6.2 Resistor Colour Code, Tolerance & Preferred Values

6.2 Resistor Colour Code, Tolerance & Preferred Values

Quick Answer: Read resistor colour bands left-to-right (tolerance band at the end): on a 4-band part, bands 1–2 are digits, band 3 is the multiplier, band 4 is tolerance. On a 5-band part, bands 1–3 are digits, band 4 multiplier, band 5 tolerance. Match colours to the standard table, then apply E12/E24 preferred values and a separate wattage rating for heat.

Fixed carbon and metal-film resistors on training boards and avionics PCBs rarely print “4700 Ω ±5%” in large type. They use colour bands. Module 3 expects you to decode them without a phone app.

Mandatory Colour Code Table

Memory aid for digits 0–9: Black Brown Red Orange Yellow Green Blue Violet Grey White — often memorised as a nonsense sentence in class; any reliable mnemonic is fine if the mapping is exact.

Gold and silver rules:

• As a multiplier band: gold = ×0.1, silver = ×0.01 (for low-ohm values).
• As a tolerance band: gold = ±5%, silver = ±10%.

Never use gold or silver as a significant digit.

Four-Band Colour Code

Order (body end with groups of bands toward the first digit; tolerance often slightly separated):

1. 1st significant digit
2. 2nd significant digit
3. Multiplier
4. Tolerance

Nominal resistance = (10 × digit1 + digit2) × multiplier.

Worked decode A. Bands: Yellow – Violet – Orange – Gold

• Yellow = 4, Violet = 7 → digits 47
• Orange multiplier = ×1 000
• Gold tolerance = ±5%
• Value = 47 × 1 000 = 47 000 Ω = 47 kΩ ±5%

Worked decode B. Bands: Brown – Black – Red – Silver

• Digits 1 and 0 → 10
• Red → ×100
• Silver → ±10%
• Value = 1 000 Ω = 1 kΩ ±10%

Worked decode C (fractional ohms). Bands: Red – Red – Gold – Gold

• Digits 2 and 2 → 22
• Gold multiplier → ×0.1
• Gold tolerance → ±5%
• Value = 22 × 0.1 = 2.2 Ω ±5%

Tolerance window

For a nominal R with tolerance t%:

Allowed range = R × (1 ± t/100)

Worked example — 47 kΩ ±5%:

• Tolerance band = 0.05 × 47 000 = 2 350 Ω
• Min ≈ 47 000 − 2 350 = 44 650 Ω
• Max ≈ 47 000 + 2 350 = 49 350 Ω

A measured 46 kΩ part can still be “in tolerance” for a 47 kΩ ±5% resistor. Module 3 loves this check.

Five-Band Colour Code

Precision resistors often use five bands:

1. Digit 1
2. Digit 2
3. Digit 3
4. Multiplier
5. Tolerance

Nominal = (100 × d1 + 10 × d2 + d3) × multiplier.

Worked decode D. Bands: Orange – Orange – Black – Brown – Brown

• Digits 3, 3, 0 → 330
• Brown multiplier → ×10
• Brown tolerance → ±1%
• Value = 330 × 10 = 3 300 Ω = 3.3 kΩ ±1%

Five-band coding is how manufacturers express tighter preferred values (for example 332 Ω) that two significant figures cannot show.

Preferred Values: E12 and E24

Manufacturers do not stock every ohm value. They use preferred number series so that, given the tolerance, the ranges of neighbouring values touch or slightly overlap.

Higher series (E48, E96) exist for precision parts; Module 3 emphasis is E12 and E24 recognition: 2.2 kΩ, 4.7 kΩ, 6.8 kΩ, and so on are preferred values, not random.

Why preferred values matter: If you need ≈5 kΩ and only E12 stock exists, 4.7 kΩ is the standard choice; a 5.0 kΩ part may be E24/E96 territory. Exam stems sometimes ask which listed value is a standard preferred value.

Wattage (Power) Ratings

The colour code states ohms and tolerance, not power. Dissipation capability is a separate marking or physical size cue:

P = I²R = V²/R = VI

Worked power check. A 100 Ω resistor drops 10 V continuously.

P = V²/R = 100 / 100 = 1.0 W.

A ¼ W part would be overloaded; choose at least 1 W, preferably with derating margin (aircraft/avionics design often derates further for altitude and ambient temperature — Module 3 at least wants you to see that 1 W of heat needs a ≥1 W rating).

Worked current form. Through 47 Ω at 0.2 A: P = I²R = 0.04 × 47 = 1.88 W → use a 2 W or 5 W class part, not a tiny signal resistor.

Practical Reading Habits for the Exam

1. Orient the resistor so the tolerance band (gold/silver or spaced last band) is on the right.
2. Decode digits → multiplier → tolerance.
3. Convert to convenient units (Ω, kΩ, MΩ).
4. If asked for limits, apply ±tolerance.
5. If asked whether a resistor can be used at a stated V or I, compute power separately from the colour code.

Exam Scenario Mindset

Scenario — Misread multiplier. Reading yellow–violet–red–gold as 47 × 100 = 4.7 kΩ instead of orange’s ×1 000 (47 kΩ) is a classic error. Slow down on the third band.

Scenario — Gold as digit. Gold never encodes “digit 0.5”; it is multiplier or ±5% tolerance only.

Scenario — Preferred value trap. “Which is an E12 value?” — pick from the E12 list (for example 27 kΩ), not an arbitrary 25 kΩ unless the stem’s series allows it.

Decode the table until it is automatic, then bolt on tolerance windows, E12/E24 logic, and wattage. That is the complete Module 3 colour-code skill set.