Physics
Potentiometers
Potentiometers are variable resistors used to control electrical potential or voltage in a circuit. They consist of a resistive element and a sliding contact, allowing for precise adjustment of the voltage. By changing the position of the sliding contact, the resistance and voltage output can be varied, making potentiometers useful for applications such as volume controls and dimmer switches.
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8 Key excerpts on "Potentiometers"
- Jack Smith(Author)
- 2005(Publication Date)
- Newnes(Publisher)
487 Digital Potentiometers and Controllable Filter C H A P T E R 21 Digital Potentiometers (usually called a “pot” in electronics speak) are a solid-state replacement for me-chanically adjustable variable resistors historically used for volume, balance and tone controls in consumer electronics, among many other applications. Instead of responding to shaft rotation, digital pots change their value—in microseconds—based upon a received command message. Digital pots are offered in a variety of control interfaces, including 1-wire, contact closure (up/down steps), parallel, increment/decrement, SPI (3-wire) and others. Some even include nonvolatile memory to retain their last setting when power is removed. We’ll look at Microchip’s MCP41xxx/42xxx family of digital pots, with an SPI interface. Other manufacturers, such as Maxim (including its Dallas Semiconductor division), have wider product ranges, and you should review their offerings if you think digital Potentiometers may be useful in your particular project. First, let’s clear up the terminology. The device we are talking about is known as a potentiometer, a variable resistor, a rheostat or a volume control, among many other terms. In reality, these terms can be boiled down into two possible connection arrangements, as shown in Figure 21-1. The potentiometer arrangement uses all three connections and is usually configured as a variable voltage divider. The variable resistor or rheostat arrangement uses two connections (the wiper—the variable connecting piece—is connected to one end of the fixed winding, but to the external circuit only two connections are seen) and operates as a variable resistance. We’ll use the term “pots” as the generic name for these devices, even though they may be connected, in certain applications, as variable resistors. One major difference between mechanical and digital pots is resolution.
eBook - ePub- Ian Sinclair(Author)
- 2000(Publication Date)
- Newnes(Publisher)
Chapter 3 Variable resistors, Potentiometers and diodes When a resistor, which can be constructed by any of the processes that have been dealt with in Chapter 2, is provided with a third contact that rubs on the resistive material and makes contact with it, then the amount of resistance between this third contact and either of the fixed contacts can be made variable since it will change when the position of the contact on the resistive surface is altered. This type of arrangement can be described as a rheostat, variable resistor, trimmer or potentiometer according to its prospective use. The common factor is that the position of the third contact is set mechanically, so distinguishing this type of resistance variation from the type that is found in photoconductive cells, where the resistivity of the material is changed by light, or in thermistors in which the resistivity of the material is changed by an alteration of temperature. Devices in which the resistivity of the material itself is changed are dealt with in Chapter 7. The three names that are used reflect the developments in the use of these components. The first radio receivers that used thermionic valves with tungsten filaments (bright-emitter valves) needed some method of controlling the filament temperature, and this was done by including a variable resistor in the circuit. Such a variable resistor or rheostat (the name means constant current) consisted of a wire winding on a ceramic tube, with one fixed contact and one moving contact (Figure 3.1), so that a variable length of resistance wire could be put in series in the circuit. This controlled the current by controlling the total resistance of the circuit
eBook - PDFSensor Systems
Fundamentals and Applications
- Clarence W. de Silva(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
In selecting a potentiometer for a specific application, several factors have to be con-sidered. As noted earlier, they include element resistance, power consumption, loading, resolution, and size. 8.3.2.3 Sensitivity The sensitivity of a potentiometer represents the change ( Δ v o ) in the output signal that results from a given small change ( Δθ ) in the measurand (the object displacement). The sensitivity is usually nondimensionalized, using the actual value of the output signal ( v o ) and the actual value of the displacement ( θ ). For a rotatory potentiometer in particular, the sensitivity S is given by S v S v o o = = ¶ ¶ D Dq q or the limit in (8.6) These relations may be nondimensionalized by multiplying by θ / υ o . An expression for S may be obtained by simply substituting Equation 8.3 into Equation 8.6. Some limitations and disadvantages of Potentiometers as displacement measuring devices are given in the following: 1. The force needed to move the slider (against friction and arm inertia) is provided by the displacement source (moving object that is sensed). This mechanical load-ing distorts the measured signal itself. 2. High-frequency (or highly transient) measurements are not feasible because of such factors as slider bounce, friction, and inertia resistance and induced voltages in the wiper arm and the primary resistor element. 3. Variations in the supply voltage cause error. 4. Electrical loading error can be significant when the load resistance is low. 5. Resolution is limited by the number of turns in the coil and by the coil uniformity (in a coil-type pot). This limits small-displacement measurements. 6. Wear and heating up (with associated oxidation) in the resistor element (coil or film) and the slider contact cause accelerated degradation. There are several advantages associated with potentiometer devices, however, including the following: 1. They are simple in design and robust. 2. They are relatively inexpensive.
eBook - ePubSensor Systems
Fundamentals and Applications
- Clarence W. de Silva(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
Note: Many other principles may be employed in Potentiometers for displacement sensing. For example, an alternative possibility for optical potentiometer is to have a fixed light source and to locate a photosensor on the moving object whose displacement needs to be measured. By calibrating the device according to how the light intensity varies with the distance between the light source and the light sensor, the distance can be measured. Of course, such a device would be quite nonlinear and nonrobust (as it will be affected by environmental lighting, etc.).8.3.3.1 Digital Potentiometer
The digital potentiometer is a device that can provide digitally incremented resistance or voltage corresponding to a digital command. The range of discrete resistance that it can provide depends on the bit size of the device (e.g., 8-bit device is able to provide 256 discrete values of resistance). The incrementing can be programmed linearly, logarithmically, etc., using a microcontroller or other digital device, depending on the application. It is clear that a digital pot is not a displacement sensor but rather a resistance splitter or voltage splitter. It is mentioned here to avoid any misconception.The potentiometer has disadvantages such as loading problems (both mechanical and electrical), limited speed of operation, considerable time constants, wear, noise, and thermal effects. Many of these problems arise from the fact that it is a contact device where its slider has to be in intimate contact with the resistance element of the pot and also has to be an integral part of the moving object whose displacements need to be measured. Next, we consider several noncontact motion sensors that do not have these shortcomings.8.4 Variable-Inductance Transducers
Motion transducers that employ the principle of electromagnetic induction are termed variable-inductance transducers. When the flux linkage (defined as magnetic flux density times the number of turns in the conductor) through an electrical conductor changes, a voltage in proportion to the rate of change of flux is induced in the conductor. This is the basis of electromagnetic induction
- Winncy Y. Du(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
2.2 POTENTIOMETRIC SENSORS 2.2.1 S ENSING P RINCIPLE Potentiometric sensors are designed based on Equation 2.1—a conductor’s resistance R (in ohms, Ω ) is a function of the resistivity of the conductor material ρ (in ohm-meter, Ω ⋅ m), its length l (in meter, m), and its cross-sectional area A (in meter square, m 2 ): R l A = ρ (2.1) 2 26 Resistive, Capacitive, Inductive, and Magnetic Sensor Technologies Although any change in l , A , and ρ will cause a change in resistance, potentio-metric sensors (also called Potentiometers or pots for short) are often designed by varying the length l only for the sake of simplicity and for saving cost. Some che-moresistive sensors are designed based on materials’ resistivity ρ change caused by chemical reactions. The resistance of a potentiometer can be evaluated using Ohm’s Law by applying an electric current I (in amperes, A) and measuring the voltage V (in volts, V) across the potentiometer: R V I = (2.2) 2.2.2 C ONFIGURATION AND C IRCUITRY Potentiometric sensors are available in two configurations: linear and rotary , as shown in Figure 2.1a and b, respectively. In both configurations, resistance change is the result of position variation ( x or θ ) of a movable contact (wiper) on a fixed resistor, resulting in an output voltage change. The circuit symbols and typical circuits of potentiometric sensors are shown in Figure 2.2a, through c. The potentiometer R 2 in Figure 2.2b functions as a voltage divider. The voltage across R 2 is the measured output: V R R R V S out = + 2 1 2 ( ) (2.3) If a load R L is placed across R 2 , as shown in Figure 2.2c, the amount of current “diverted” from R 2 will depend on the magnitude of R L relative to R 2 . The output voltage across R 2 (which is also the load voltage) is then V R R R R R R R R V L L L S out = + + 2 1 2 1 2 (2.4) (a) (b) 3 2 1 1 2 3 θ V 1–3 = V in V 2–3 = V out 3—grounding FIGURE 2.1 Linear (a) and rotary (b) potentiometer configurations.
- Lazo M. Manojlovic(Author)
- 2019(Publication Date)
- Arcler Press(Publisher)
The potentiometer resistance is proportional to its length. In order to convert the actual position of a movable contact of the potentiometer into a corresponding electrical counterpart, a constant voltage is applied across the potentiometer fixed contacts by using an external voltage supply. The potentiometer output signal is the voltage between the movable contact, i.e., the wiper arm, which slides along the coil (or film), and the reference voltage terminal of the coil, as presented schematically in Figure 4.1. The potentiometer output voltage v o is proportional to the movable contact displacement x as: , (4.1) where k is the proportionality factor. Figure 4.1. Schematic diagram of a potentiometer. Analog Sensors and Transducers 71 The relationship between the output voltage of the potentiometer and the wiper arm displacement, given by the equation 4.1., didn’t take into con-sideration that the output terminals of the potentiometer can be loaded thus providing a zero output current. However, in the real-life situations, there is a load at the potentiometer output that has finite impedance . Typically, at the potentiometer output terminals, there is the circuitry into which the potentiometer signal is fed, such as conditioning, interfacing, processing, and control circuitry, as schematically presented in Figure 4.2. Due to the leakage of the output current, there is the drop of the output voltage to v’ o even if the reference voltage V R remains constant under load variations. The corresponding effect is known as the electrical loading of the transducer. Therefore, the linear relationship, given by equation (4.1,) cannot be valid anymore thus causing an error in the displacement measurement. Loading effect can influence the displacement measurement in two ways. Due to the finite internal resistance of the reference voltage source, the load can affect the reference voltage, i.e., by loading the voltage source.
- Dennis L. Eggleston(Author)
- 2011(Publication Date)
- Cambridge University Press(Publisher)
The more common and versatile type with three leads is called a potentiometer or a “pot.” Schematic symbols for resistors are shown in Fig. 1.6 . One must also select the proper power rating for a resistor. The power rating of common carbon resistors is indicated by the size of the device. Typical values are 1 8 , 1 4 , 1 2 , 1, and 2 watts. 6 Basic concepts and resistor circuits Table 1.3 Standard color scheme for resistors Color Digit Multiplier Tolerance (%) none 20 silver 0.01 10 gold 0.1 5 black 0 1 brown 1 10 red 2 100 2 orange 3 10 3 yellow 4 10 4 green 5 10 5 blue 6 10 6 violet 7 10 7 gray 8 white 9 Resistor Rheostat Potentiometer Figure 1.6 Schematic symbols for a fixed resistor and two types of variable resistors. As noted in Eq. (1.3) , the power consumed by a device is given by P = VI , but for resistors we also have the relation V = IR . Combining these we obtain two power relations specific to resistors: P = I 2 R (1.7) and P = V 2 / R . (1.8) 1.2.1 Equivalent circuit laws for resistors It is common practice in electronics to replace a portion of a circuit with its functional equivalent. This often simplifies the circuit analysis for the remaining portion of the circuit. The following are some equivalent circuit laws for resistors. 1.2.1.1 Resistors in series Components connected in series are connected in a head-to-tail fashion, thus forming a line or series of components. When forming equivalent circuits, any 1.2 Resistors 7 V R 1 R 2 R 3 I V R eq I Figure 1.7 Equivalent circuit for resistors in series. number of resistors in series may be replaced by a single equivalent resistor given by: R eq = i R i (1.9) where the sum is over all the resistors in series. To see this, consider the circuit shown in Fig. 1.7 . We would like to replace the circuit on the left by the equivalent circuit on the right. The circuit on the right will be equivalent if the current supplied by the battery is the same.
eBook - PDF- Morris A. Colwell(Author)
- 2016(Publication Date)
- Newnes(Publisher)
The individual currents will be determined by the formulae given above in the normal way. Fig. 5b shows the equivalent circuit that could apply here for the purposes of the calculation. The equivalent is a potentiometer which is also shown; the sliding wiper has the choice of position within the range of overall resistance so that the voltage or current through the load can be preselected. Potentiometers are usually recognisable as controls with knobs or presettable screwdriver slots, so that a d.c. voltage or a.c. signal can be adjusted, for example, a volume control. The principles are the same, although rarely does anyone need to go to the extent of cal-culating exactly the current and voltages under various situations of the wiper setting. It is by experience in the practice of electronics that potentiometer values are determined according to the application, bearing in mind the possible effects of shunting a high impedance source or risking excessive current flow when the control is at one end of its travel. There are often known values that are adopted by con-vention for various applications and these will become familiar to the constructor. Output voltage i ^ C± Load , t VR'll (R1) Input < J _ _ voltage utage p i c _ i (R2) Output voltage 22 It has been for a long time the convention to arrange the carbon or wirewound tracks of Potentiometers in a circular form so that the controls can be rotated and the whole component contained in a fairly small space economically. However, there is a growing trend in dom-estic equipment to adopt the style used for a number of years in studios, whereby the track is straight and the moving wiper is arranged to travel in a straight line. There are no rules or factors that dictate which method is adopted; it is largely a question of aesthetics or personal whim. A range of rotary t y p e Potentiometers that are representative of generally avail-able styles.
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