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Transducers Devices which convert non-electrical quantities (e.g. temperature, pressure) into electrical ones (e.g. voltage, resistance) whose value depends on the value of the nonelectrical quantity. Transducers exist for almost every imaginable quantity! We will concentrate on three example transducers and the circuits which can be used to apply them. 1. Light level -- the photodiode Forward direction -- operates like a normal diode. Reverse direction -- much larger leakage current than normal; its value is dependent on the light level. The operating principle is that the junction region is exposed to incident light (unlike a normal diode where the device is contained in an opaque packaging). The photons of light cause electron-hole pairs to be produced, which give rise to the light-dependent current. The current-light level relation is nonlinear; in particular, there is a leakage current known as the “dark current” when there is no light falling on the device. We could produce a voltage dependent on the light level like this: E R Voltage proportional to diode current Phototransistors also exist -- they give rather more current than photodiodes for a given light level because of the current amplification they produce, but they also have a greater “dark current”. 2. Temperature -- Resistance Thermometers and Thermistors These have in common that their resistance changes with temperature, but the difference is how rapidly! Platinum resistance thermometer -- resistance increases by 0.39% per degree C Thermistor (negative-temperature-coefficient) -- resistance DECREASES strongly with temperature rise. The platinum resistance thermometer is normally used in an unbalanced Wheatstone Bridge circuit to give a temperature-dependent voltage. E Pt element To differential amplifier If all the resistances are of nominal value R, it can be shown by circuit theory that the voltage at the differential amplifier input (assuming its Rin is high) is ER/(4R) when the thermometer resistance has changed by R. We only need to use such an arrangement with a thermistor if it is measuring very small temperature ranges. More usually, something like this will suffice. E Vo Hall-effect devices The Hall effect is observed quite strongly in semiconductors and its principle is very like that of an electric motor. Consider the rectangular block of semiconductor material below in which a magnetic field is “flowing” into the paper. Current I Semiconductor (assumed p-type for simplicity) Hall Voltage EH Voltage source producing the current I + - The principle is that the current I, flowing at right-angles to the magnetic field of flux density B, experiences a force in a direction at right-angles to both the field and the current according to Fleming’s Left-Hand Rule. The “holes” carrying the current are therefore deflected to the side, creating a Hall Voltage EH as shown. (You may like to confirm whether the polarity of the voltage is correctly marked). We find that: EH = RHBI where RH is a constant known as the Hall Coefficient. Would the Hall Effect also occur in an n-type material? If so, what would be different ? Hall-effect transducers are most frequently used for measuring magnetic fields. The transducer is placed in the field and a known current is passed through it, resulting in a Hall voltage proportional to the field strength. It is otherwise surprisingly difficult to measure constant magnetic fields -- alternating ones are easier as we can insert a coil and measure the alternating voltage developed across it. Hall-effect transducers are also used to measure large currents non-invasively in “Clip-on” devices where a loop of magnetic material containing a Hall-effect device is clipped round the wire carrying the current. In each case, the Hall-effect device is often manufactured with an amplifier in the same package, which allows a much higher output voltage to be obtained for a given field strength and current. Transducer Examples 1. A particular thermistor has a resistance which can be expressed in terms of temperature by the following graph. Resistance, ohm 1200 1000 800 600 400 200 0 0 10 20 30 40 50 Temperature, C 60 70 80 90 100 It is to be used to produce a voltage which will vary from 0 V at 10 C to 5 V at 50 C. Design a suitable circuit. 2. A similar device is to be produced using a platinum resistance thermometer having a resistance of 100 ohm at 20 C. Design a suitable circuit. 3. A photodiode having a dark current of 50 nA and a reverse current of 100 uA on a sunny day in summer is to be used to make a lightmeter for use by cricket umpires. The cricket club is short of funds and it must therefore utilise a 0-5 V panel meter of resistance 10 k which the wicket-keeper has found in his garden shed. The No. 11 batsman, anxious to persuade the selectors of his claim to a higher place in the batting order, has donated a 4.5-volt battery and a large box of resistors, fixed and variable, of most conceivable preferred values. A free ticket (normal cost £0.00) for the opening match has been offered to the designer of the first successful design opened ... (a) Design a suitable circuit. (b) The No. 11 batsman now additionally offers a second 4.5-V battery, an operational amplifier and a light-emitting diode. Modify your circuit to illuminate the LED when there is insufficient light. 4. RS offer (or offered !) a Hall-effect i.c. incorporating amplification which provides an output voltage of 7.5 - 10.6 mV/mT when supplied with a current of 3.5 mA. How could it be used in conjunction with the meter from Question 3 and any other necessary components to read directly in mT ?