Pot and Trimmer

Pot and Trimmer

Potentiometers ("pots") are adjustable resistors with a knob, and trimmers are tiny ones you adjust with a screwdriver. This tab reads their markings.

What do the markings mean?

A pot's marking has two parts: the total resistance and a letter for the taper.

Total resistance is the resistance from one outer pin to the other. It can be a 3-digit code (the same as SMD resistors) or a short number.

• 103 means 10 kΩ.
• 504 means 500 kΩ.
• 10K also means 10 kΩ.

Taper is how the resistance changes as you turn the knob.

• Linear (B): the resistance changes evenly with rotation. Halfway around gives about half the resistance. Good for brightness, tone, and as a voltage divider.
• Logarithmic or audio (A): the resistance changes slowly at first and faster at the end, which matches how our ears hear loudness. Good for volume controls.
• Reverse logarithmic (C): rare. It changes fast at first and slowly at the end.

How do I use the tab?

B103 is a 10 kΩ potentiometer with a linear taper.

1. Click Pot / Trimmer.
2. Choose Panel pot or Multi-turn trimmer to match what you have (this changes the picture and the typical tolerance).
3. Type the marking, for example B103, A504, 10KB, B50K, or 3296W-103.
4. Read Resistance, Minimum, Maximum, and Taper.

The tool also understands the words LIN and LOG if you type them.

Examples

Marking	Result
B103	10 kΩ, linear taper
A504	500 kΩ, logarithmic (audio) taper
10KB	10 kΩ, linear taper
3296W-103	10 kΩ, multi-turn trimmer (W marks a linear multi-turn series at Bourns)

Gotchas

• The letters are not universal. Parts made in Asia and the US usually follow A = log, B = linear, C = reverse log. Some European parts use A for linear and B or C for log. If you are not sure where a part came from, test it.
• How to test the taper. With a meter across one outer pin and the middle pin, turn the pot to the middle of its travel. A linear pot reads about half its total value. A log pot reads much less, often 10 to 20 percent.
• Tolerance is loose. Panel pots are typically ±20%, and trimmers ±10%. Do not expect exactly 10.00 kΩ.
• The middle pin is the wiper. The wiper is the adjustable output. The outer two pins are the ends of the resistance.

Related guides

Resistor Networks, Resistor Bands.