I am far from an expert on the use of potentiometers and writing this blog for a site like Planet Analog is a prime example of "leading with my chin." I expect any number of corrections, but since no one else seems to have decided to tackle this lowly subject and since I have worked quite a bit with pots, I thought I would try to put something on paper.
As I understand it there are two different reasons for using an adjustable resistor. In the first we are trying to control a device through its complete range of operation. Such a device might be a lamp dimmer and typically these devices tend to be high wattage. Modern design tends to frown on this simple solution, preferring additional circuitry to improve efficiency, simplify operation, and add features. In the second approach we (or at least I) design a circuit using standard parts and then improve the circuit's performance by using a potentiometer (pot) to trim the circuit values.
Pedantically, a device that acts as a voltage divider is called a potentiometer and is a three terminal device. The pin that allows for the variable resistance is called the wiper which is named for the way the mechanical connection is made to the fixed resistor. A variable resistor is a rheostat and is a two terminal device. With mechanical pots I am not sure if there ever was a pure rheostat, but with the advent of electronic pots it is possible to realize one. Of course it is very easy to configure a potentiometer into a rheostat as shown in Figure 1. Having said all of that, it is rare to hear the word rheostat used nowadays, being almost always being replaced by pot, potentiometer, or even trimmer.
Two methods of configuring a pot as a rheostat. At first glance they are identical, but the configuration of (b) keeps a current flow even if the wiper of the pot goes open circuit (a not uncommon event).
When trying to create a trimmer circuit, it is not normally a good idea to use the setup of Figure 2 (a) for use in the rheostat mode because the thermal characteristic of the pot is normally several orders of magnitude worse than a fixed resistor even for an expensive multi-turn pot. In Figure 2(b) the value of the pot is normally much smaller than the fixed resistor and so its thermal performance has a much reduced effect on the overall resistance.
How to improve the thermal performance of a rheostat configure potentiometer.
As a potentiometer you might be tempted to use the arrangement in Figure 3a. It may be cheap, but the resolution of 1% is difficult even with multi-turn pots and the temperature drift will probably preclude precision design. Figure 3b is an improvement since the overall percentage error contributed by the pot is much less provided its value is much less than the fixed resistors. However the thermal variation of the end-to-end value of the potentiometer will affect the output.
I understand that this parameter is far worse than the ratiometric drift. In addition, the value of the pot tends to be relatively small and may be difficult to obtain. I have only seen figure 3c in a 1987 Maxim seminar (the source of the whole of this block of information), and I think it is worthy of a repeat performance. The absolute value of the trimmer has no effect on the output, and large pot values can be used. Also the resolution of the adjustment is maximized to the center of the range.
Potentiometer mode. In ©, the initial value of the resistive divider R1/R2 is set to the expected value. To set the value of R3 consider at one end of the range of Rp, the minimum voltage will be the resistive divider created by R1 and R3||R2. Conversely the maximum will be the resistive divider of R1 ||R3 and R2.
Now what happens if you want to adjust the gain of an amplifier by adjusting a pot? There are several application notes that deal with this in far more detail than I can in this blog. I would recommend AN1316 from Microchip, Tutorial 864 from Maxim, and Circuit Note CN-0112 from Analog Devices. All these refer to electronic pots, but the deliberations are the same for mechanical ones working with the specifications for the device you want to use.
Here I am at the end of Part 1 and I am still trying to figure out where to insert the fact that the resistance value adjustment (the taper) can be either linear or logarithmic. Logarithmic pots are often used in audio volume control circuits and are sometimes called audio pots. Generally you will know from your application which taper to use.
In Part 1 of this blog I have tried to treat the potentiometer as a generic device. In Part 2 I will move on to tackling the different types and the advantages of both mechanical and electronic pots.