Potentiometers (variable resistors) are common components used in electrical circuits. They are usually circular in form and incorporate a resistance track in the form of an arc of about 270° traversed by a wiper that can be rotated over the track by turning a spindle ( ill. 3-1).
The resistance track is usually carbon, although it can also be a winding of resistance wire. Carbon-track potentiometers are cheapest, but they are generally suitable only for low-power loads (1/4-1/2 W for general circuit application). Wirewound potentiometers can have much higher power ratings, depending on the diameter size of the wire.
Another basic difference between the two is the available resistance values. Lowest values for carbon-track potentiometers are usually 0-100 Ohm extending up to 4.7 M-Ohm or higher. Wirewound Potentiometers may be produced with full-load resistance of only a few ohms and ranging up to about 47 k-Ohm.
Both carbon-track and wirewound potentiometers are also produced in linear form, that is, with a straight track traversed by a sliding wiper ( ill. 3-2). Both circular and linear potentiometers are also produced with resistance elements of deposited thin film (conductive plastic).
| AMAZON multi-meters discounts AMAZON oscilloscope discounts |
Potentiometer characteristics can be linear (not to be confused with linear geometry) or logarithmic ( ill. 3-3). With linear response, equal movements of the wiper produce equal changes in resistance.
ill. 3-1: Construction of a rotary potentiometer.
ill. 3-2: Construction of a slide potentiometer.
ill. 3-3. Linear (A) and logarithmic (B) potentiometer responses.
With logarithmic response, incremental movements of the wiper produce changes in resistance proportional to the logarithm of the increment.
Potentiometers can also be made with characteristics intermediate between these two, such as semilog or linear tapered. In theory it’s possible to design a potentiometer to have any response characteristics required. For transducers, however, linear characteristics are normally preferred. Most slide-type potentiometers have linear characteristics. Annular types produced as radio components are more likely to have logarithmic characteristics unless linear characteristics are specified when buying.
The slide-type (or “geometrically” linear) potentiometer also has another advantage as a transducer. A linear movement applied to it produces an equal linear movement of the slider. To apply a linear movement to a circular potentiometer, we must fit an arm to the potentiometer spindle. Spindle rotation with applied linear movement won’t produce equal “linear” rotation of the spindle. For equal increments of linear input movement, spindle rotation will progressively decrease ( ill. 3-4).
ill. 3-4. A rotary potentiometer is not ideally suited for linear measurement of displacement.
Of course, a similar problem can arise if the transducer is to detect rotary movement. In this case it’s the circular potentiometer that will produce “equal” movement response, whereas the slider type will not.
An advantage of the potentiometer as a transducer is that it’s a simple device, easy to calibrate and able to provide a high signal output. It can also be used with either dc or ac excitation. On the other hand, its sensitivity can be low if it’s a high resistance type, and it can be subject to wear and generation of spurious noise, being particularly susceptible to vibration. Other major disadvantages are its high friction and a tendency to wear that limits usable frequency response. In addition, it must be regarded as a limited-life device.
These disadvantages apply mainly to standard electronic circuit potentiometers. Potentiometers produced specifically as transducers overcome many of these limitations. Quite complex designs have been produced to overcome the effects of vibration on the wiper system and to exclude dirt and other abrasive materials that could aggravate wear. Cost is still relatively low, however, compared with other types of transducers.
One other point worth noting when choosing a potentiometric transducer is that carbon-track and conductive-Plastic types provide infinite resolution. Wirewound types normally provide an output in small increments, determined by the wire diameter. This may or may not be significant in the application involved.
The following are further numerous potentiometer types.
CARBON-TRACK POTENTIOMETERS
Where carbon is used as the resistive element, a carbon film may be sprayed onto an insulated form or ceramic base, or a heavier coating may be used, which is subsequently compressed and molded at a high temperature into a sheet on a phenolic or ceramic base. These types are known as sprayed carbon and molded carbon, respectively. Sprayed carbon potentiometers are generally non-precision types with limited life and stability. Molded carbon potentiometers are heavy-duty types. Both types can be produced with linear or nonlinear characteristics, the latter generally having logarithmic characteristics.
WIREWOUND POTENTIOMETERS
Wirewound potentiometers consist of bare insulating wire wound around an insulating form in either single-turn or multi-turn configuration. Both precision and non-precision types are produced. Potentiometer characteristics of this type of element are close linearity (although nonlinear windings can also be produced), high power ratings, low temperature coefficients, and close resistance tolerances. Wirewound potentiometers can also be expected to have good stability and long mechanical life and are suitable for working over a wide temperature range.
CONDUCTIVE-PLASTIC POTENTIOMETERS
The element in conductive-plastic potentiometers consists of a mixture of conductive and nonconductive plastics in a thermosetting plastic binder deposited on a ceramic substrate. Particular advantages are small size, infinite resolution, long life, and high reliability. Because the resistive element is very smooth, conformity can be nearly perfect to any required function.
The excellent life of conductive-plastic potentiometers makes them particularly suitable for servo applications where the potentiometers may be operating consistently over a few degrees of arc (which will provide high wear rates on wirewound and carbon potentiometers).
It’s a general characteristic of nonwirewound potentiometers that they are less suitable for working in the voltage-divider mode because of “dc offset.” This applies particularly in the case of conductive-plastic potentiometers.
CERMET POTENTIOMETERS
Cermet resistive elements contain a film of precious metal and glass fired onto a ceramic substrate. Such elements can be produced with a wide range of resistances with both linear and nonlinear characteristics, with virtually infinite resolution and high power ratings. Cermets can also be operated at higher temperatures than other types of potentiometers. A possible disadvantage with cermet potentiometers is a high temperature coefficient, although this has been substantially reduced with modern types to become almost comparable with wirewound potentiometers. Its life is limited by the life of the wiper, which may be subject to relatively high wear because of the hardness of the ceramic track.
TEMPERATURE COEFFICIENT OF RESISTANCE
The temperature coefficient of resistance is the unit change in resistance per degree Celsius from a stated reference temperature. It does not follow, however, that the same coefficient applies over the whole of the working temperature range. For critical applications special resistance elements may be employed (such as wirew0u types with special resistance wire to provide low temp. coefficients).
NOISE
Noise can be defined as spurious variations in electrical output not representative of the input drive to transient resistors appearing between the wiper and resistive track. It can be determined quantitative in terms of actual transient resistance values. For non-wirewound potentiometers such spurious deviations are referred to as smoothness and are measured in terms of voltage variations from theoretical values for specific travel movements. Smoothness values are quoted as a percentage of the input voltage.
WORKING MODES
Potentiometers may be used in the variable-current or voltage- divider modes. In the variable-current mode, shown in ill. 3-5, the adjustability RA is given by
where IA is the current reading achieved
RT is the total resistance of the potentiometer.
RA is then the adjustability expressed as a percentage of the total resistance of the potentiometer after setting the potentiometer to 50 percent of its total resistance. Adjustability is read as signal range.
The voltage-divider mode is shown in ill. 3-6. Here the adjustability R the other adjustment to 50-percent voltage ratio, is given by
Rv = (VA - 0.5) x 100
where VA is the voltage ratio achieved.
ill. 3-6. Voltage-divider mode.
This performance may be modified by the effective resistance in the divider leads and also by contact-resistance variations. Precision-type potentiometers commonly employ multiwire wipers to minimize the latter effect.
POTENTIAL-DIVIDER CIRCUIT
The potential-divider circuit is shown in ill. 3-7, where R1 and R2 represent the (variable) resistance values available on either side of the wiper of a potentiometer. Because the current flowing through the potentiometer must be the same as the current through R1 and R2, the following relationship applies:
V1 = source voltage (such as battery voltage)
By adjustment of the potentiometer, virtually any rate of volt age V2 or V3 can be set up, less than the supply voltage V. Also, for any initial setting of the potentiometer any change in its setting (i.e., “transducer” movement) will vary both V2 and V3 proportionately. Either V2 or V3 can, therefore, be tapped as a signal source corresponding to movement, because an output signal could be used for direct meter indication or in an alarm circuit. In the latter case the alarm circuit is designed to switch on at a particular threshold voltage level extracted from either V2 or V3.
ill. 3-7. A potential-divider circuit.
PREV: Simple Electromechanical Transducers
NEXT: Strain Gauges