ELECTROTECHNOLOGY III

1
ELECTROTECHNOLOGY III

• Farah Khalid
• TEMPERATURE TRANSDUCERS

2
TRANSDUCERS

• What is a transducer ?
• A transducer is an electronic device that
  converts energy from one form to another.
• Common examples include microphones,
  loudspeakers, thermometers, position and pressure
  sensors. Although not generally thought of as
  transducers, photocells, LEDs (light-emitting
  diodes), and even common light bulbs are
  transducers.
• Efficiency is an important consideration in any
  transducer. Transducer efficiency is defined as
  the ratio of the power output in the desired form
  to the total power input. Mathematically, if P
  represents the total power input and Q represents
  the power output in the desired form, then the
  efficiency E, as a ratio between 0 and 1, is
  given by
• E Q/P

3
TEMPERATURE

• Resistance is a function of temperature.
• Thermistor.
• Block of semiconductor material.
• Resistance Temperature Device (RTD)
• Strip of metal.
• Thermocouple.
• Potential across two metals is a function of
  temperature.

4

• THERMISTOR
• Temperature range -100 C to 200 C
• ConstructionBlock of semiconductor material
  (without junction).
• Principle of Operation
• As with all semiconductors
• electron energy levels divided into two bands
  valence and conduction.
• Electrons in conduction band participate in
  current flow.
• Electrons in valence band do not
• The no. of electrons in conduction band increases
  with temperature, reducing resistance.
• Resistance is determined by the density of
  conduction electrons.
• Desirable Characteristics
• SensitiveSmall change in temperature yields an
  easily readable change in resistance
• Can be made very small Small devices react to
  temperature changes quickly
• High resistance Easier to design conditioning
  circuit
• Undesirable Characteristics
• Delicate Can be damaged (de-calibrated) by
  excessive heat.

5

• RESISTANCE TEMPERATURE DEVICE (RTD)
• Temperature range about 200C to 750C
  (Platinum).
• Construction Transducer is a metal, usually
  platinum. Metal is wound into a long coil or
  printed on a ceramic substrate in a serpentine
  pattern.
• Principle of Operation
• In normal operation a current is flowing through
  RTD.
• As the temperature increases, electrons collide
  more frequently with metal atoms increasing
  resistance.
• Desirable Characteristics
• Accurate. (An RTD used to define part of
  ITS-90.)
• Stable.
• Wide temperature range.
• Reasonably linear.
• Available in standard types.
• Undesirable Characteristics
• Low sensitivity Small resistance change with
  temperature.
• Expensive.

6

• THERMOCOUPLES
• Temperature range about 270C to 1820C
• Construction
• Two metals joined (e.g., welded together).
• Transducer is junction (weld).
• Leads, connecting to metals, specially
  constructed.
• Principle of Operation
• When dissimilar metals joined, an EMF develops
  across the junction. (As
• in semiconductor PN junctions.)
• Strength of EMF is a function of metals used and
  junction temperature.
• Temperature is determined by measuring voltage.
• Measuring this voltage is not as easy as one
  might think
• Desirable Characteristics
• Very wide temperature range freezer to furnace.
• Rugged, Accurate.
• Highly standardized.
• No self-heating Conditioning circuit does not
  warm transducer.
• Undesirable Characteristics
• Difficult to measure voltage.

7

• There are three thermoelectric effects that play
  a large part in the generation of electricity by
  way of temperature
• Seebeck effect Named after Thomas Seebeck
  (German physicist). He fused two dissimilar metal
  wires together on both of their ends. He then
  heated one of the junctions and found that
  electrical current flowed from one wire to the
  other. He caused electrons to flow from a copper
  wire to an iron wire. This effect developed into
  what we know now as the thermocouple.
• Peltier effect Named after Jean Peltier (French
  physicist). He applied current to two dissimilar
  metal wires attached together at their ends. As
  electrons moved from the copper wire to the iron
  wire that junction became warm (hot). As the
  electrons moved from the iron wire to the copper
  wire, that junction became cool. The reasoning is
  that electrons moving from a higher energy state
  (iron) to a lower energy state (copper) create an
  excess of energy, therefore heat.
• Faraday effect Named after Michael Faraday found
  through experimentation that in certain materials
  resistance is decreased as temperature increases
  that is, they have a negative temperature
  coefficient. The resistance of oxides such as
  cobalt, manganese, and nickel is decreased when
  they are heated.
• Thermoelectric materials are often chosen for
  their ability to provide a uniform
  voltage-temperature relationship.