Download 640251Lecture18Temperature

640251 Lecture 18
• Temperature Sensors
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Thermistor,
Diode
ThermoCouple
Pyroelectric Sensors
LM335 – IC Kelvin calibrated linear temperature
sensor
Electronic Sensors
• Convert a physical property (Eg pressure,
temperature, distance, angle, speed, time)
into an analogue or digital voltage
representing the measured quantity.
• We convert analog voltages to a digital
signal for convenient numerical
manipulation in a computer or
microcontroller.
Electronic Thermometer
Temperature Sensors
• 4 Main Types of temperature sensor:
– CONTACT BASED – sensor must be in thermal
contact with device being measured
• Thermistor
• Diode characteristic based
– Example LM335Z IC temperature sensor
• Thermocouple or Thermopile
– Non-Contact based
• Pyroelectric Sensors
Thermistors
• 2 Main classes:
– PTC – Positive Temperature Coefficient
– NTC – Negative Temperature Coefficient
PTC Thermistors
(1)
– PTC – Positive Temperature Coefficient
• Fault-Current Limiting
– Application Example:
– PTC can be used as series protection element in telephone
line equipment
– Protects against accidental mains connections faults
– Protects against induced currents during lightning strikes
– Telephone or Modem Device may go immediately back
into service with no damage (providing the protection
devices were properly chosen and operated correctly).
PTC Thermistors
(2)
– PTC – Positive Temperature Coefficient
• Used for temperature sensing
• ΔR = kΔT as a first approximation.
• The above is often insufficiently accurate so
quadratic or cubic approximation is needed.
• Alternatively – linear interpolation using tables of
values can result in accurate temperature conversion
• For tables and examples see datasheets:
http://www.murata.com/catalog/r44e9.pdf
http://en.wikipedia.org/wiki/Thermistor
NTC Thermistors
– NTC – Negative Temperature Coefficient
– NTC THERMISTOR – applications
• Automatic LCD contrast control
• Inrush current limiting
– After a device is switched on, the thermistor input resistance is
high and the current limiter cold.
– The device power supply capacitors need to charge up, high
current flows.
– NTC heats up, and allows a greater current to flow.
– Effectively prevents initial massive surge current.
– Disadvantage is the device is always hot during operation – thus
power is lost and maximum efficiency is not achieved – unless
the device is shorted out…
Diode Junction based thermometer
(1)
Diode Junction based thermometer
Diode Equation:
I = current through diode
Is saturation current
q = magnitude of electron charge
V = applied voltage
k = Boltzmans constant
T = temperature (Kelvin)
(2)
Diode Junction based thermometer
Rearrange and solve for constant current:
Eg = Energy gap of silicon at 0 Kelvin
So diode voltage is linearly related to Temperature.
Diode based temperature transducers operate over the
temperature range -40C to 160C. Temperatures in
excess of 160 may cause the device or packaging to fail.
(Remember solder melts at approx 180 degrees C)
(3)
Diode Junction based thermometer
(4)
With a constant current source across our diode, we have a
Negative Slope of Diode Voltage drop versus Temperature
Thermocouple
(1)
• A thermocouple is a common type of temperature
sensor used in electronics and control systems.
• Thermocouples are contact based temperature
difference sensors that create a voltage difference.
• The Seebeck (Thermo electric) Effect occurs
where different metals are in contact with each
other.
• A common example is an Iron/Copper connection.
• The difference in fermi levels causes current to
flow until the electron energy levels equalise.
Thermocouple
(2)
• Using microcontrollers Cold junction
compensation can be performed or
improved using:
– look-up tables and linear interpolation, or
– approximation using polynomial interpolation.
• There are many types of Standard
thermocouple with varying accuracy and
temperature ranges.
Type K Thermocouple
(1)
Type K Chromel-Alumel is the most commonly used general
purpose thermocouple.
It is inexpensive and, owing to its popularity, available in a
wide variety of probes.
They are available in the −200 °C to +1350 °C range.
The type K was specified at a time when metallurgy was less
advanced than it is today and, consequently, characteristics
vary considerably between thermocouples.
The characteristic of the thermocouple undergoes a step
change when a magnetic material reaches its Curie Point.
This occurs for this thermocouple at 354°C. (Curie point –
temperature that the metal loses its ferromagneticity – becomes paramagnetic)
Type K Thermocouple
(2)
• Sensitivity is approximately 41 µV/°C.
• Suitable for use in gas and liquid, both with very good
accuracy.
• Typical Accuracy:
• ±1.5 ° C
−40 °C to 375 °C
• ±0.004×T between 375 °C and 1000 °C
• A potential problem arises in some situations since one of
the constituent metals, Nickel, is magnetic.
• Using a Type K thermocouple within a strong alternating
magnetic field would induce sensor vibration (cause metal
fatigue failure) and electrical noise pickup. (Need to shield
the wires to decrease induced voltages and currents
Thermocouple circuit
• Example: Thermocouple circuit with cold
junction compensation
InfraRed Thermometer
(1)
• Infrared thermometers measure temperature
using (infrared) radiation emitted from objects.
They are sometimes called laser thermometers if
a laser is used to help aim the thermometer, or
non-contact thermometers to describe the
device’s ability to measure temperature from a
distance. By knowing the amount of infrared
energy emitted by the object and its emissivity the
object's temperature can be determined.
InfraRed Thermometer
(2)
• The most basic design consists of a lens to
focus the infrared energy on to a detector,
which converts the energy to an signal that
can be displayed in units of temperature
after being compensated for ambient
temperature variation.
• This configuration enables temperature
measurement from a distance without
contact with the object to be measured.
InfraRed Thermometer
(3)
• Some typical circumstances are where the
object to be measured is moving;
– where the object is surrounded by an
electromagnetic field, as in induction heating;
– where the object is contained in a vacuum or
other controlled atmosphere;
– or in applications where a fast response is
required
– Detecting clouds for remote telescope operation
Black Body Radiation
(1)
Planks Law:
The higher the temperature,
higher the peak frequency
(the shorter the peak
wavelength) radiated.
Strip radiators look red
Incandescent lamps white
High temperature halogen
Car headlamps blue white
Black Body Spectrum
Black Body Radiation
(2)
• Contact free IR thermometers need to be
calibrated using black body radiation sources.
•
See also: www.uni.edu/morgans/ajjar/Astrophysics/blackbody.html
InfraRed Thermal Imaging
• Checking mechanical equipment or electrical
circuit boards, circuit breaker boxes or outlets for
hot spots
• Checking heater or oven temperature, for
calibration and control purposes
• Checking for hot spots in fire fighting situations
• Monitoring materials in process of heating and
cooling, for research and development or
manufacturing quality control situations (eg is the
transistor thermally bonded to its heatsink?)
• Checking for avian flu symptoms at airports
LM335Z – Kelvin IC sensor
If we want to measure temperatures
from -40C to 160C, the anticipated
voltage across this sensor is 2.33V to
4.33V. The datasheet recommends
current from 400uA to 4mA; So we
design for about 500uA at 160C.
If V+ is 5V, and our output is 4.5V, then
0.5V over a 1K ohm resistor will
produce 500uA. At -40C, current will be
(5 – 2.33)/1000 = 2.67mA – within the
recommended operating range 
Acknowledgements
• Geoff Taylor
• Paul Main - sea.net.au, May 2008