User talk:Phampie2

ABSTRACT A thermistor is a type of resistor whose resistance varies with temperature. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements. Thermistors THERMal resISTORS Thermistors are special solid temperature sensors that behave like temperature-sensitive electrical resistors. No surprise then that their name is a contraction of "thermal" and "resistor". There are basically two broad types, NTC-Negative Temperature Coefficient, used mostly in temperature sensing and PTC-Positive Temperature Coefficient, used mostly in electric current control. There's even more history of the name and development of thermistors and facts about some key NTC parameters at the Kele Electronics website, just be prepared for some strong opinions about one brand of thermistor. They are mostly very small bits of special material that exhibit more than just temperature sensitivity. They are highly-sensitive and have very reproducible resistance Vs. temperature properties. During the last 60 years or so, only ceramic materials (a mix of different metal oxides) was employed for production of NTC thermistors. In 2003, AdSem, Inc. (Palo Alto, CA) developed and started manufacturing of Si and Ge high temperature NTC thermistors with better performance than any ceramic NTC thermistors. Thermistors, since they can be very small, are used inside many other devices as temperature sensing and correction devices as well as in specialty temperature sensing probes for commerce, science and industry. Some of those new-fangled digital medical thermometers that get stuck in one's mouth by a nurse with an electronic display in her other hand are based on thermistor sensors. They are probably inside your cell phone, automobile, stereo and television, too, but you'd never know it unless you were an engineer or visited here. Thermistors typically work over a relatively small temperature range, compared to other temperature sensors, and can be very accurate and precise within that range, although not all are. Thermistor Terminology A glossary slightly modified from that given in a US government publication: MIL-PRF-23648D. Note that the term being described is in bold typeface. A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the voltage drop produced. The standard reference temperature is the thermistor body temperature at which nominal zero-power resistance is specified, usually 25°C.

The zero-power resistance is the dc resistance value of a thermistor measured at a specified temperature with a power dissipation by the thermistor low enough that any further decrease in power will result in not more than 0.1 percent (or 1/10 of the specified measurement tolerance, whichever is smaller) change in resistance. The resistance ratio characteristic identifies the ratio of the zero-power resistance of a thermistor measured at 25°C to that resistance measured at 125°C. The zero-power temperature coefficient of resistance is the ratio at a specified temperature (T), of the rate of change of zero-power resistance with temperature to the zero-power resistance of the thermistor.

A NTC thermistor is one in which the zero-power resistance decreases with an increase in temperature.

A PTC thermistor is one in which the zero-power resistance increases with an increase in temperature.

The maximum operating temperature is the maximum body temperature at which the thermistor will operate for an extended period of time with acceptable stability of its characteristics. This temperature is the result of internal or external heating, or both, and should not exceed the maximum value specified. . The maximum power rating of a thermistor is the maximum power which a thermistor will dissipate for an extended period of time with acceptable stability of its characteristics.

The dissipation constant is the ratio, (in milliwatts per degree C) at a specified ambient temperature, of a change in power dissipation in a thermistor to the resultant body temperature change.

The thermal time constant of a thermistor is the time required for a thermistor to change 63.2 percent of the total difference between its initial and final body temperature when subjected to a step function The resistance-temperature characteristic of a thermistor is the relationship between the zero-power resistance of a thermistor and its body temperature.

The temperature-wattage characteristic of a thermistor is the relationship at a specified ambient temperature between the thermistor temperature and the applied steady state wattage.

The current-time characteristic of a thermistor is the relationship at a specified ambient temperature between the current through a thermistor and time, upon application or interruption of voltage to it.

The stability of a thermistor is the ability of a thermistor to retain specified characteristics after being subjected to designated environmental or electrical test conditions. Thermistors are thermally sensitive resistors and have, according to type, a negative (NTC), or positive (PTC) resistance/temperature coefficient. Thermometrics product portfolio comprises a wide range of both types. Manufactured from the oxides of the transition metals - manganese, cobalt, copper and nickel, NTC thermistors are temperature dependant semiconductor resistors. Operating over a range of -200°C to + 1000°C, they are supplied in glass bead, disc, chips and probe formats. NTCs should be chosen when a continuous change of resistance is required over a wide temperature range. They offer mechanical, thermal and electrical stability, together with a high degree of sensitivity. The excellent combination of price and performance has led to the extensive use of NTCs in applications such as temperature measurement and control, temperature compensation, surge suppression and fluid flow measurement. PTC thermistors are temperature dependent resistors manufactured from barium titanate and should be chosen when a drastic change in resistance is required at a specific temperature or current level. PTCs can operate in the following modes: •	Temperature sensing, switching at temperatures ranging from 60°C to 180°C, e.g. protection of windings in electric motors and transformers. •	Solid state fuse to protect against excess current levels, ranging from several mA to several A (25°C ambient) and continuous voltages up to 600V and higher, e.g. power supplies for a wide range of electrical equipment. •	Liquid level sensor. The unique patented design Composite Thermistor contains 2 NTC and 1 PTC thermistors and has a resistance temperature characteristic similar to a single NTC but with a region of constant resistance. Designed for driving automotive coolant temperature gauges, the composite sensor resistance is virtually constant over a specified range, which results in a steady centre dial gauge reading during normal engine operation. Hot and cold zone sensitivity are retained, so that motorists are warned of abnormal conditions. With careful selection of the plateau region, the same type of device can be used in a wide variety of operation systems, so that the production economics are compatible with the requirements of our automotive customers. Composite Thermistors can be custom-designed to match the electrical and thermal characteristics of gauges and probe housings. 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. Benefits of Using a Thermistor Accuracy Thermistors are one of the most accurate types of temperature sensors. OMEGA thermistors have an accuracy of ±0.1°C or ±0.2°C depending on the particular thermistor model. However thermistors are fairly limited in their temperature range, working only over a nominal range of 0°C to 100°C.

Stability Finished thermistors are chemically stable and not significantly affected by aging. Types of Thermistors Thermistor Elements The thermistor element is the simplest form of thermistor. Because of their compact size, thermistor elements are commonly used when space is very limited. OMEGA offers a wide variety of thermistor elements which vary not only in form factor but also in their resistance versus temperature characteristics. Since thermistors are non-linear, the instrument used to read the temperature must linearize the reading. 44000 Series Thermistor Element Linear Response Thermistor Elements For applications requiring thermistors with linear response to temperature change, OMEGA offers linear components. These unique devices consist of a thermistor composite for temperature sensing and an external resistor composite for linearizing. Linear Response Thermistor Thermistor Probes The standalone thermistor element is relatively fragile and can not be placed in a rugged environment. OMEGA offers thermistor probes which are thermistor elements embedded in metal tubes. Thermistor probes are much more suitable for industrial environments than thermistor elements. TJ36 Thermistor Probe