Lab Report I.E Chapter 2 [PDF]

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CHAPTER 2 POSITIONAL RESISTANCE TRANSDUCERS OBJECTIVES :  Describe the basic construction of rotary and slider variable resistors.  State that the resistance section may be either a carbon track or wirewound.  Describe the difference between a logarithmic and a linear track.  Draw the basic characteristics of output voltage against variable control setting.  Describe the effect on the output voltage of loading the output circuit.  Compare the application of a carbon track variable resistor to the wirewound type. EQUIPMENT :  DIGIAC 1750 Transducer and instrumentation Trainer.  4mm connecting Leads.  Digital Multimeter.



2.1 Variable Resistor Construction A variable resistor consists of a “track” having a fixed overall resistance with a “wiper” which can be moved to make contact with any point along the track. In the carbon type, the total track resistance is varied by adjusting the proportion of non-conducing material to carbon in the compound during manufacture. This will produce a track of constant resistance along its length, so that any section of the track will have the same resistance as any other similar section. The track will be linear. Variable resistors intended for use in audio applications, where subjective appreciation of sound amplitude (loudness) is proportional to logarithmic scales, are made with similar logarithmic (non-linear) scales. The resistance along the track is not a linear relationship, increasing with the square of the rotation of the spindle, or movement of the side wiper (R ∞ S2, where S is the setting of the Wiper). A close approximation is made to the ideal logarithmic characteristics by using three or four sections of track with different resistance slopes.



Non-Linear variable resistors are not suitable as positional transducers and are therefore not included on the DIGIAC 1750 Trainer facilities.



The track can be laid out on a rotary or a straight base, as in Fig 2.1



Fig 2.1 For higher power applications the track may be wirewound. With the wiper making contact with the top edge of a coil of resistance wire. Note: All variable resistors on the D1750 have a 1kΩ resistor in series with the wiper. This resistor is not swon on schematic diagrams.



2.2 Linear Variable Resistor Characteristics A variable resistor can be used to provide a voltage. A steady voltage is applied across the ends of the fixed track. The wiper then picks off a variable voltage at the contact point with the track (with respect in the end of the track). Used in this way the variable resistor is calles a potensiometer.



Fig 2.2



With a dual polarity voltage source, the polarity and magnitude of the output voltage will depend on the direction of movement of the wiper from its central position, as shown in Fig 2.3 Fig 2.3



Note that the position of the variable resistor spindle (or slider) setting is indicated by the output voltage from the potentiometer.