The part used for dials that adjust brightness, radio volume, and sensor reference voltages is the potentiometer. It looks like a small part with a knob to turn, but inside it has a resistive strip and a contact (the wiper) that moves along it, and depending on how you connect it, it plays two roles. This article covers the structure of a potentiometer, the two ways to connect it, choosing a type and value, and tips for mounting it on perfboard.
The three-pin structure
A potentiometer has three pins.
- Pins 1 and 3: the two ends of the resistive strip. The resistance between them is the total resistance (for example, 10 kΩ), which does not change when you turn the knob.
- Pin 2 (the wiper): the contact that moves along the strip. When you turn the knob, the resistance between pins 2 and 1 and between pins 2 and 3 change in opposite directions, and the sum of the two is always the total resistance.
Two ways to connect it
1. As a variable resistor (using 2 pins)
Connect only the wiper (pin 2) and one end (pin 1 or 3) to the circuit and it becomes a variable resistor that varies from 0 Ω up to the total resistance. It is safer to tie the unconnected opposite end to the wiper. That prevents the resistance from going to infinity the moment the wiper loses contact.
- Uses: brightness control (in series with an LED), timer time adjustment (the R of a 555), simple current adjustment.
- Be sure to add a series protection resistor. Set a minimum resistance so that excess current cannot flow when you turn the potentiometer all the way to 0 Ω. For example, when adjusting an LED at 5 V, connecting a
330 Ω fixed resistor + 1 kΩ potentiometerin series makes the current vary between about 9 mA and 2.3 mA.
2. As a voltage divider (using 3 pins)
Connect pin 1 to the input voltage (Vin) and pin 3 to ground, and take the output from the wiper (pin 2), and you get a continuously adjustable voltage from 0 V to Vin. It is the most common way to make sensor reference voltages, volume controls, and analog inputs to a microcontroller. The principle of dividing is explained in Designing Voltage Dividers and the Loading Effect.
- If the input resistance of the circuit connected to the output is 10 times or more the total value of the potentiometer, there is almost no effect.
- 10 kΩ is the usual choice. To save current go up to 100 kΩ, and to be strong against noise go down to 1 kΩ–10 kΩ.
Types
Taper: A and B
How the resistance changes with the knob angle differs.
| Marking | Taper | Use |
|---|---|---|
| B (linear) | Changes in a straight line with angle | General adjustment, dividers, reference voltages |
| A (audio taper, log) | Slowly at first, quickly near the end | Volume (matches how people hear sound) |
"B10K" is a 10 kΩ linear type, and "A100K" is a 100 kΩ log type. At first, picking a B type is fine for most cases.
Shape
- Knob type (panel mount): turned often by hand. Solder it directly to the board or connect with wires.
- Trimmer (preset) resistor: has a small screw slot and is for setting once with a screwdriver. A number code like
103is printed on the surface, and 103 is 10 kΩ (10 × 10³ Ω). - Multi-turn trimmer: turns many revolutions for fine adjustment. Used for precision reference voltages.
Choosing a value
| Use | Recommended value |
|---|---|
| Reference voltage, divider | 10 kΩ |
| Adjusting an analog sensor input | 10 kΩ – 100 kΩ |
| Audio volume | 10 kΩ – 100 kΩ (A type) |
| LED brightness control | 1 kΩ (with a series protection resistor) |
| 555 timer timing adjustment | 100 kΩ – 1 MΩ |
The rated power of a potentiometer is small, about 0.1 to 0.5 W. Do not use it where large current flows (such as motor speed control); use it together with a transistor.
Using it on perfboard
- Pin spacing is often 2.5 mm (1 hole) or 5 mm (2 holes), so choose one that fits the holes.
- The potentiometer in Patterndo (3-pin, 10 kΩ by default) is placed in pin order 1-2-3, and you can change the wiper position (0 to 100%) in its properties, so you can check the change in output voltage by simulation.
- Remember that the wiper pin is in the middle. While wiring, it is easy to connect the middle pin to the input by mistake.
- If heat is applied for too long during soldering, the internal contact is damaged. Finish within 3 seconds per pin.
Common mistakes
| Mistake | Result | Prevention |
|---|---|---|
| Driving an LED with a potentiometer without a series protection resistor | LED overcurrent near 0 Ω | Add a fixed resistor in series |
| Leaving the end opposite the wiper unconnected | Resistance jumps to infinity on a bad contact | Tie it to the wiper |
| A low input resistance load on a divider output | The output drops a lot | A buffer (op-amp) or a larger potentiometer value |
| Pin order mix-up | It works in the opposite direction | Swap pins 1 and 3 |
Summary
A potentiometer is a variable resistor if you use only the wiper (the middle pin) and one end, and a voltage divider if you use all three pins. For general adjustment, pick a B-type 10 kΩ, and when using it with an LED or a timer, add a series protection resistor too. Next, reading Reading Resistors and LED Current-Limiting Resistors together will help you decide values.