Our last two lessons covered fixed-value resistors. In this lesson we’ll cover potentiometers – devices whose resistance varies when you turn or slide a knob. At the end of the lesson we’ll head into the lab and demonstrate what we learned (see the video at the end of this post) and that the measured results match our design predictions.
Potentiometers have a multitude of uses; they serve as volume controls, are used to adjust contrast and brightness on monitors, they set the output voltage and current on power supplies, can be trimmed to calibrate various instruments, and many more.
Potentiometers, often abbreviated as pot(s), come in single, as well as multi-turn options. Some stack more than one potentiometer in a single device – two potentiometers linked to the same shaft so they rotate and adjust together. Some integrate a switch with the potentiometer; like the volume control on a car stereo. Some potentiometers are rotary and some are sliders. Some are designed to be panel mounted, and others, primarily used for internal adjustments like calibration (trim pots), are mounted internally on a circuit board.
Some potentiometers vary their resistance linearly. For example, 5 degrees of rotation changes the resistance by the same amount at the start of the pot’s rotation range as at the end of the pot’s rotation range. Others vary their resistance logarithmically. For example, 5 degrees of rotation at the start of the pot’s range may result in a 10Ω change while a 5 degree rotation at the end of the pot’s range may result in a 10kΩ resistance change. Logarithmically scaled pots are often used in audio circuits.
Like resistors, potentiometers are available in a wide range of resistance values, tolerances, styles, sizes, current, and voltage ratings.
It’s important not to confuse potentiometers with the newer rotary encoders that are used in many applications where potentiometers were previously used. Potentiometers are analog devices that change resistance through rotation or linear motion. The potentiometer has an internal resistance element, either carbon or wire-wound, and a wiper that moves along the material thereby varying the resistance. Rotary encoders are digital devices that send a series of pulses to a decoder, normally a microcontroller, that decodes the signal and then takes some action based on the number of pulses and direction of rotation. The two are not interchangeable.
The potentiometer’s rated/stated resistance value is the total resistance from A to B. This portion of the potentiometer behaves like any other resistor. It’s the wiper (C) that makes a potentiometer unique.
Internally, a potentiometer is an adjustable voltage divider. The schematic symbol is shown below. The wiper (C) moves along the resistive material forming the voltage divider. Turning, or sliding, the pot in one direction increases the amount of resistive material between A and C while at the same time decreasing the amount of resistive material between B and C. As a result, the resistance from A to C increases and the resistance from B to C decreases, and vice versa. Turning the potentiometer all the way to one end results in a resistance of 0 from A to C and a resistance equal to the potentiometers maximum resistance from B to C.
Notice that a potentiometer is not necessarily a variable resistor. It is a variable voltage divider. The resistance between A and B never changes. A 1kΩ pot for example, will always measure 1kΩ between the end terminals (A-B).
However, we can use a potentiometer as a variable resistor by using only two of the terminals, only A to C for instance, or only B to C. Which two you use will determine if the resistance increases or decreases with clockwise rotation. When we use a potentiometer as a variable resistor, we normally connect one terminal (either A or B) to the wiper (C). This reduces noise and increases reliability. When we connect the variable resistor into the circuit, we can simply connect to the end terminals since whichever terminal is connected to C is electrically the same as terminal C. Now we can vary the resistance between 0Ω and the potentiometer’s stated value, which, in the below example, would be 1kΩ.
Let’s look at a sample (potentiometer as a variable resistor) circuit that we can demonstrate and test in the lab. Just be sure to use an input voltage of 3.5V or less or you’ll burn out the LED. Alternately, you could include a separate dropping resistor, in series with the pot, to limit the maximum voltage across, and current through, the LED.
Now let’s go back and look at our 3.3V voltage divider from the last lesson. Recall that we designed a voltage divider that would create a 3.3V supply from a 5V input.
Since a potentiometer is a variable voltage divider, we could have used a potentiometer as the voltage divider. This would have the added advantage of allowing us to easily adjust the voltage if we discovered that the supply voltage (5V) was a bit higher or lower than what we designed for. Here’s the circuit.
While this will work, there’s a serious problem with it. Recall that the maximum voltage we could put across the load without damaging it is 3.6V.
As designed, this circuit will allow the load voltage to go all the way up to 5V. That doesn’t bode well for our connected device (the load).
We can address that by putting a dropping resistor before the potentiometer so that we limit the maximum voltage at the top of the pot to 3.5V which includes a 0.1V safety margin. We’ll set the total resistance so that we have the same current flow as in the previous design, 3.5mA. Here’s the circuit.
While this circuit will work as is, there’s really no reason for the output voltage to range all the way down to zero. If we assume a single turn potentiometer with 270 degrees of rotation, then with a range of 0 to 5V we end up with a precision of 3.5/270 = 0.0129 volts per degree of rotation. We can get much finer granularity in our adjustments if we limit both the upper and lower ends of the potentiometer’s range. For example, if we limit the lower range to 3.0V, then the precision increases to (3.5V – 3.0V)/270 degrees or 0.00185V/degree of rotation, making it much easier to set a precise value. See the circuit below.
The resistor values specified in the above circuit are non-standard values. To complete the design, and end up with something we can actually build, we need to choose standard resistor values that are as close as possible to our design values. Since we are using a potentiometer, which allows us to adjust the voltage precisely, we can go with 5%, or even 10% tolerance resistors. Consulting a table of standard resistor values, and a supplier website, provides the following readily available options for 5% tolerance components (430Ω, 200Ω potentiometer, 820Ω). Let’s plug these values in and redo the calculations to make sure we still meet our design specs.
Here’s the completed circuit with our standard resistor values substituted and the current and voltages calculated using Ohm’s Law. Using these values allows us to adjust the output voltage between about 2.8V and 3.5V.
Now let’s go take a look at the earlier LED dimmer circuit and this new adjustable voltage divider circuit in the lab. See the video below.
As you can see from the video, our design works and the actual lab results track very closely to our engineering predictions.
Well, that’s it for this lesson. Stay tuned for the next lesson which will complete our look at resistors. If you found this useful and want to know when new topics are available then please like and subscribe.
Cheers
Dominick
Note: Depending on the nature of the connected device (load), this circuit may or may not be sufficient for supplying a steady voltage. If this were a microcontroller, for example, internal and external switching activity would cause large swings in the device’s current requirements and a voltage divider would’nt be able to regulate the voltage given the dynamically changing current. For that we’d need a voltage regulator, which we will cover in future lessons.
