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Encyclopedia > Thermistor
NTC thermistor, bead type, insulated wires
NTC thermistor, bead type, insulated wires
Thermistor symbol
Thermistor symbol

A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. The word is a combination of thermal and resistor. Samuel Ruben invented the thermistor in 1930, and was awarded U.S. Patent No. 2,021,491. Image File history File links NTC_bead. ... Image File history File links NTC_bead. ... Image File history File links Thermistor_symbol. ... Image File history File links Thermistor_symbol. ... Resistor symbols (non-European) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ... For other uses, see Temperature (disambiguation). ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... Thermal can mean: Clothing worn in extreme cold to conserve body heat Exothermic reaction in chemistry Heat Thermal, California, a small town near Indio, California Thermal neutron Thermal power station, a power station that produces electricity using heat Thermistor, a type of resistor used to measure temperature Thermodynamics or Thermal... Resistor symbols (non-European) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ... Samuel Ruben (1900-1988): made lasting contributions to Electrochemistry and solid-state technology. ...


Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then: For other uses, see Linear (disambiguation). ...

ΔR = kΔT

where

ΔR = change in resistance
ΔT = change in temperature
k = first-order temperature coefficient of resistance

Thermistors can be classified into two types depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have the smallest possible k, so that their resistance remains nearly constant over a wide temperature range. The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K (kelvin). ... A negative temperature coefficient (NTC) occurs when the thermal conductivity of a material rises with increasing temperature, typically in a defined temperature range. ...


Thermistors differ from resistance temperature detectors in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges. A resistance temperature detector measures the relationship between electrical resistance and temperature. ...

Contents

Steinhart-Hart equation

In practice, the linear approximation (above) works only over a small temperature range. For accurate temperature measurements, the resistance/temperature curve of the device must be described in more detail. The Steinhart-Hart equation is a widely used third-order approximation: The Steinhart-Hart equation is a model of the resistivity of a semiconductor at different temperatures. ...

frac{1}{T}=a+b,ln(R)+c,ln^3(R)

where a, b and c are called the Steinhart-Hart parameters, and must be specified for each device. T is the temperature in kelvins and R is the resistance in ohms. To give resistance as a function of temperature, the above can be rearranged into: For other uses, see Kelvin (disambiguation). ... The ohm (symbol: Ω) is the SI unit of electric resistance. ...

R=e^{{left( beta-{alpha over 2} right)}^{1over 3}-{left( beta+{alpha over 2} right)}^{1over 3}}

where

alpha={{a-{1over T}}over c} and beta=sqrt{{{{left({bover{3c}}right)}^3}+{{alpha^2}over 4}}}

The error in the Steinhart-Hart equation is generally less than 0.02°C in the measurement of temperature. As an example, typical values for a thermistor with a resistance of 3000 Ω at room temperature (25°C = 298.15 K) are:

a = 1.40 times 10^{-3}
b = 2.37 times 10^{-4}
c = 9.90 times 10^{-8}

B parameter equation

NTC thermistors can also be characterised with the B parameter equation, which is essentially the Steinhart Hart equation with c=0.

frac{1}{T}=frac{1}{T_0} + frac{1}{B}ln left(frac{R}{R_0}right)

where the temperatures are in kelvin. Using the expansion only to the first order yields: For other uses, see Kelvin (disambiguation). ...

R=R_0e^{Bcdot (1/T-1/T_0)}

or

R=r_infty e^{B/T}

or

T={Bover { {ln{(R / r_infty)}}}}

where

R0 is the resistance at temperature T0 (usually 25 °C=298.15 K)
r_infty=R_0 cdot e^{-{B/T_0}}

Conduction model

Many NTC thermistors are made from a pressed disc or cast chip of a semiconductor such as a sintered metal oxide. They work because raising the temperature of a semiconductor increases the number of electrons able to move about and carry charge - it promotes them into the conducting band. The more charge carriers that are available, the more current a material can conduct. This is described in the formula: This article is about the manufacturing process. ... A semiconductor is a solid whose electrical conductivity is in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ... This article or section does not cite its references or sources. ... An oxide is a chemical compound containing at least one oxygen atom and other elements. ... For other uses, see Electron (disambiguation). ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... In electricity, current refers to electric current, which is the flow of electric charge. ...

 I = n cdot A cdot v cdot e

I = electric current (ampere)
n = density of charge carriers (count/m³)
A = cross-sectional area of the material (m²)
v = velocity of charge carriers (m/s)
e = charge of an electron (e=1.602 times 10^{-19} coulomb)


The current is measured using an ammeter. Over large changes in temperature, calibration is necessary. Over small changes in temperature, if the right semiconductor is used, the resistance of the material is linearly proportional to the temperature. There are many different semiconducting thermistors sizes ranging from about 0.01 kelvin to 2,000 kelvins (-273.14°C to 1,700°C). Wire carrying current to be measured Spring providing restoring force An ammeter is a measuring instrument used to measure the flow of electric current in a circuit. ... For other uses, see Kelvin (disambiguation). ...


Most PTC thermistors are of the "switching" type, which means that their resistance rises suddenly at a certain critical temperature. The devices are made of a doped polycrystalline ceramic containing barium titanate (BaTiO3) and other compounds. The dielectric constant of this ferroelectric material varies with temperature. Below the Curie point temperature, the high dielectric constant prevents the formation of potential barriers between the crystal grains, leading to a low resistance. In this region the device has a small negative temperature coefficient. At the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply. At even higher temperatures, the material reverts to NTC behaviour. The equations used for modeling this behaviour were derived by W. Heywang and G. H. Jonker in the 1960s. Fixed Partial Denture, or Bridge The word ceramic is derived from the Greek word κεραμικός (keramikos). ... Barium titanate is an oxide of barium and titanium with the chemical formula BaTiO3. ... The relative dielectric constant of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. ... In physics, the ferroelectric effect is an electrical phenomenon whereby certain ionic crystals may exhibit a spontaneous dipole moment. ... The Curie point is a term in physics and materials science, named after Pierre Curie (1859-1906), and refers to a characteristic property of a ferromagnetic material. ... The relative dielectric constant of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. ...


Another type of PTC thermistor is the polymer PTC, which is sold under brand names such as "Polyfuse", "Polyswitch" and "Multiswitch". This consists of a slice of plastic with carbon grains embedded in it. When the plastic is cool, the carbon grains are all in contact with each other, forming a conductive path through the device. When the plastic heats up, it expands, forcing the carbon grains apart, and causing the resistance of the device to rise rapidly. Like the BaTiO3 thermistor, this device has a highly nonlinear resistance/temperature response and is used for switching, not for proportional temperature measurement. A polymer (from Greek: πολυ, polu, many; and μέρος, meros, part) is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. ... A PolySwitch™ is a Polymer form of Positive temperature coefficient resistor (PPTC) which acts as a self repairing fuse. ... For other uses, see Carbon (disambiguation). ... For other uses, see Plastic (disambiguation). ... In science and engineering, conductors, such as copper or aluminum, are materials with atoms having loosely held valence electrons. ...


Self-heating effects

When a current flows through a thermistor, it will generate heat which will raise the temperature of the thermistor above that of its environment. If the thermistor is being used to measure the temperature of the environment, this self-heating effect will introduce an error if a correction is not made.


The electrical power input to the thermistor is just

P_E=IV,

where I is current and V is the voltage drop across the thermistor. This power is converted to heat, and this heat energy is transferred to the surrounding environment. The rate of transfer is well described by Newton's law of cooling: Heat flow along perfectly insulated wire Conduction is the transfer of heat or electric current from one substance to another by direct contact. ...

P_T=K(T(R)-T_0),

where T(R) is the temperature of the thermistor as a function of its resistance R, T0 is the temperature of the surroundings, and K is the dissipation constant, usually expressed in units of milliwatts per °C. At equilibrium, the two rates must be equal.

P_E=P_T,

The current and voltage across the thermistor will depend on the particular circuit configuration. As a simple example, if the voltage across the thermistor is held fixed, then by Ohm's Law we have I = V / R and the equilibrium equation can be solved for the ambient temperature as a function of the measured resistance of the thermistor: A voltage source, V, drives an electric current, I , through resistor, R, the three quantities obeying Ohms law: V = IR Ohms law states that, in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference (i. ...

T_0=T(R) -frac{V^2}{KR},

The dissipation constant is a measure of the thermal connection of the thermistor to its surroundings. It is generally given for the thermistor in still air, and in well-stirred oil. Typical values for a small glass bead thermistor are 1.5 mW/°C in still air and 6.0 mW/°C in stirred oil. If the temperature of the environment is known beforehand, then a thermistor may be used to measure the value of the dissipation constant. For example, the thermistor may be used as a flow rate sensor, since the dissipation constant increases with the rate of flow of a fluid past the thermistor.


Applications

  • PTC thermistors can be used as current-limiting devices for circuit protection, as replacements for fuses. Current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating. This creates a self-reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device.
  • PTC thermistors can be used as heating elements in small temperature-controlled ovens. As the temperature rises, resistance increases, decreasing the current and the heating. The result is a steady state. A typical application is a crystal oven controlling the temperature of the crystal of a high-precision crystal oscillator. Crystal ovens are usually set at the upper limit of the equipment's temperature specification, so they can maintain the temperature by heating.
  • NTC thermistors are used as resistance thermometers in low-temperature measurements of the order of 10 K.
  • NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purpose designed for this application.
  • Thermistors are also commonly used in modern digital thermostats and to monitor the temperature of battery packs while charging.

200 A Industrial fuse. ... A Crystal Oven is a temperature-controlled chamber used to maintain constant temperature of electronic crystals, in order to ensure stability of operation. ... Resistance thermometers, also called resistance temperature detectors (RTDs), are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. ...

References

  • Measurement Specialties thermistor technical data sheet: Temperature

Manufacturers

  • Measurement Specialties
  • Thermometrics-GE
  • Sensor Scientific, Inc.
  • Blackbeads Electronics, Inc. - Philippines
  • Betatherm
  • Ametherm, Inc.
  • Quality Thermistor

See also

A voltage source, V, drives an electric current, I , through resistor, R, the three quantities obeying Ohms law: V = IR Ohms law states that, in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference (i. ... Resistor symbols (non-European) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ... In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. ... Bi-metallic thermostat for buildings A thermostat is a device for regulating the temperature of a system so that the systems temperature is maintained near a desired setpoint temperature. ... A resistance temperature detector measures the relationship between electrical resistance and temperature. ...

External links

  • Thermistor App Note
  • Thermistor ADC using Parallel Port
  • The thermistor at bucknell.edu
  • Measurement Specialties thermistor technical data sheets

  Results from FactBites:
 
Thermistors and Thermistor Elements and Thermistor Probes (242 words)
A thermistor is a temperature-sensing element composed of sintered semiconductor material which exhibits a large change in resistance proportional to a small change in temperature.
Thermistors usually have negative temperature coefficients which means the resistance of the thermistor decreases as the temperature increases.
Finished thermistors are chemically stable and not significantly affected by aging.
Thermistors, Probes, PTC and NTC temperature sensors for military and commercial applications, by Quality Thermistor ... (165 words)
Thermistors, Probes, PTC and NTC temperature sensors for military and commercial applications, by Quality Thermistor Inc.
QTI has an extensive library of thermistor specifications as well as a complete test lab that can be used to test and identify hard to find or obsolete parts.
Quality Thermistor is your source for axial leaded, radial leaded and surface mount NTC & PTC thermistors.
  More results at FactBites »

 
 

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