ELECTRONIC INSTRUMENTATION EKT 314/4 WEEK 4 : CHAPTER 2 TRANSDUCERS

Contents Definition of Transducer Type of Transducers Electrical Transducers Parameters Advantage/Disadvantages Classification Application Selection of Transducers Examples of Transducers

Photoelectric Transducers Primary types Photoemissive Cell Photoconductive Cell Photovoltaic Cell Photojunctions

Photoemissive Cell Electron emission due to the incident radiation upon photo emissive surface. Also known as phototube Three basic types of phototube: Vacuum Gas-filled Photomultiplier

Photoemissive Cell (cont.) Vacuum Phototube Consists of rod anode and curvature cathode in the vacuum glass. Cathode is coated with emissive materials that emit electron when light radiation occur on them. Stable, consistent characteristics over time when operate at low voltage and protected against excessive light. Moderate sensitivity due to small current flow in the vacuum tube.

Photoemissive Cell (cont.) Gas-filled Phototube Same construction as vacuum phototube except the presence of inert gas, usually argon into tube. Emitted electrons are accelerated by the electric field and cause ionization. Anode current increases due to high collision. Provide gain over response up to 10 compared to vacuum phototube. Not stable as vacuum type, the characteristic are not linear.

Photoemissive Cell (cont.) Photomultiplier Phototube Consist of evacuated glass envelope containing photo cathode, anode, and dynodes (additional electrodes). Each dynode is at higher voltage than the previous dynode. Electron emitted by cathode are attracted to first dynode and emitted again (secondary emission) to the following dynodes. High sensitivity and high frequency response but large in size and expensive.

Photoconductive Cell Fabricated from semiconductor materials such as Cadmium Sulphate (CdS) or Cadmium Selenide (CdSe). The increase of current with light intensity while the voltage remain constant makes the resistance of semiconductor decrease. Also known as Photo Resistor, Light Dependent Resistor (LDR). Simple, high sensitivity and low cost. Variations of temperature may affect resistance for a particular light intensity.

Photoconductive Cell (cont.) Construction Typical Curves of Resistance vs. Illumination.

Photoconductive Cell (Example) The relay of (a) of figure above is to be controlled by a photoconductive cell with the characteristics shown in (b) in figure above. The potentiometer delivers 10mA at a 30V setting when the cell is illuminated with 400l/m2 and is required to de-energized when cell is dark. Calculate (i) the required series resistance, and the (ii) dark current level.

Photoconductive Cell (Example) i) From the characteristic in (b), cell resistance at 400l/m2 is 1kΩ. Therefore, So

Photoconductive Cell (Example) ii) The cell dark resistance is 100kΩ Dark current =

Photovoltaic Cell Semiconductor junction devices for converting radiation energy into electrical energy (voltage). Also known as solar cell. Conversion efficiency depends on the spectral content and illumination intensity. Main advantages is its ability to generaee voltage at fast response. Can be used as energy converter (power provider) directly.

Photovoltaic Cell (cont.)

Photojunctions Photodiodes Silicon diode with the lens on its case to focus the incident of the light to the junction. Without bias, operates like photovoltaic devices or voltage source. When reverse bias operates like photoconductive device. Important advantage is fast response compared to photoconductive device. It is also small and inexpensive.

Photojunctions (cont.) Phototransistor NPN device by addition of junction to photodiode. Provide larger output current compared to photodiode for a given amount of illumination on a very small area. More sensitive than a photodiode up to the factor of 100.

Contents Electrical Transducers Classification Selection of Transducers Photoelectric Transducers Temperature Transducers

Temperature Transducers Transducers that can be used to measure temperature. Resistance Temperature Detectors (RTD) Thermocouples Thermistors Other temperature transducers

Resistance Temperature Detector (RTD) Usually make use of platinum, nickel or resistance wire elements. Resistance varies with the change of temperature. Almost all metals give high resistance when temperature increase. High value of temperature coefficient is required to sense a small changes in the temperature.

Resistance Temperature Detectors (cont.) The temperature ranges and coefficient of resistane of various resistane wire can be tabulated as in table below. Material Range (°C) Coefficient of Resistance (Ω/Ω/°C) Platinum -200 ~ 850 0.0039 Copper -200 ~ 260 0.0067 Nickel -80 ~ 300 0.0043

Resistance Temperature Detectors (cont.) Expression below relate the resistance of the conductors and the temperature: Rt is resistance of the conductor at temperature t°C. Rref is resistance of the reference temperature (typically 0°C) a is temperature coefficient of resistance Dt is the difference between operating and reference temperature.

Resistance Temperature Detectors (cont.) Platinum RTD is the most widely used. Advantages: Wide operating temperature range. Stability at high temperature. Linearity. Disadvantages: Low sensitivity. Expensive. Easily affected by contact resistance.

Resistance Temperature Detectors (cont.) Two Lead Wire RTD Uses Wheatstone’s Bridge* to measure the resistance. Low cost but in order to achieve high accuracy, the circuit must be stable and insensitive to the variations of ambient temperature. Es is supply voltage, Eo is output voltage, R1, R2 and R3 are fixed value resistors, RL1 and RL2 are the Resistance of the two leads And RT is resistance of RTD.

Resistance Temperature Detectors (cont.) Three Lead Wire RTD Practical and accurate method for most industrial applications. The bridge circuit automatically balance resistance change due to ambient temperature change. Third lead has no effect on the bridge ratios and balance.

Thermocouples Consist of two different materials that are in thermal and electrical contact. Thermoelectric effect – Seebeck effect. Direct conversion of temperature differences to electrical voltage.

Thermocouples (cont.) The junctions at different temperature causes a circulation of current. Open circuit will generate voltage that is relative to the Seebeck current. Electromagnetic force (Thomson and Peltier) come from the conductors where the density of free charge carriers increases with temperature. Materials used for wire and sensing junction temperature effect the magnitude of the voltage generated. Reference temperature junction of thermocouple also known as cold junction.

Thermocouples (Cont.) Common thermocouple materials combination: Platinum/Platinum-Rhodium Chromel/Alumel Chromel/Constantan Copper/Constantan Iron/Constantan

Thermocouples (Cont.) Thermocouple output voltage with respect to temperature for various thermocouple material.

Thermocouples (Cont.) If in thermocouple, the voltage generated by the reference junction is the same with the one generated by the sensing junction, this give null output provided that both junctions is at the same temperature. This can be solved by a process known as cold junctions compensation. The reference junction is now at 0°C.

Thermocouples (Cont.) The isothermal block is made of material that is good conductor of heat but poor in electricity. Industries often use an isothermal block that contains a thermistor and two reference junctions. This setup known as electronic ice point reference.

Thermocouples (Cont.) Advantages Disadvantages High speed Good accuracy Cheap Rugged Disadvantages Low accuracy Placed remote from measuring devices. Reference junctions compensation.

Thermistor Also called thermal resistor as the resistance varies as a function of temperature. Manufactured in the form of beads, discs and rods. Most thermistors have a negative coefficient (NTC) of temperature resistance. Three important characteristics: Resistance – Temperature Voltage – Current Current – Time

Thermistor (cont.) The resistance of thermistor, RT at a temperature T can be formulated as follows: From the equation above, a and b are constants that can be determined by the structure and material of thermistor.

Resistance versus temperature of a NTC Thermistor Thermistor (cont.) Resistance versus temperature of a NTC Thermistor

Thermistor (cont.) Major applications of thermistors are measurement and control of temperature. Other applications of thermistors: Measurement of power at high frequencies. Measurement of thermal conductivity. Measurement of level, flow and pressure of liquids. Measurement of composition of gases. Vacuum measurement.

Thermistor (cont.) advantages Limitations High sensitivity especially in NTC region. Fast response over narrow temperature range. Cold junction compensation is not required. No problems on contact and lead resistance. Low cost and small size. Limitations Characteristics of resistance are non-linear. Not recommended for wide temperature range application. Need a shielded power lines or filters and low excitation current.

Thermistor (example) The circuit of figure below is used for temperature measurement. The thermistor is 4kΩ type. The meter is 50mA meter with a resistance of 3Ω, Rc is set to 17Ω, and supply voltage Vt is 15V. What will be the meter reading at 77oF (25oC) and at 150oF. (Kalsi2005, pg. 398)

Thermistor (example) From the plot of temperature versus resistance, the resistance at 25oC is 4kΩ. So, the current at 25oC is.

Thermistor (example) At 150oF, the resistance is approximately 950Ω. The meter reading will be:

Other Temperature Transducers Bimetallic strip Two strips of different metals welded together, in the form of straight cantilever beam with one end fixed. Due different thermal coefficient, one of the metals expands more than the other when heated. Commonly used as thermostat. Simple, cheap and robust.

Other Temperature Transducers (cont.) Integrated circuit Temperature sensing element and signal conditioning electronics in single monolithic integrated circuit package. Advantages; linearity; cheap and high output. Disadvantages; require power supply, small range, self-heating, slow and limited configuration.

Other Temperature Transducers (cont.) IC Type Transducer

Other Temperature Transducers (cont.) Radiation Pyrometers Operation based on Stefan-Boltzmann law – sense the temperature from the energy radiated from heated blackbody optically. Construct such that a heat is focused onto the hot junction of thermocouple. Used for measure very high temperature. Two types of pyrometer that are fixed focus and variable focus.

ELECTRONIC INSTRUMENTATION EKT 314/4 WEEK 4 : CHAPTER 2 END