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Potentiometers are renowned for wearing out (at least in my experience); the little wiper eventually just wears its contact and nolonger has a firm electrical connection. For an audio device, this can manifest as a crackle when changing volume. The wear isn't necessarily even and there may be positions that have worse contact than others. I've noticed it be worse usually near the upper limit (full volume; full brightness; etc.), but the wear distribution can probably mostly be attributed to how the device has been used.
    Having a component with such friction seems like a very bad idea to me (and evidently it is) and I often wonder whether there are commercially available designs that do not have a sliding contact (excluding digital potentiometers[1]), and whether they're economical. I envision that one such wiperless design would be based on ball-bearings or epicyclic gears, with at least one of the balls or planet gears being conductive, the rest being insulative, and the tracks in which they roll, or the annulus or star/sun gear, having the resistive gradient element(s). But is anything like this currently available?


Note 1: It should behave similarly to an ordinary passive potentiometer. Digital potentiometers require a power supply and draw power so, as I understand, are not necessarily drop-in replacements (a 3-pin digital potentiometer would require that the end pins double-up as the power supply, which isn't always the case). I'm specifically interested in knowing whether such components as wiperless passive potentiometers exist which in their simplest form have 3 pins where the sum of the resistances between pins 1 and 2 and between pins 2 and 3 is intended to be constant (i.e. a 2-pin variable resistor is not by itself a potentiometer).

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    \$\begingroup\$ James, I've never come across or heard of epicyclic (planetary) potentiometers. You could have come up with something novel. \$\endgroup\$ – Nick Alexeev Mar 8 '15 at 20:10
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    \$\begingroup\$ You mean like a strain gauge? \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 8 '15 at 20:15
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    \$\begingroup\$ Why would a pot 'wear out' usually at the full volume setting where it is rarely set? Crackles in pots in audio are usually due to dirt or DC. I repait vintage audio, 40-60 years old, and I've never seen the failure mechanism you describe. I've only seen the wiper lift off the track completely, or mechanical failure of an associated power switch. \$\endgroup\$ – user207421 Mar 8 '15 at 21:37
  • \$\begingroup\$ @EJP Pots crackle the most at the ends of travel, because that's where you shove all the dirt to. Ideally, there should be no current flowing through a pot's wiper - whether DC or AC. This makes it extremely hard to apply a potentiometer in a low-noise audio circuit: high impedance buffers are plenty noisy. \$\endgroup\$ – Kuba Ober Mar 8 '15 at 21:39
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    \$\begingroup\$ Just a question - are digital pots out of the question for your design? They are controlled by a microcontroller so you can get around the "scratchiness" issue with the tradeoff of a microcontroller input/output and perhaps a way to simulate linear/log control of it. \$\endgroup\$ – cowboydan Mar 8 '15 at 22:13
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How to get the most out of a potentiometer?

In many precision, low-noise designs, it's a bad idea to start with to even have the signal routed through the front panel. So, at the very least, the control element should merely produce a voltage signal that governs a voltage-controlled amplifier/attenuator. With a potentiometric source, you can buffer and low-pass-filter the control signal, so that wiper dropout effects are minimized.

schematic

simulate this circuit – Schematic created using CircuitLab

Here, a voltage reference feeds the potentiometer. The variable wiper resistance is modeled by Rw, which can vary by 9 orders of magnitude, but is mostly "low" and on the order of an Ohm. R2 keeps the time constant above 50ms. Since R2>>R1, R1's influence is small. C2 forms a low pass filter with R1+R2, but also acts as a hold capacitor. U2 is an op-amp set up in non-inverting mode, so that its input has a very high impedance. The output of U2 goes to a voltage-controlled amplifier.

C2 should be a low-leakage type with NP0 or plastic dielectric, and U2 should have a FET or CMOS input stage. So, don't use 741 for U2 with expectation that it would work all that great - although it will still work better than the naked potentiometer would.

If the wire from R1 to the circuit is long, you might need a bootstrapped shield. Some experimentation is necessary to ensure stability of the circuit then, though, as the shield-to-signal capacitance adds positive feedback to the system.

That already gives you a much better performing circuit than using a potentiometer directly on the signal. Even with a fairily short 50ms time constant, you can get rid of crackle even on the most ridiculously dirty potentiometers. You can always trade off response time for insensitivity to crackle.

Routing audio to front panels is usually an EMI nightmare and it's often not cheap at all to do it properly.

Voltage Controlled Gain

A good bang-for-the-buck voltage-controlled gain element can be made by using a photoresistor illuminated by a LED. Photoresistors, if you select them, can have very low voltage coefficient of resistance and thus very low distortion, certainly beating most simple multiplier circuits by an order of magnitude or more. They are available as self-contained units, known as Vactrols, from Excelitas. They need to be applied with some care, as you don't want to exceed about 100mV across the photoresistor, but otherwise they are wonderfully powerful devices for about $5 each.

There are decent integrated voltage-controlled amplifiers, such as the last-time-buy (sadly) SSM2018, or newer AD8338, THAT2181, etc.

How about rolling contact?

If you still have a mechanical mouse, open it up. Take the ball out and look at the rollers. Invariably they'll be covered with a hardened track of grime. Rolling contact isn't all it's cracked up to be if you can't control the environment quite well. Sliding contacts have a self-cleaning property. Rolling contacts, in a potentiometer, would have the exactly opposite behavior - they'd be self dirtying. That'd be a very bad idea.

Mechanically there's another aspect you seem to forget: rolling contact is wonderful at concentrating the stresses, and requires sufficiently hard surfaces to prevent wear. It's kinda hard to make a low-power resistive sensor where the surface needs to interface with a metal ball/roller while having any sort of useful life expectancy.

If you really don't care about the power of the circuit, you're fee to make the resistive track, C-shaped, out of hardened steel. Feed it a couple of amperes, in pulses, use a sample-and-hold circuit to get the pulse amplitude, and you're set. It'll work as long as you house it in a dust proof enclosure. Note that dust-proof is usually harder than water-proof (!).

TL;DR: The rolling contact would be possibly the worst thing you'd ever wish in a potentiometer wiper.

So, what other options are there?

You can obtain the signal from other sources. They all work by converting shaft angle into a voltage, using a variety of techniques. I present them in no particular order.

Noncontact Potentiometers

Suppose that you start with a basic, C-shaped resistive track of a potentiometer. Choose a large one, so that it's easy to work on. Open it up. Bend the wiper so that it's lifted up from the track, but just ever so slightly. Feed the track with an AC signal, say a 1MHz square wave, with the other end of the track at 0V. The wiper is capacitively coupled to the track, and will pick up a signal whose amplitude is proportional to the position on the track. You will need to tweak it to get rid of the worst parasitic capacitances, but work it will. You can use a FET follower or an op-amp to lower the impedance of the wiper's signal, then use a synchronous demodulator to convert the amplitude back to baseband. It might sound fancy, but for such a simple sensor you can do it on a couple dollars worth of parts, nothing fancy needed at all. You don't need any better than 10% linearity anyway.

Variable Transformers

A very precise, and perhaps an over-the-top source would be a RVDT (a rotary cousin of an LVDT). For a one-off "vanity" project, it'd be a nice choice - these things are virtually indestructible, and with luck you can get them cheaply from surplus. For a volume control, you could make a very simple RVDT conditioner (the circuit is same as for an LVDT).

Variable Capacitors

Another vanity option would be an old, heavy, rotary capacitor. The better ones have a pair of ball bearings. Similar to an RVDT, they have no other contacting parts to wear out. Put the capacitor in a multivibrator circuit, hook up to a voltage-to-frequency converter circuit (LT app notes have plenty of those), and you're set.

Magnetic Sensors

A much lower cost option would be a Hall sensor. Suppose you have a magnet oriented radially on a shaft, and a Hall transducer next to it. As you rotate the shaft, the magnetic flux passing through a properly placed sensor will vary. This is a good source of a control voltage - cheap to implement, too.

Optical Sensors

You can also have an optical sensor: print a V-gap, with X-Y mapped to polar coordinates, on a sheet of transparency foil. Install on the shaft. Put a LED-photodectector pair so that it "sees" through the gap. Condition the photodetector (either a transistor or a diode) with an op-amp.

Another optical option that doesn't need a V-gap would be to have a tilted disc mounted on the end of a shaft, so that it's not quite perpendicular to the shaft's axis. Then use a reflective sensor (LED + photodetector) to obtain a continuous signal proportional to the angle.

Another optical option is to have a multiphase pattern printed on a cylinder on the shaft, and use multiple optical sensors, with their outputs summed, provide the output. The pattern might look as follows:

axial distance
^
|   █████████
|      ██████
|         ███
|0---------360--> angle

As the cylinder turns above the sensors, their outputs get progressively lower. By judiciously tweaking off the number of detectors/stripes, and the detection distance, you can get by with a simple black-and-white pattern. Sometimes that's easier to manufacture than something fancier.

Strain-to-angle Converters

Yet another option, quite sensible if you know how to deal with strain gages, would be to have the shaft interface with a long spiral spring. Slap a 4-gage strain gage bridge somewhere on the spring, with the sensitive axis along the spring's length, and you get a very nice signal proportional to the shaft angle. You'll need to add a bit of friction into the mechanical circuit so that the shaft stays put when you release the knob.

Odds-and-ends

Yet another option, if you want to get funky, would be to have a variable acoustic capacitor. Have the shaft go through a flat toroidal box. It can have a rectangular cross section, of course. Make a radial slot through the inside of the box, and extend a radial pin from the shaft through the radial slot. Attach a paddle that almost fills up the box's cross section to the end of the pin. At the zero point in the box, add a partition and an acoustic transducer. Attach it to an oscillator, and you've got an electro-acoustic angle-to-period converter.


The above are only the things I have tried, with some degree of success, at some point in life. There's almost an infinite supply of other ideas, if you want to have some transduction fun.

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  • \$\begingroup\$ If you can handle the additional complexity, a rotary optical encoder is probably your best bet. They're used extensively in the space industry because they're low power, reliable, and don't wear out. (I'm working on a payload that's using a potentiometer to save cost, but let me tell you it's not worth it.) \$\endgroup\$ – 2012rcampion Mar 8 '15 at 21:49
  • \$\begingroup\$ @2012rcampion The major problem with encoders is that they offer a discrete output. If one cares, as some people do, about a truly step-less output, that's only time-discrete if at all, all those other methods can work quite well. Everything depends on the degree of overboardiness you wish the project to be. For a vanity project, the funkier the better :) \$\endgroup\$ – Kuba Ober Mar 8 '15 at 21:54
  • \$\begingroup\$ You're right, I wouldn't use an encoder to replace a pot in any analog circuit (say, the volume pot on a guitar for example). I'd use it if you're digitizing the output anyway (e.g. to read into a uc or to control the output gain on a dac). \$\endgroup\$ – 2012rcampion Mar 8 '15 at 22:00
  • \$\begingroup\$ @2012rcampion Fankly said, I would not apply a pot directly to the audio signal even on a guitar. In fact, I would never ever apply it directly to an audio signal period. It never is a durable solution, and replacing crackly pots is not my favorite pastime. Probably the simplest low-distortion controlled resistance is a photoresistor - it would be a way better element for volume control than a pot. \$\endgroup\$ – Kuba Ober Mar 8 '15 at 22:03
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    \$\begingroup\$ You can build a fully continuous optical encoder by gradually occluding a light or changing its incident angle. It's just that then you're hostage to the linearity of your detector. \$\endgroup\$ – pjc50 Mar 9 '15 at 12:40
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No, they don't exist. Simply because they can't.

A potentiometer consists of a carbon track with a wiper moving up and down it. You can't have that wiper moving over the carbon track without friction. Yes, you could reduce the friction with bearings and such, but there will always be that friction.

So people use a rotary encoder instead - most often an optical one if you want low friction - a disc with slots in it that breaks a number of infra-red beams.

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    \$\begingroup\$ Typical potentiometer has sliding friction. If I understand the the O.P correctly, he is proposing to use different mechanics that would replace sliding friction with rolling friction. This may or may not be feasible or economical. But the idea seems neat, at least from a blue-sky standpoint. \$\endgroup\$ – Nick Alexeev Mar 8 '15 at 20:15
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    \$\begingroup\$ “You can't have that wiper moving over the carbon track without friction.” – Sure, a car's tires have a tiny bit of rolling resistance, due to deformation of the rubber, cornering, imperfect shape and angle of the wheel and road, minor misalignment with the other wheels, etc., but this is somewhat different to an exhaust pipe scraping behind on the road. :-] \$\endgroup\$ – James Haigh Mar 8 '15 at 20:32
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    \$\begingroup\$ Roll a ball bearing over a piece of carbon for a while. Watch the groove appear. Then imagine how you are going to prevent that ball bearing ending up loose and breaking contact. Pressure. Increased pressure. That equals increased friction, and increased groove depth. \$\endgroup\$ – Majenko Mar 8 '15 at 20:36
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    \$\begingroup\$ Only the cheapest of potentiometers use a carbon track. There are conductive polymer designs, as well as cermet ones. All are way too fragile to roll anything on them. Sliding contact actually is the most gentle way to interface with the resistive track. Most potentiometers, improperly applied, don't wear out but merely get crackly. That's due to dust trapped on the track, between the slider and the track. It doesn't indicate wear, but merely the reality of how hard it is to keep the dust out - and is is hard. The OP somewhat chases a solution without defining the problem. \$\endgroup\$ – Kuba Ober Mar 8 '15 at 20:54
  • \$\begingroup\$ If you use a rolling cylinder instead of a ball bearing, then you would have a lot more surface area spreading out the applied force and reducing friction. To see what the professionals do to make a precision decade resistance box, you'll see that they don't avoid friction: EEVblog #461 - Genrad Decade Resistance Box Teardown - at youtube.com/watch?v=fKrvtYS_6fI&t=10m18s \$\endgroup\$ – MicroservicesOnDDD Feb 25 at 0:10
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It is very difficult to avoid having wiper resistance vary arbitrarily with wiper position. In a good design, however, wiper resistance will have minimal effect on circuit behavior. Each tenfold reduction in the amount of current carried by the wiper will cause a tenfold reduction in the amount of voltage superimposed by its resistance. Likewise each tenfold increase in the voltage carried by the pot will cause a tenfold reduction in the significance of any voltage superimposed by the resistance.

If a device tries to drive a 1/8 watt 8-ohm speaker (1VRMS) using a 10-ohm pot as a volume control, a one-ohm variation in wiper resistance will manifest itself as a 1/8-volt variation in the signal. Nasty. If one were to use a 50:1 step-up transformer to scale the voltage from 1V 1/8A to 50V 1/400A before passing it through a 500-ohm pot, then a one-ohm variation in wiper resistance would manifest itself as a 1/400-volt variation in the signal at the pot; passing it through a 1:50 step-down transformer to drive a speaker would make it appear there as a 1/20,000-volt signal (a 2,500-fold reduction versus controlling the speaker directly). A major improvement.

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On an more engineering aspect, to achieve the effect of a "friction-less pot", you can control a digital pot (or something similar) with a contactless measurement tool.

For example, you can get one of those sonar modules and control a d-pot by translating the distance between the sensor and the moving target as measured contactlessly using the sonar into a resistance (or wiper position) on the d-pot.

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