Potentiometers, Rheostats and Trimmers

Variable resistors are electromechanical components and, therefore, expose not only to the weaknesses of fixed resistors but also to all the failure possibilities of electromechanics. Thus the reliability is comparatively poor.

The total resistance seldom is critical and usually follows the European E3 series or 1-2-5-10 (USA).

Figure 1. Definition of electrical and mechanical travel in potentiometers

Figure 2. End resistance and minimum resistance of potentiometers.

Figure 3. Potentiometer end and minimum voltage.

Figure 4. Schematic sketch of different potentiometer travels.

Figure 5. Potentiometer and rheostat connections.

This formula applies also to the potentiometer.

Figure 7. Potentiometer output voltage versus wiper travel.

If we compare Figure 6. with 7. we may imagine how the terminal metallization acts like a landing strip for the output voltage (the plane parts at the beginning and the end of the actual electrical travel).

Specification of the independent linearity is used, for example, on such potentiometers where the user wants to adjust the gradient of the output voltage. It is done by means of built-in resistor elements connected to the potentiometer track.

Figure 9. Potentiometer independent linearity – non-wirewound

Figure 11. Potentiometer terminal-based linearity. Wirewound only.

Taps

Certain applications require additional terminals. These are called taps and exist in both wire wound and non-wire­wound potentiometers. In the latter, they have either of two functions: Current taps and voltage divider taps. The former ranges over the width of the whole resistance track and disrupts the linearity. The latter are located at the outermost edge of the track and don’t influence the linearity.

Gang potentiometers

Sometimes several potentiometer elements called cups are coupled together on a common shaft. The assembly is called a gang. The technique requires some definitions.

Phasing point

To meet the requirement of simultaneous conformity or tracking, gang cups are phased together at a common reference position of the shaft. For linear potentiometers, that shaft position is usually chosen where the output ratio e/E of the electrical elements is approximately 50%.

Figure 13. Potentiometer tracking error = difference in output ratio Δe

Figure 14. Potentiometer loading error.

Resolution

From MIL-R-39023, we quote the definition of resolution: ”A measure of the sensitivity to which the output ratio of the potentiometer may be set”. The same MIL standard also defines voltage resolution as “The maximum incremental change in output ratio with shaft travel in one direction in any specified portion of the resistance element” (applicable to wire-wound only). The resolution is expressed in per cent of the input voltage E. The higher resolution, the lower percentage.

Descriptions of non-wirewound potentiometers sometimes discuss “In­fi­nite Resolution”. More literally, it should be written in “infinitely high resolution”. The resolution of a wire-wound potentiometer with N number of turns may be approximated to the reciprocal 1/N.

Adjustability

The resolution of trim potentiometers has the name adjustability, which means setting accuracy or suitability. It is stated in per cent of the total applied voltage E (or of Rtot) and thus measures the accuracy with which a desired voltage (or resistance) may be set.

Potentiometer noise

CRV Contact Resis­tance Variation

CRV stands for Contact Resis­tance Variation. Before we discuss this concept, let us begin with Contact Resistance in general. The contact between the wiper and track has a certain resistance, the value of which depends on the contact pressure, the track material and its nature, the current through the wiper, etc. The Contact Resistance (CR) increases, for example, exponentially with decreasing wiper current somewhere below 1 to 0.1 mA. The more metal in the resistance element, the less pronounced the phenomenon. Figure 15. shows in principle the CR of a cermet potentiometer, but the curve could equally well be applied to a wire-wound potentiometer.

In addition, the wiper current varies from point to point along the travel track. In addition to the contact resistance, there is in non-wire wounds also a certain track material resistance (Figure 16.).

We can call the sum of all these static resistances Contact Resistance, CR. In the endpoints they are part of the end resistance ER respectively, the minimum resistance MR. When the shaft then is moved over the track, arbitrary resistance changes or dynamic Contact Resistance Variations, CRV, occur that in principle may look like the record shown in Figure 17.

Figure 17. Schematic of contact resistances in a potentiometer.

The figure applies in principle to both wirewound and non-wirewound potentiometers. Wirewounds have a fixed contact resistance (CR) that usually lies considerably below 1 ohm. Non-wire-wound CRs are in the ohms range (See explanatory Figure 16.).

ENR Equivalent Noise Resistance

For wirewound potentiometers the Equivalent Noise Resistance, ENR, is specified. Usually, it is measured according to the setup in Figure 18. The current generator feeds 1 mA through the wiper. The voltage drop across the contact then may be graded directly in ohms (1 mV corresponds to 1 ohm).

Figure 18. Test circuit for determination of the ENR in wirewound potentiometers according to MIL-R12934.

In conclusion: For mutual comparisons between potentiometer types the above mentioned specification is good. ENR information from application conditions, however, may require additional measurements.

Output Smoothness

Output Smoothness means the maximum instantaneous variation of the output voltage compared to the ideal output. This parameter is applicable to non-wirewound precision potentiometers only. The high resistivity of the track material and the geometry of the wiper contribute to a considerably higher contact resistance (CR) than the one in wirewounds. There would be considerable power generated in the CR if we used the test equipment for ENR in Figure 18. Instead the test circuit in Figure 19. is used.

Figure 19. Determination of potentiometer Output Smooth­ness according to MIL-R-39023.

Figure 20. Definition of potentiometer Output Smoothness.

Summary of output voltage errors

The different types of output voltage errors we have dealt with may be divided into static errors (deviations from the ideal output ratio) and dynamic ones (ENR, CRV and Output Smoothness). The latter is superimposed on the static ones, as shown in the following schematic.

Figure 21. Potentiometer static and dynamic output voltage errors.

The following formalized figures indicate the principal methods for determining mechanical properties. Different standards may state deviating forces and measures. US military specifications state, for example, ½ pound instead of 2 N and the distances are stated in inches (25.4 mm). The circle with a pointer symbol refers to a dial indicator for recording of mechanical movement. The figures are designed to be fully descriptive. Further detailed instructions have to be found in the standard in question.

Figure 22.-26. Potentiometer mechanical configuration from left: Shaft End Play; Runout; End Play; Lateral Runout; Pilot Diameter Runout

Backlash

If we turn the shaft in one direction until the output voltage has attained a specified value that we call e₁ at a travel angle Θ = α₁ and continue the movement a little before we turn back, we have to pass α₁ a certain way before we attain e = e₁. This will happen at a travel angle Θ = α₂ and depends on mechanical play in the wiper system. Backlash is defined as the maximum difference between α₁ and α₂ and thus is expressed in degrees. The case is illustrated in Figure 27. and describes a mechanical hysteresis.

Figure 27. Backlash or wiper play = (α1-α2)°.

Figure 28. Wirewound power potentiometer.

Figure 29. Example of a bushing mount potentiometer. Manufacture Vishay Sfernice.

A phenomenon that threatens components with cavities if they are subjected to temperature changes in humid environments is the so-called pumping effect. An insufficiently sealed potentiometer with large holes inside the housing is particularly subject to this phenomenon. If moist air in the cavities is cooled, the water vapour condenses and creates a negative pressure at the subsequent temperature rise.

New humid air is sucked in, etc. Instead of having such a half-tight potentiometer, a hole in the bottom would be better, so condensed water could run out…Thus, use qualitatively sealed potentiometers if the environment requires them. Especially those with a carbon track, conductive plastic or wire winding are sensitive to moisture. A particular type of carbon track potentiometer is the slider pot with a linear motion. It is usually used in audio controls in-studio mixer panels.

It is essential that the bulk track is moulded and thus relatively wear-resistant, partly to allow prolonged use and partly so that noise-producing wear will be prevented. Caution! If one has chosen some slider potentiometer, finding any equivalent second source is usually challenging.

Current derating

The current through the wiper should be kept low in potentiometers with a carbon or conductive plastic track to minimize noise and wear, preferably with values less than 0.25 mA. Wirewound potentiometers, on the other hand, should draw a certain minimum current through the wiper – more than 0.1 mA – to reduce noise and contact disturbances. The contact resistance in cermet potentiometers has the sensitivity of the metals to small currents and should likewise carry a certain minimum current. An excellent minimum value might be 25 µA. The data sheets also state a maximum current through the wiper, for example, 100 mA at resistances below 100 ohms. At higher resistance values, the contact resistance CR increases, which could lead to local overheating if the wiper current is not kept down.

Thus, the wiper current is determined by the resistance value and nominal power, resistance track material and rotational life (number of shaft turns). In the summary tables, we shall describe some guiding formulas for the wiper current confined within certain limits. The highest limit is the potentiometer’s maximum power current, which may be calculated from the equation.

Next maximum limit – that never must be greater than the maximum power current – is calculated by means of a formula that contains the rotational life N expressed in number of million shaft revolutions (megarevolutions, “MR”) and the resistance in question expressed in kohms. The result is obtained in mA

Finally there exists a commonly recommended minimum limit. Example. Suppose a cermet potentiometer with P_R = 0.5 W, N = 100 000 revolutions = 0.1 MR, R = 1 MΩ = 1000 kΩ and the minimum limit for cermet = 25 µA. If we insert 0.1 and 1000 respectively in the applicable formula

we obtain

This value is well below the absolute maximum current derived from

Thus the wiper current should be adjusted to lie between 0.4 mA and 25 µA.

Table 1. TYPE 2 POTENTIOMETERS CHARACTERISTICS

The disturbances also increase at decreasing temperatures and seem more pronounced when below –30 °C. Thus, the wiper requires a certain current to get a noise-free operation, preferably more than 0.1 mA. Lubricants To reduce wear and prevent oxidation, both on track and wiper, the way is, in most cases, lubricated with some grease or oil. The most common are silicone compounds with relatively constant viscosity over a broad temperature range. The silicones, however, show a rapidly increasing thickness when greases reach temperatures below –5⋅⋅⋅ -15 °C and oils below –30⋅⋅⋅ -40 °C. In addition, at a moderate wiper speed, the wiper tends to slide on top of the lubricant film, causing the ENR values to rise unacceptably. Silicone lubricants have a characteristic that, in specific applications, may be disastrous to the surroundings: they “creep”.

Their surface tension is so low that the lubricant “wets” adjacent surfaces contaminated with a thin film. We talk about a silicone infection. A small dab of grease will, spread over large square meter areas within months. The phenomenon causes problems in the following ways:

• The lubricant film at dry circuit conditions might electrically isolate contaminated surfaces.
• Contaminated contacts with a braking function accompanied by arcovers will be isolated by the silicone granules formed when the miniature arcs decompose the silicone film.
• Contaminated lens systems in high-energy lasers will have their lens surface damaged by the laser beam when it burns the silicone film.
• Glueing of components on contaminated surfaces is made impossible.

Figure 30. single-turn servo potentiometer

Figure 31. Cutaway view of a multi-turn bushing mount potentiometer. Bourns

Figure 32. Principle of the hybrid potentiometer element.

Non-wire wounds, Type 1

Normally non-wirewound precision potentiometers are manufactured as single-turn devices. Conductive plastic cermet is used, as well as bushing mount styles, in servo applications. The design from Figure 30. could as well be applicable to a nonwirewound servo potentiometer. Just as in servo applications, the rotational life has to be high and puts high demands on the wiper design, the contact pressure and the track, which therefore has to be polished. The polished CP track allows a number of revolutions of several tens of millions.

Then, however, the wiper current must be limited. Otherwise a kind of welding effects on the contact surfaces along the track results, that will destroy both wiper and surface and increase wear and noise. The wiper current should be limited to maximum of 0.25 mA. Example: A serious manufacturer promised a rotational life of 100 x 10⁶ revolutions at 2 mA. But he recommended a maximum wiper current of 10 µA! The 0.25 mA rule, however, is not unconditional. Material, resistance value, nominal power and the number of revolutions of the rotational life come into play. The wiper current in wirewound and cermet potentiometers should comply with Formula 2.

The contact function of cermet potentiometers is, in its nature, suggestive of that of the metals. Therefore the wiper current should, just as in wirewounds, also be limited in low values. An approximate minimum value might be 25 µA. (Figure 15). In the summary tables, some guiding formulas are stated.

Linear motion potentiometers

A parallel to the slider potentiometer but in Type 1 design is the linear actuated precision potentiometers or linear position transducers. Thus, they have a straight resistance track and a piston or guide bar that holds the wiper and transforms linear motions. The mechanical travels vary between 10 mm and 1.2 m. The resistance tracks are made of wirewound or hybrid elements, of cermet or conductive plastic. As to other parameters they have the same characteristics and specifications that apply to corresponding rotary potentiometers.

Table 2. PRECISION POTENTIOMETERS / TYPE 1 POTENTIOMETERS CHARACTERISTICS

Wirewounds still exist due to their better tolerances and lower TCR. But this design suffers sensitivity to a fundamental mechanical problem. On the one hand, the wire has to be thin to achieve high resistance values; on the other, the contact pressure from the wiper needs to be increased to give a stable setting and a low noise. After a few trim operations, the wire might be pulled in two. Moreover, the corrosion risk is high if moisture gets inside the housing of the trimmer. With the better and better characteristics that characterize the cermet trimmers of today, the need for wire wounds has strongly decreased. If we need better characteristics, we should preferably use metal foil trimmers, despite their higher price. Like conventional potentiometers, trim potentiometers are also encapsulated with O-ring seals for more severe environments.

SM trimmers subjected to washing fluids should have a design that is tight enough to withstand the rinsing process. Otherwise, rinsing products like flux residues may deposit on the track, which can cause severe contact disturbances. Hole mount types may entice someone into bending the leads. Then it is essential that the lead inlets in the potentiometer housing are relieved. Otherwise, forces are easily transferred to the joints with the resistance track and substrate. Consequently, the substrate or the solder joint cracks, thus resulting in an open-circuit.

Regarding cracks, the long multi-turn rectangular 1¼” trimmers of the same basic design as the one in Figure R636 are more sensitive to exterior mechanical forces than the shorter ¾” design. Of course, the latter also has to be handled with care. The ¾” size, fortunately, has more and more replaced the 1¼” style and is being replaced by the more common 3/8” or ½” square styles. The square types exist in single-turn designs and ¼” and 4 mm diameter potentiometers. SMDs exist as single-turn styles in sizes of 3 or 4 mm square, the latter in a multi-turn format, just as hole mount potentiometers, the trimmers are marked with movement direction (Figure 34.).

Figure 34. Circuit diagram with movement direction marked on a trimmer.

The wiper current must not exceed specific specified maximum values that for wire wound and cermet potentiometers may range as high as 100 mA. Furthermore,  wiper current should be kept above specific minimum values. B to minimise the noiseoth the maximum and minimum values are determined by rated resistance and track material. The following formulas may serve as guidance simultaneously as they contain certain derating information. If the resistance value in question is called R and is expressed in kΩ when we insert it in the “0.4 power fraction”, we get the following conditions.

Carbon track and conductive plastic:  

Example. R=100kΩ gives I=[10/100]0.4 ≈ 0.4mA.

Figure 36. Worm gear trimmer. Bourns 3296

Figure 38. Geared spindle drive 1.25 inch potentiometer. Bourns

Source: Passive Blog Components