Potentiometer Marking A: Linear or Logarithmic? Bourns, Alpha, Piher, Vishay Taper Codes Explained

User question

Is marking A on a potentiometer linear or logarithmic

Artificial Intelligence Response

Direct answer to the question

• In most modern components from Asian or North-American suppliers, the letter “A” on a potentiometer means a logarithmic (audio) taper.
• Under the older IEC/DIN (mainly European) convention, however, “A” can mean linear and “C” (or “B/C”) denotes logarithmic.
• Therefore, the marking alone is not universally reliable; verify the datasheet or measure the part if the provenance is uncertain.

Detailed problem analysis

1. Taper definitions
    • Linear: resistance is directly proportional to shaft angle — 50 % rotation ≈ 50 % total resistance.
    • Logarithmic (audio): resistance follows an exponential/log curve that approximates the ear’s loudness perception — 50 % rotation ≈ 10–20 % (or 80–90 % for reverse-log) of total resistance.

2. Marking systems in use
    a) JIS / ANSI (dominant in Asia-Pacific and most U.S. distributors)
     A = Log, B = Linear, C = Reverse-log
    b) Old IEC 60393 / DIN 41 320 (legacy European)
     A = Linear, B or C = Log, F = Reverse-log
    c) Manufacturer deviations
     Specialty vendors (guitar, pro-audio, military) may publish their own codes or append “LIN”, “LOG”, “AUD”, etc.

3. Why the conflict persists
    • Globalisation moved most production to Asia, so the JIS code (A = log) became de-facto standard for new stock.
    • Repair of vintage European or U.S. gear still encounters the IEC/DIN convention.
    • Some catalogue databases (e.g., Mouser, Digi-Key) translate everything to JIS to minimise confusion, but the silk-screen or body stamping on the pot may still use IEC letters.

4. Practical impact
    • Selecting the wrong taper can make a volume or dimmer control “all-or-nothing” over a small part of its rotation.
    • Linear pots used in feedback networks or position sensing will behave predictably only if the taper is correct.

Current information and trends

• Datasheets from major vendors—Bourns, Alpha, Piher, Vishay—now label logarithmic units as “A” almost exclusively.
• Boutique audio and guitar-pedal suppliers sometimes specify “A-log”, “B-lin” explicitly to avoid ambiguity.
• Some e-commerce platforms add suffixes: “100 k A(lin)” or “100 k A(log)”—always read the full listing.

Supporting explanations and details

Measuring unknown taper:

1. Isolate the potentiometer.
2. Measure total resistance \(R_T\) between the two outer terminals.
3. Set the shaft to its mechanical midpoint.
4. Measure \(R{mid}\) between the wiper and one outer terminal.
    • \(R{mid} \approx 0.5\,RT\) → Linear.
    • \(R{mid} \approx 0.1–0.2\,RT\) → Logarithmic (A under JIS).
    • \(R{mid} \approx 0.8–0.9\,R_T\) → Reverse-log (C under JIS).

Example: a pot marked “A100 k” bought from an Asian vendor shows 11 kΩ at mid-rotation → clearly logarithmic.

Ethical and legal aspects

• No specific legal issues, but mis-labelled parts can compromise product safety (e.g., lamp dimmers not meeting ergonomic or EMC standards).
• RoHS/REACH compliance still applies to the device regardless of taper.

Practical guidelines

1. If the source is Asian, assume A = log unless the datasheet says otherwise.
2. For vintage European repairs, treat A as linear until proven differently.
3. When designing new PCBs, annotate the schematic (“100 k, log (JIS-A)” or “100 k, lin (IEC-A)”) to prevent later confusion.
4. Keep a DMM on your bench; one 30-second measurement eliminates doubt.

Possible disclaimers or additional notes

• Some miniature SMD or slider potentiometers omit the letter entirely; absence often defaults to linear, but always confirm.
• Special tapers (W, S, MN, detented) follow their own codes beyond the A/B/C scheme.

Suggestions for further research

• Review IEC 60393-1 and JIS C 5260 for official definitions.
• Experiment with logarithmic law approximations using two linear resistors to synthesise custom tapers.
• Investigate digital potentiometers (digipots) with programmable tapers as an alternative.

Brief summary

“A” most often means a logarithmic (audio) potentiometer today, yet historical European parts use “A” for linear. Check the component’s origin, read the datasheet, or measure the resistance profile to be certain.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.