2.11
Practical Resistors: Potentiometers
A potentiometer (sometimes just called a “pot”) is a mechanically adjustable resistor.
In general, a resistive strip of material is exposed with a terminal at either end. Then, a wiper is mechanically moved to make contact somewhere along that strip, with one terminal on the movable wiper. Overall, the potentiometer has three terminals, and the schematic symbol represents the idea of a wiper moving across a resistor:
Just like a resistor, a potentiometer is defined with a specified resistance and power rating.
There are many variations of potentiometers:
Sometimes the wiper movement is accomplished with a rotary motion, like a knob, and sometimes with a linear motion, like a slider.
Sometimes the wiper motion is basically linear with resistance. Other times, the strip is designed to give a certain profile of position versus resistance (for example, a logarithmic scale).
Sometimes the potentiometer is hidden in a place that is not adjustable to the end user of the electronic device. In this case it may be adjusted by manufacturing and test personnel, and is sometimes called a “trimmer.” In other cases, the potentiometer is a user-adjustable feature.
Sometimes, there are multiple mechanically connected potentiometers, where a single knob controls two (or more) independent potentiometers. For example, for left and right channels of an audio signal, there might be two independent electrical potentiometers connected to a single mechanical knob.
Potentiometer as a Two-Terminal Device
The potentiometer can be used as a two-terminal device. In this case, it’s just an adjustable resistor.
We can pick the wiper terminal plus either of the outside terminals. Either is fine, so we can choose which is best based on our desired mechanical layout.
The remaining terminal will be left open. Since no current can flow, all the resistive material outside the range of the two selected terminals has no effect.
For example, we can use a potentiometer to make an adjustable low-pass filter:
Exercise Click to open and simulate this circuit. How does the output change as we virtually turn the potentiometer knob?
In the simulation above, we use R1.K as the Sweep Pararameter, telling the simulator to re-run the simulation for each of
.
In the transient response plot, we can see that when the adjustable resistance is larger ( in this example), the 1 kHz square wave is dramatically reduced in amplitude at the ouptut of the low-pass filter. But if we turn the potentiometer toward its other end ( in this example), the square wave passes through mostly undisturbed. Over this parameter range, we are changing the effective resistance by a factor of:
which changes the low-pass filter RC time constant proportionally by the same factor of 19.
We repeat the same parameter sweep for the frequency-domain Bode plot, and find that the low-pass cutoff frequency indeed moves substantially by tweaking the potentiometer.
In the real world, this means that we can make filters that are user-adjustable (or adjustable at manufacturing time if the potentiometer is hidden).
Potentiometer as a Three-Terminal Device
The potentiometer can also be used as a three-terminal device.
In this case, the wiper effectively forms a voltage divider, splitting the total resistance into two pieces, and if is the position of the wiper from 0 to 1.
If the current drawn from the wiper terminal is kept small, and both ends of the resistive material are at known voltages, then the overall effect is that turning the potentiometer knob selects a voltage in between the two extremes.
Exercise Click to open and simulate this circuit. How does the output voltage change as we virtually turn the potentiometer knob?
The output voltage changes linearly all the way from to as we sweep the potentiometer from one end to the other ( ). The total current stays constant because the total resistance is constant.
See the Voltage Dividers section for more thorough coverage of this circuit.
Fine Tuning: Limiting Potentiometer Range
In both the two-terminal and three-terminal examples above, the potentiometer was the only resistor present, so its ability to adjust over the entire range from 0 to its full resistance gave us a wide variation in outputs.
However, in many cases, we want the user to only be able to adjust a resistance over a more narrowly-limited range.
For example, let’s suppose that we want to modify the three-terminal voltage divider above to only give us variation between, 4 V and 5 V. We can accomplish that by adding two additional fixed resistors, one to either end of the potentiometer:
Exercise Click to open and simulate this circuit. How does the output voltage range compare to the circuit above (without R2 and R3)?
Since resistors in series simply add their resistances, these additional resistances form a voltage divider circuit that effectively has a terminal (at the wiper) in between two resistances:
- The upper resistance contains both the fixed R3 and the upper part of potentiometer R1.
- The lower resistance contains both the fixed R2 and the lower part of potentiometer R1.
By choosing appropriate values for R2 and R3, we can turn the same potentiometer into a circuit that makes fine-tuning easier, because an adjustment in the potentiometer knob causes now a much smaller adjustment , making it easier to adjust more precisely.
This fixed-plus-adjustable arrangement is very common whenever you see a potentiometer, in both two-terminal and three-terminal applications.
Potentiometers Drawbacks
Potentiometers are mechanical devices and rely on good contact between the movable wiper and the resistive material.
As a result, potentiometers wear out. They can suffer from corrosion, or even simply a loss of spring pressure over time. They’re also especially noisy when they’re being moved.
For these reasons, potentiometers are less common than they once were. In trimmer and other manufacturing adjustment situations, they’re being replaced by digital adjustments where possible. And in user-facing scenarios, like knobs or joysticks, optical or magnetic solutions are becoming inexpensive and reliable enough to take over.
Potentiometers also get hot due to resistive heating, and are subject to temperature coefficients and maximum power ratings.
What’s Next
In the next section, Resistors in Series and Parallel, we’ll look specifically at building circuit networks out of resistors and how currents and voltages behave as multiple elements are combined.