A Report By Joseph Magaro

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• A Report By Joseph Magaro

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Thermistor Basics

• Thermally Sensitive Resistors
• According to type, a negative (NTC), or positive
  (PTC) resistance/temperature coefficient.
• Operating over a range of -200C to 1000C
• Supplied in glass bead, disc, chips and probe
  formats

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Thermistors Basics
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Thermistor Basics
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Thermistor History

• Michael Faraday (1791-1867), the British chemist
  and physicist, in his less familiar 1833 report
  on the semiconducting behavior of Ag2S (silver
  sulfide), discovered the first considered
  thermistor.
• Early 1940s, Bell Telephone Laboratories
  developed techniques to improve the consistency
  and repeatability of the manufacturing process of
  Thermistors

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Thermistor History

• Some of the first commercial thermistors were the
  disc type which were used primarily for
  regulation, protection, and temperature
  compensation of electronic circuits.

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Thermistor Construction

• Thermistors are made from combinations of metal
  oxides of manganese, nickel, cobalt, copper,
  iron, and titanium.
• A mixture of milled semiconductor oxide powders
  and a binder are first shaped into the desired
  geometry.
• The mixture is dried and sintered (under
  pressure) at an elevated temperature.
• Lastly, the wire leads are attached, and the
  combination is coated with glass or epoxy.

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Thermistor Construction
The mechanical cut-out of a thermistor
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PTC Thermistors

• PTC thermistors are temperature dependent
  resistors manufactured from barium titanate
• Should be chosen when a drastic change in
  resistance is required at a specific temperature
  or current level.

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NTC Thermistors

• Temperature dependant semiconductor resistors.
• 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.

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NTC Thermistors

• NTCs should be chosen when a continuous change of
  resistance is required over a wide temperature
  range.
• Offer mechanical, thermal and electrical
  stability, together with a high degree of
  sensitivity.

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Composite Thermistor

• Composite Thermistors can be custom-designed to
  match the electrical and thermal characteristics
  of gauges and probe housings.
• The unique patented design Composite Thermistor
  contains 2 NTC and 1 PTC Thermistor 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

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Temperature vs. Resistance

• Two fundamental ways to change temperature
  Internal and External
• A simple change in current will result in an
  Internal change in temperature.
• External change in temperature would require
  changing the temperature of the surrounding or
  immersing the device in hot or cold solution.

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Temperature vs. Resistance
This graph illustrates the R/T characteristics of
some typical NTC thermistors and a platinum RTD.
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Thermistor in Action

• Power Tr heats up by increasing its output.
  Continuous operation beyond the specified
  temperature may damage the Tr. POSISTOR",
  connected to the Tr by thermal contact, will
  prevent damage to the Tr by limiting the Base
  current by increasing the "POSISTOR" resistance.
  When the temperature of the Tr decreases, the
  resistance returns to the initial value.

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Bibliography

• Electronic Devices and Circuit Theory 6th Edition
  by Boylestad and Nashelsky.
• www.murata.com
• www.globalspec.com

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Thank You