|Elliott Sound Products||Beginners' Guide to Potentiometers|
Copyright © 2001 - Rod Elliott (ESP)
Page Created 22 Jan 2002
The humble potentiometer (or pot, as it is more commonly known) is a simple electro-mechanical transducer. It converts rotary or linear motion from the operator into a change of resistance, and this change is (or can be) used to control anything from the volume of a hi-fi system to the direction of a huge container ship.
The pot as we know it was originally known as a rheostat (or reostat in some texts) - essentially a variable wirewound resistor. The array of different types is now quite astonishing, and it can be very difficult for the beginner (in particular) to work out which type is suitable for a given task. The fact that quite a few different pot types can all be used for the same task makes the job that much harder - freedom of choice is at best confusing when you don't know what the choices actually are, or why you should make them. This article is not about to cover every aspect of pots, but is an introduction to the subject. For anyone wanting to know more, visit manufacturers' web sites, and have a look at the specifications and available types.
The very first variable resistors were either a block of carbon (or some other resistive material) with a sliding contact, or a box full of carbon granules, with a threaded screw to compress the granules. More compression leads to lower resistance, and vice versa. These are rare in modern equipment, so we shall limit ourselves to the more common types .
It is worthwhile to have a look at a few of the common pot types that are available. Figure 1 shows an array of conventional pots - both PCB and panel mounting.
Figure 1 - Some Examples of Pots
Note that these are not to scale, although the relative sizes are passably close. Apart from the different body shapes and sizes, there are also many "standard" mounting hole and shaft sizes. Probably the most common of all is the one in the centre of the picture. A panel mount, 25 millimetre (1") diameter pot. This uses a 10mm (3/8") mounting hole, and has a 6.35mm (1/4") shaft. These pots have been with us almost unchanged for 40 years or more.
The remainder show a few of the many variations available. The fluted shaft types are commonly referred to as "metric", but will accept a standard 1/4" knob - albeit with a little play (it is less than a perfect fit, but is acceptable if the grub screw is tight enough). Metric pots are also available in 16mm round and 25mm round formats.
Most rotary pots have 270 degrees of rotation from one extreme to the other. A "single turn" pot is therefore really only a 3/4 turn device, despite the name. There are some other rotary types with only 200 degrees or so, and some specialty types may have less than that again.
|The standard schematic symbol for a pot is shown to the left (although some people insist on using zig-zag lines for resistors and pots, I don't and never have, so don't expect me to start now . A little later, we shall look at the many ways a standard pot may be wired, as well as some further explanations of the different "law" or taper used. Project 01 has been on this site for a long time now, and is a simple and effective way to create an almost logarithmic taper from a linear pot - but I am getting ahead of myself here.|
First, we need to continue with the examination of the basic types (and you thought the above small sample was enough ) Well, as they say ... "You ain't seen nothin' yet!"
Before we look at other pot types, a quick sample of knobs. Yes, I know that everyone has seen knobs, but a dissertation on pots would be less than complete if I didn't include these.
Of these, only one deserves special mention - the one on the left. This is a multi turn vernier readout (analogue in this case) for a standard pot. Typically used with precision wirewound or conductive plastic pots, these used to be common on equipment where very accurate (and repeatable) settings were required. They are expensive, but in their day were almost indispensable. Now, a digital panel meter is cheaper, and considered much more "high tech" - such is progress, but at the expense of the "olde worlde" charm of a mechanical contrivance. And yes, you can still get them!
The remainder are perfectly ordinary knobs, and again, are but a very small sample of those available from a multiplicity of manufacturers. Most cheap knobs are plastic, but they are available with brass inserts, in solid aluminium (brushed, anodised, etc.), plastic innards with a thin aluminium outer shell or just an insert. You can even buy
audiophool audiophile solid wood knobs, optionally coated with special lacquer that is designed to make you think the sound has improved (nudge-nudge, wink-wink ). The list is endless, but I shall end it here.
Then of course, there are trimpots (aka trimmers) - pots designed for "set and forget" applications. They are used for "trimming" the value of a resistor, and are commonly used for calibrating instruments, setting the bias current on power amplifiers, and a host of other areas where a circuit cannot be relied upon to give an exact gain, output voltage, or current. Naturally, a normal panel pot can be used, but these are very much bigger, and any calibration or setup control should not be made available for everyone to fiddle with as they please.
This is a very small sample of those available. The first and fourth are multi-turn types, and these should be used when a very precise setting is required. Because they are sealed, they are relatively immune from contamination, and for all but the most trivial application, should be used instead of the open types (#2 and #5). Trimpots (as shown) are generally available as vertical or horizontal - the choice is usually made based on ease of adjustment of the final circuit.
The taper (also called "law") of a pot is important. We need not worry with trimpots, since they are almost always linear, and I do not know of a supplier of anything other than linear trimpots. For all panel pots, we must be aware of the use the pot will have, and select the correct type accordingly.
The most common use of a pot in audio is as a volume control. Since our hearing has a logarithmic response to sound pressure, it is important that the volume control should provide a smooth variation from soft to loud, such that a given change in position of the pot causes the same sensation of volume change at all levels.
First, the term "taper" needs some explanation. In the early days, when an audio taper (logarithmic, or just log) was needed, the resistance element was indeed tapered, so that it provided a different resistivity at different settings. By changing the physical taper, it was possible to make a pot provide the exact gradient of resistance needed. By definition, a linear pot has no taper as such (the resistance element is parallel sided), but the term has stuck, so we might as well get used to it.
The violet curve in Figure 4 shows an antilog or reverse audio taper pot. These are quite uncommon, but used to be used for balance controls using a log/antilog dual section (commonly called dual gang) pot. It is shown on the graph mainly for its interest value, but they are generally an historical component now.
All this tapering proved a rather expensive exercise, so manufacturers economised ("they" won't notice the difference!), and worked out a method of using two resistance elements of differing resistivity, and joining them to create what I referred to as the "Commercial log" taper. In short, it doesn't work (not properly, anyway), and the discontinuity where the two sections join is almost always audible with cheap "log" pots.
Project 01 showed how this can be fixed, and I will explain the logic a little more as we progress. In the meantime, I suggest that you get an old pot and dismantle it so that you can see exactly what is inside. I could show you some photos, but there is nothing like doing it yourself to really get to know the subject.
Now, this should be dead easy - a simple code to indicate the resistance and law of a pot should cause no grief to anyone, right? Wrong! It wouldn't have been so bad if someone hadn't decided to change it, and even then, it wouldn't have been so bad if there was no overlap between the "old" and "new" "standards" ... I think you can see where this is headed by now.
|Taper||Old Code||New Code||Alternate|
Wasn't that a nice thing to do? It is obviously important to check before you make assumptions, or you can easily get the wrong type - especially if working on older equipment.
At least the resistance marking is usually sensible, so a 100k pot will be marked as 100K - but not always. The coding system used for capacitors is sometimes used as well (especially on small trimpots), so a 100k pot could also be marked as 104 - 10, followed by 4 zeros, or 100000 (100k) ohms.
Because they are variable, there is a much smaller range of potentiometer values, almost always in a 1, 2.5, 5 sequence. Common values for panel pots are 1k, 5k, 10k, 25k, 50k, 100k, 500k and 1M - 2.5k and 250k went missing along the way, and these are not stocked by very many distributors. 25k pots are becoming harder to get as well. Not all values are available in log and linear, and in some cases you may even find that for a particular type, you can get them in any value you want, as long as it's 100k (for example).
Trimpots suffer a similar fate. The only way to know what you can get from your local supplier is to check their catalogue. In reality, everything is available, but you may have to go a very long way to get it.
For most audio applications, these are of little on no consequence. In many other applications however, exceeding the specified ratings could lead to the destruction of the pot or yourself! Neither can be considered a good thing.
Power - A pot with a power rating of (say) 0.5W will have a maximum voltage that can exist across the pot before the rating is exceeded. All power ratings are with the entire resistance element in circuit, so maximum dissipation reduces as the resistance is reduced (assuming series or "two terminal" rheostat wiring). Let's look at the 0.5W pot, and 10k is a good value to start with for explanation.
If the maximum dissipation is 0.5W and the resistance is 10k, then the maximum current that may flow through the entire resistance element is determined by ...
P = I² * R ... therefore
I =√P / R ... so I = 7mA
In fact, 7mA is the maximum current that can flow in any part of the resistance element, so if the 10k pot were set to a resistance of 1k, current is still 7mA, and maximum power is now only 50mW, and not the 500mW we had before.
Voltage - Two separate issues here. One is directly related (in part, at least) to the power rating, and is important to ensure that the life of the pot is not reduced. Knowing about the other might save your life.
Voltage across resistance element - The maximum voltage across the example pot from above is 7mA * 10k, or 70V. This will rarely (if ever) be achieved in an audio system, but is easy with many other designs. As the resistance increases, so does the voltage - a 0.5W 1M pot will pass only 700uA at maximum power rating, but the voltage needed to create this current is 700V. Unless the pot is actually rated to withstand 700V across the resistance element (rather unlikely), it will fail - maybe not today, or tomorrow, but it will fail eventually. Special pots are made (custom jobs, of course) for high voltages, and standard pots should never be used beyond their rating - assuming that you can find out what the rating is, of course.
Dielectric Voltage - The dielectric (insulation of pot "guts" to the body) rating is especially important if the pot is connected to mains operated, non-isolated equipment. Wall mounted lamp dimmers and such are typical examples. This is not commonly specified, but for safety, should be at least 2.5kV. A common way to achieve this is to use a plastic shaft, with the body of the pot insulated from the chassis, and inaccessible by the user - even if the knob falls off or is removed! This point cannot be stressed highly enough.
Most standard pots will safely withstand (maybe) 100V or so between the resistance element and terminals, and the body and shaft. Miniature types will usually be less than this. Never, ever, use a standard pot with a metal shaft to control direct mains operated equipment. Even if the pot case is earthed, the voltage rating between the internal element(s) and terminals to the case is often unspecified, and is almost always completely unsuited to mains voltages.
"But we already covered that, didn't we?" Not really - I merely glossed over the basics. Now, we shall look at a few examples of pots you may come across. Firstly, there is the resistive material and some typical characteristics ...
|Material||Manufacturing Method||Common uses||Power (Typ)|
|Carbon||Deposited as a carbon composition ink on an insulating (usually a phenolic resin) body||Most common material, especially for cheap to average quality pots. Has a reasonable life, and noise level is quite acceptable in most cases. (DC should not be allowed to flow through any pot used for audio control)||0.1 to 0.5W|
|Cermet||Ceramic/metal composite, using a metallic resistance element on a ceramic substrate||High quality trimpots, and some conventional panel mount types (not very common). Low noise, and high stability. Relatively limited life (200 operations typical for trimpots)||0.25 to 2W|
|Conductive Plastic||Special impregnated plastic material with well controlled resistance characteristics||High quality (audiophile and professional) pots, both rotary and linear (slide). Excellent life, low noise and very good mechanical feel||0.25 to 0.5W|
|Wire wound||Insulating former, with resistance wire wound around it, and bound with adhesive to prevent movement||High power and almost indefinite life. Resistance is "granular", with discrete small steps rather than a completely smooth transition from one resistance winding to the next. Low noise, usually a rough mechanical feel.||5 to 50W|
Bear in mind that the above list is a rough guide only, and is not intended to be be the "last word" on the different resistive types or their characteristics. In all cases, if you really want to know the full details about any one of those listed, get the manufacturers' data for the pot - it will be a lot more accurate (and specific) than the brief explanations above.
In addition to the resistive materials, there is also the physical type of pot. I am not going to describe the size and shape, but how the pot is configured mechanically and electrically.
|Rotary||Single gang||Single turn||Single channel controls for monoblock amplifiers, guitar amps, or anywhere that a single control is sufficient for the application.|
|Rotary||Single gang||Multi turn||Precision trimpots for critical applications. The resistance range is covered in anything from 10 to 25 turns of the screwdriver slotted actuator. There are some multi turn panel pots, but these are quite rare and expensive. Multi turn dual pots are also very uncommon.|
|Rotary||Dual gang||Single turn||Stereo applications, or anywhere it is desirable to change two separate resistances at once. Nearly all dual gang pots have equal resistances and tapers, but it may be possible to rebuild a dual gang pot using intestines removed from another pot of the same make and type (not needed very often though)|
|Rotary||Dual concentric||Single turn||Commonly used in (cheap) car radios and some consumer goods. These feature dual concentric shafts, allowing a single pot position to provide (for example) volume and "tone". The knobs are designed to fit the separate shafts (which are usually of different diameters). Almost impossible to buy from retail outlets or manufacturers in small quantities. (Usually to special order)|
|Linear||Single gang||Slide||Commonly used as "faders", unless they are of high quality, best just called slide pots. They are available in a variety of lengths, from 30mm to 100mm or more of linear travel. True faders will normally be relatively long, and generally are conductive plastic (and rather expensive )|
|Linear||Dual gang||Slide||As above, but for stereo mixers. Otherwise identical comments apply.|
Again, this is a simplified listing. If you are willing to pay for 10,000 units, most pot makers will quite happily build you a triple gang pot with unequal resistances and different tapers, or an eight gang pot so you can build a variable stereo crossover network. In fact, almost any configuration is possible, but for various reasons may not be plausible or sensible.
Nearly all manufacturers and distributors have settled on a limited range of "standard" values and types, based on the most common uses for their products. That other configurations used to be available but were withdrawn due to lack of consistent sales is a lamentable fact, brought about by "economic rationalisation", which basically means that if they don't sell them in good quantities, they will be neither made nor stocked by anyone (unless you are happy to pay through the nose, of course).
Most problems of this type can be solved by throwing money at them until the problem disappears, but few of us can afford this approach - besides, I think the military establishments of the world have a patent on that method :-)
A standard single gang pot is shown in Figure 5. The important external bits are shown so you can refer to them as needed. I have (somewhat arbitrarily) numbered the terminals as 1, 2 and 3. Terminal 2 is the wiper. For a "standard" volume control application, 1 is normally connected to ground, the input is applied to 3, and the output taken from 2 (wiper) allowing the output to be varied from ground (no signal) to input (maximum signal).
In addition, there are a few odd-ball additions to the list. These include pots with integral switches (as used in small transistor radios - hint as to where to get one if you need it badly enough). The switches may be rotary, so in the minimum volume position, the switch is off, or they may use a push-pull switch. Older car radios often use a combination switch and dual concentric pot, so that power, volume and tone can all be controlled with one knob complex.
As before, the possibilities are almost endless, limited only by imagination and budget. There are few mechanical constraints that will prevent a special design from being feasible, although expecting accurate tracking on a 100 gang pot might be asking too much :-) Could it be made though? But of course - leave your large sack of money at the door, sir, and come on in.
Oh yes, I almost forgot. Motorised pots. Standard (or high quality) rotary or slide pots that are driven by small DC motors to allow remote control. Even a cheap pot will usually outperform an expensive "digital" volume control, with the added advantage that it can be operated by hand or with the remote. These are quite common, and even some of the (relatively cheap) Chinese made subwoofer "plate" amps use them for remote control.
Ah! Another one ... Most pot "gangs" are 3 terminal types, but there are some with a tapping partway along the resistance element. This was used in the bad old days to create a "loudness" control, where the bass and treble are increased at low levels to compensate for the way our hearing reacts to different levels. Because there was rarely (if ever) any attempt to match the acoustic power levels, the loudness control was always wrong. To get it right requires source, preamp, power amp and speakers to have a known gain/ sensitivity, and ideally a preset control would have been incorporated to ensure the system could be calibrated. This was never done by the vast majority of manufacturers - Yamaha appears to be the only maker who even made an attempt (I don't know how good it was, never having seen a system that used it).
Well, that part is simple, isn't it? Judging from the number of e-mails I get asking about how to wire pots, the answer is obviously "no". Being 3 terminal devices (for a single gang), there are quite a few different ways that they can be wired. Connection to a single terminal is rather pointless, so at least that eliminates three "possibilities". At this point, a diagram is needed ...
Figure 6 - Potentiometer Terminals and Connections
As shown in Figure 6, a pot is usually wired using all three terminals, and I have used the same numbering scheme as in Figure 5. One terminal (1) is earthed (grounded) for use as a volume control - the most common usage. This allows the wiper to be turned all the way to zero signal for maximum attenuation. Note that if the earth terminal were to be left disconnected, all we have is a variable series resistance, whose effectiveness will be minimal in typical circuitry. This is still a common usage however, but for different reasons (see below).
Turning the shaft clockwise (CW - by convention, to move pin 2 physically closer to pin 3, and increase (for example) volume) will select a different point along the resistance element, and forms a voltage divider, so the attenuation of the signal is proportional to the rotation of the shaft. At the fully clockwise position, there is close to zero ohms in series with the signal, and the full resistance of the pot to earth. Attenuation at this setting is zero (assuming a zero or low impedance source - this is often overlooked!), and this is full volume (maximum signal level).
The source impedance should normally be no greater than 1/10th (0.1) of the pot's stated resistance. Further, the load resistance or impedance should be 10 times the pot's resistance to prevent the taper from being adversely affected. You may (of course) be deliberately loading the pot as described below, but the following stage must still present a high impedance unless its impedance has been included in your calculations.
The second form of connection is a variable resistor. Not usable as a volume control, but still extensively used for other applications. It is common (and preferable) to join two of the leads together - the wiper, and one end or the other. The actual connection depends on what you are trying to achieve, and since there are so many possibilities, I won't even try to explain them all. When used in this mode it is most commonly refererred to as a variable resistance or resistor - the word rheostat is somewhat dated (to put it mildly) and is not a term that I use in any of my articles.
To get an idea of the different configurations that are in common use, have a look at the ESP Projects pages, and those on other web sites. The number of possibilities is actually not that great, but people use different conventions as well. For example, in Australia, we use the term "anti-clockwise" or ACW. In the US, this is "counter-clockwise" or CCW. At least the term clockwise seems to be common to both countries :-) Naturally enough, these are only two conventions, and I am unsure of the terminology in other countries - especially if they don't use English (and why would they, if they already have a language of their own).
As a completely irrelevant side issue, the Web is changing this quite quickly, as the majority of web sites are in English.
Figure 7 - Volume and Balance Controls
Figure 7 assumes the use of a log pot for volume. The balance control can be done in many different ways, with that shown being but one. Quite a lot of Japanese equipment uses a dual gang pot for balance, but the resistance element only goes for half the travel. When set in the centre position, there is no loss at all, and rotation in either direction attenuates the appropriate channel, but leaves the other unaffected. This is yet another type of custom pot, made for a specific purpose. I know of no manufacturer that sells such an item through the normal distribution channels, so home builders have to come up with different ways to achieve the same (or similar) things. The balance control as shown above (with the values shown) will give a response very similar to the more complex version described in the next section.
Using pots can be done in the conventional way, or you can get adventurous and achieve a lot more. A good example is the "Better Volume Control" shown in Project 01. The other ideas presented also show how you can make modifications to the way a pot behaves, just by adding a resistor (R). The "ideal" value by calculation is 22k for a 100k pot, and
this gives a maximum deviation of +1.58 and -1.7dB from a real log curve. This is contrast to the original article, where 15k was suggested, and although the error is greater (+2.89dB and -1.12dB), the overall behaviour is almost ideal in listening tests.
Figure 8 - A Better Volume Control
Take a look at the balance control (below) as an example. The conventional balance control requires either a log/antilog pot (virtually impossible to obtain), or one of the special types commonly used in Japanese consumer hi-fi gear. About the only way you'll get one of those is to remove it from the equipment - again, they are virtually impossible to get from normal hobbyist suppliers.
Add a couple of resistors to a dual gang linear pot, and the problem is solved. Not only is the pot heavily "centre weighted", but will also maintain a relatively constant sound level as the balance is changed from full Left to full Right. The centre weighting means that for most of the pot's travel, the balance is shifted subtly, so it provides a very fine resolution around the central position - there is little requirement for only one channel (other than testing), but that is still available. In short, lots of benefits, and few drawbacks.
Figure 9- Centre Weighted Balance Control
Needless to say there are many other configurations that can be used, and this is but one. The resistor value (RL and RR) is fairly important - it really should be 35k for a 100k pot, but the error when using 33k is minimal (about 0.16 dB at centre position).
One of the goals of circuit design is to utilise available components. This is not necessary if you make 10,000 of something, since at these quantities special orders will cost little or no more than the normally available components. When you are making one for yourself (or perhaps two - one for a friend for example), specially designed components are not an option due to the setup costs (this could easily be thousands of dollars / euro / pounds). Even in quantities of several hundred, available components are still (usually) cheaper.
The balance control above is an example of a dual log / reverse log pot, created with a standard dual gang pot and a couple of resistors ... and it works better than a commercial offering is likely to - even if you managed to find one.
For more information on this configuration, see Project 01. Note that as shown, the balance control here is not optimised for any significant impedance at the output, so its performance will change if you connect a volume control to the output.
Figure 10 - Creating an "S" Curve for Lighting
Another example of modifying a pot to make it do what you want is shown in the LX-800 Lighting Controller. The faders need an "S" curve, to compensate for the non-linear behaviour of lamps and our eye's sensitivity to light levels. This is also achieved with a couple of resistors across a normal linear pot.
If you don't like the shape (because of the lamps you use, for example), you can simply change the resistor values and modify the curve to suit your exact needs. Since even ordinary log pots are not actually logarithmic anyway, can you imagine getting a pot that would give you an S-Curve? Even worse, if you found that it was not suited to certain lamps, then you would be hard pressed to modify the law to get what you needed. In some cases it would be impossible.
|Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2002/ 2003. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.|