aR25 power level which causes a resistance change that can • Thermal constant � Rated Power Resistance the resistor and greater surge current protection. be ignored relative to the measurement error as a whole. Since the resistance values are high and change in to R the �between is the in direct thermal capacity (C)thermistor of the thermistor and of in inverse ratio to the dissipation coefficient, namely: �� theratio initial temperature of the and that its values are generally great, the errors created by measurement and long lead wires can be ignored. The NTC thermistor is especially suited for use as a temperature sensor due to its hig surroundings subjected to ina seconds steppedneeded change tempera-to register a change of 63.2% of the The thermal timewhen constant is the time forin a thermistor difference �� operating temperature range of –55°C to +300°C, it is ideally suited for measurement a The rated resistance of thermistor which is the zero power resisbetween the initial temperature of the thermistor and that of its surroundings when subjected to a stepped change in ture under zero power conditions. also relatively easy to monitor and of low cost to purchase. tance measured at 25°C and indicated on the thermistor. This is Application Guide for temperature under zero power conditions. 1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop NTC thermistors should be selected according to the following criteria: the most common value used to describe the resistance value of a Temp. Measurement τ is in direct ratio to the thermal capacity (C) of the thermistor • Rated Zero Power Resistance R25 - The required range of temperature and Control 2. Rated resistance of power NTC Thermistor R is: thermistor. - The required range of resistance �and is inindirect ratioratio to thetothermal capacity (C) of the thermistor and in inverse ratio to the dissipation coefficient, namely: inverse the dissipation coefficient, namely: - The required measuring accuracy The rated resistance of thermistor which is the zero power resistance measured at 25°C and indicated on the thermistor. Temperature Measurement and Control The NTC thermistor is - Environment (medium of heat transfer) Basic Characteristic & Application Example This is the most common value used to describe the resistance value of a thermistor. Basic - The expectedsuited time constant especially for use as a temperature sensor due to its high BasicCharacteristic Characteristicand andApplication ApplicationExample Example • Max. Steady State Current Beta Value of Power NTC Thermistor - The dimensions ofofPower PowerNTC NTCThermistor Thermistor levelgeometrical of accuracy. Within the operating temperature range of –55°C The maximum allowable continuous current allowed to pass through the thermistor at 25 deg. C. +300°C, it to is use ideally suited for measurement and control of A to practical circuit for temperature measurement with a NTC thermistor B or beta value is an indication of the slope of the curve which could be a Wheatstone Bridge in which a NTC thermistor forms one leg ofcost the Power PowerLoad-Temperature Load-TemperatureCharacteristic CharacteristicCurve Curve temperature, and is also relatively easy to monitor and of low • Beta Value bridge. Power Load-Temperature Characteristic Curve represents the relationship between the resistance and the to purchase. In the equation: temperature of a particular thermistor measured under zero power If the sensor temperature changes in the balanced bridge circuit, a measurable E is the loop voltage, Im is the surge current B or beta value is an indication of the slope of the curve which represents the relationship between the resistance • Max. Steadyand State State Current Current Max. Steady For conversion power, reversion power, switch power, UPS power Im = 100 times operating current. current will pass through the ammeter. In some cases a variable resistor R3 is Resistance-Temperature Characteristic conditions. Theof higher the thermistor Beta value the greater in the temperature a particular measured underthe zerochange power conditions. The higher the Beta• value the greater the NTCand thermistors be selected according to the For filament, heater, etc. add the loop Im = 30 times operating current. used, according toshould the resistance value of R3 from which wefollowing can infer to the change in resistance per degree C. can calculate the RT2 using this resistance per degree C. You temperature measured (In the balanced state). The maximum allowable continuous currentcurrent allowed allowed to pass through the thermistor at 25 deg. C. criteria: The maximum allowable continuous to pass The R/T characteristic is the relationship between the zero power resistance of the thermistor and its temperature. Since 3. When the B value is higher, the final resistance and the temperature rise will be less. formula: You can calculate the RT2 using this formula: - The required range of temperature this relationship is non-linear described through the thermistor at it25is deg. C. by the R/T curve. Also, NTC thermistors and sensors which are used in conjunction with relays or magn 4. Generally, the greater the product of the time constant and the dissipation coefficient result in a larger thermal capacity - The required range of resistance appropriate alarm and protection equipment and are used in applications requiring tem of the resistor and greater surge current protection. R-T curve of NTC thermistor The required measuring accuracy • Resistance-Temperature Characteristic - Environment (medium of heat transfer) - The expected time constant The R/T characteristic is the relationship between the zero power resistance of the thermistor and its temperature. Since Here: B=3380 T1=25T1=25 RT1=10Kohm Here: B=3380 RT1=10Kohm RT1 - The zero power resis�� - The geometrical dimensions RT1 - The zero power resistance at T1 this relationship is non-linear it is described by the R/T curve. Resistance-Temp. Characteristic tance atzero T1 power RT2resistance - The zero RT2 - The at T2 power resistance at T2 �� R-T curve of NTC thermistor A practical circuit to use for temperature measurement with a Unless otherwise indicated, the B value is calculated using the zero power resistance at 25 deg. C (298.15K) and at 50 is the relationship between the zero The R/T characteristic Unless otherwiseThe indicated, B value is calculated using the deg. C (323.15K). Beta valuethe is not a rigorous constant and is temperature dependant within a small range of of the thermistor and its temperature. Since �� NTC thermistor could be a Wheatstone Bridge in which a NTC power resistance operating temperatures. The NTC thermistor is especially suited for use as a temperature sensor due to its high level of accuracy. Within the Sketch Map of Surge Current Protection in Circuit zero power resistance at 25 deg. C (298.15K) and at 50 deg. C thermistor one leg of this relationship is non-linear it is described by the R/T curve. operating temperature range of –55°C to +300°C, it is ideally suited for measurement and controlforms of temperature, andthe is bridge. Sketch Map of Surge Current Protection in Circuit of Power NTC Thermistor Sketch Map of Surge Current Protection in Circuit of Power NTC Thermistor of Poweralso NTC Thermistor (323.15K). The Beta value is not a rigorous constant and is temrelatively easy to monitor and of low cost to purchase. perature dependant within a small range of operating temperatures. If the sensor temperature R-T curve of NTC thermistor NTC thermistors should be selected according to the following criteria: - The required range of temperature changes in the balanced bridge • Temperature Coefficient of Zero Power Resistance T � - The required range of resistance circuit, a measurable current will Temp. Coefficient of Zero - The required measuring accuracy The temperature coefficient or alpha (symbol) at a specified temperature is the average percent change of the zero power pass through the ammeter. In - Environment (medium of heat transfer) resistance per degreeResistance C to the rated resistance - The expected time constant Power T(R25). some cases a variable resistor - The geometrical dimensions R3 is used, and according to the The temperature coefficient or alpha (symbol) at a specified temA practical circuit to use for temperature measurement with a NTC thermistor resistance value of R3 from which could be a Wheatstone Bridge in which a NTC thermistor forms one leg of the perature is the average percent change of the zero power resiswe can infer to the temperature bridge. tance per degree C to the rated resistance (R25). measured (In the balanced state). Introduction α If the sensor temperature changes in the balanced bridge circuit, a measurable current will pass through the ammeter. In some cases a variable resistor R3 is used, and according to the resistance value of R3 from which we can infer to the temperature measured (In the balanced state). Namely: Namely: Where: Where: � T - The temperature coefficient of the zero power resistance at T RT - The zero power resistance at T - The temperature coefficient of the zero power resistance T -T Temperature B RT- B- value The zero power resistance at T α at T T - Temperature B - B value • Dissipation Coefficient � • Static V-I Characteristic Also, NTC thermistors and sensors which are used in Typical Application- Power NTC Thermistor Circuit conjunction Also, NTC thermistors and sensors which are used in conjunction with relays or magnetic amplifier loops of the with relays or magnetic amplifier loops �� appropriate alarm and protection equipment and are used in applications requiring temperature control. When the of the appropriate alarm and protection equipment and are used in applications requiring temperature control. When the temperature changes, the resistance of the NTC thermistor will also change, which will cause the bridge circuitry to become unbalanced and a current will pass through the control circuit which sense the current, so the temperature in the controlled area will be adjusted. �� The dissipation coefficient is the ratio of the rate of change of the power consumption of a thermistor to �� the change of it’s �� corresponding temperature, namely: �� The value of � will change for different ambient temperatures and transfer mediums and should be used for reference �� 1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop 1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop 2. Rated resistance of power NTC Thermistor R is: 2. Rated resistance of power NTC Thermistor R is: �� Linearization of the NTC Thermistor Characteristic Curve �� The change in the resistance of a NTC thermistor is �� remarkably non-linear. If a nearly linear resistance �� curve is required while measuring a wide range of �� temperature, such as in a dial thermostat, a resistor �� in�� will �� connected series in ��parallel provide an�� or �� of �� approximation linearity however the temperature �� range�� exceeds� 50 to�� 100 Kelvin. �� a) Linearization of NTC thermistor by paralleled resistor ��b)�� Signal linearization circuitry �� The �� will �� voltage Ve and power consumption Pv of�� a linearized NTC NTC��thermistor reduce the �� thermistor �� NTC of an accuracy. �� The R/T curve of linearization of a NTC thermistor by means of a �� parallel connection of a�� resistor �� The combination of a NTC thermistor and a resistor connected in parallel will produce an S shaped characteristic curve. The best linearization will be obtained if the inflection is placed in the middle �� of the operating temperature range. Under these conditions, the �� resistance of the connection be approximated by �� parallel � �� can �� 1)�� Sample of a simple �� amplifier circuit applying an exponent: �� voltage �� at the load resistor R(sub L) as a function of �� 2) The output �� the temperature �� HOW TO CHOOSE AND WORK WITH NTC THERMISTORS Advantages of the NTC thermistor �� The NTC thermistor and temperature sensor compared with other �� sensors in temperature measurement and control applications: �� The resistance of which are in parallel connection is: �� RT, ��RP �� 1) Reliable performance; �� 2) High precision, Good tolerances and interchangeability; 3) Large temperature coefficient of resistance, High accuracy 4) Low cost, especially for middle-or-low temperature measure�� ment and control. �� at �� In ��the equation: RTM is�� the NTC�� thermistor resistance average �� 5) High dissipation coefficient: Test current can be greater than of �� temperature TM�� B is�� the B value of NTC�� thermistor��(linear) slope �� of�� traditional sensors, simplified circuitry. �� curve: �� of��the characteristic �� Application Notes: �� of�� the �� 1. Please supply�� all characteristics the application. Include resistance and tolerance, B value, dimensions, length of wire and application temperature range etc. 2. If you are not certain of the characteristics, please provide the following data: 1) Purpose, application details 2) Environmental conditions 3) Range of temperature measurement and control 4) Dimensions 5) Testing power 6) The zero power resistance and errors at two or more temperatures 3. Insulation and housings can be added according to the requirement of users high-dissipation coefficient. Test current can be far larger that that of an other type of sensor which will simplify the circuitry. Special builds are available according to your requirements (characteristics, dimensions and wire) WARNINGS 1. Avoid sudden changes of ambient temperature of the thermistors and temperature sensors, this could cause premature aging. 2. Excess current passing through the thermistor will cause the components to self-heat and result in a variation of the temperature reading. This factor should be considered before selecting. (When the heat of the component is 1/ 10 of dissipation coefficient (mW/°C) the temperature variance will be 0.1°C., when it is 1/100 of diss.coef. the temperature difference will be 0.01°C) 3. The excess current caused by bad insulation, electrostatic induction, poor contacts to circuitry will damage the thermistor. Pay particular attention to the method of connection, and that too much current is not allowed to pass through thermistor. 4. Measurements should be taken only after 5 to 7 seconds. 5. Small size and short time constant should be selected if the application requires fast response and high precision. 6. If water, dust or ionic compounds are between the ends of the lead wire or on the surface of insulation, the resistance will decline and become unstable causing a difference in the temperature reading. Moisture protection and insulation precautions should be taken to insure dryness. 2010/Sep NTC 8415 Mountain Sights Avenue • Montreal (Quebec), H4P 2B8, Canada Tel: (514) 739-3274 • 1-800-561-7207 • Fax: (514) 739-2902 E-mail: sales@cantherm.com Website: www.cantherm.com | Division of Microtherm

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