Tech Reference Sheet: Potentiometer Laszlo Szocs, MAE 221 Wednesday lab A potentiometer (colloquially referred to as a “pot”) is a three- terminal resistor with a sliding “wiper” that forms an adjustable voltage divider. There are many ways to connect a pot (schematic symbol shown in Figure 1), one of the most common ways being as a voltage divider (Figure 2) as previously mentioned. Additionally, the implementation of a potentiometer as a variable resistor is also very frequent (Figure 3). In order to connect it as such, only two terminals may be connected and by adjusting the wiper one can alter the two resistances, ! ! and ! ! that are formed as the wiper “splits” the pot into two different resistors. Adjusting its displacement alters the output voltage, ! ! , according to the equation ! ! ! ! =! ! ! ! !! ! , which can be used to calculate the final output potential. The wiper acts to essentially split the potentiometer into two resistors of varying length. Since the length is directly proportional to resistance, the movement of the wiper thus changes it as it moves up and down the potentiometer. The resistance of the potentiometer can be calculated by using Ohm’s Law (V=IR) and resistor addition, noting that any two resistors in series must have the same current running through them, so the voltage drop across each is directly dependent on their resistance, and in this way it may be used as a voltage divider . Another common pot application, which is shown in Figure 4 (schematic in Figure 5), is as a dynamic displacement-measuring device. By measuring ! ! and calibrating the instrument to correlate a specific displacement of the ball with a voltage, it is possible to use the voltage output as an indicator of displacement. In the potentiometer shown, the wire wrapped around the PVC tube has a known resistance per length ( ≈ .15Ω/!). By measuring the output voltage and knowing the displacement the ball has from a chosen origin, it is possible to fit a linear function relating the two quantities. Once calibrated, displacement can be calculated by just providing a voltage. An example line fit to the voltage-displacement function for the potentiometer in Figure 4 is displayed in Figure 6. Commercially manufactured potentiometers Figure1:PotentiometerschematicsymbolFigure2:Thepotentiometerasvoltagedivider,usingoutputvoltagetolightanLEDFigure3:Thepotentiometerasavariableresistor! ! ! ! Figure4:Thepotentiometerasadynamicdisplacement- measuringdevice70cm
