640251 Lecture 18 Temperature Sensors Thermistor Diode Thermo

640251 Lecture 18 • Temperature Sensors – Thermistor, – Diode – Thermo. Couple – Pyroelectric Sensors – LM 335 – 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 LM 335 Z 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/r 44 e 9. 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, 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 -40 C to 160 C. 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 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

Infra. Red 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 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.

Infra. Red 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.

Infra. Red 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

Infra. Red 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

LM 335 Z – Kelvin IC sensor If we want to measure temperatures from -40 C to 160 C, the anticipated voltage across this sensor is 2. 33 V to 4. 33 V. The datasheet recommends current from 400 u. A to 4 m. A; So we design for about 500 u. A at 160 C. If V+ is 5 V, and our output is 4. 5 V, then 0. 5 V over a 1 K ohm resistor will produce 500 u. A. At -40 C, current will be (5 – 2. 33)/1000 = 2. 67 m. A – within the recommended operating range

Acknowledgements • Geoff Taylor • Paul Main - sea. net. au, May 2008