Recipe

Read a resistor's colour code

Use when: a through-hole resistor shows coloured bands and you need its value and tolerance

Goal: the resistance in ohms and the range its real value may fall in

Taught in Resistors in practice

1. 1

   Hold the resistor so the gold, silver or set-apart band is on the right.

   Example

   Bands yellow, violet, red, gold: gold goes on the right.

   Why: the value is read left to right, and gold or silver never start a value.

2. 2

   Four bands means two digits; five bands means three digits.

   • 4 bands — digits are bands 1 and 2, the multiplier is band 3.
   • 5 bands — digits are bands 1 to 3, the multiplier is band 4.

   Why: precise parts need a third digit, so they carry one extra band.

3. 3

   Turn each digit band into its number, then add as many zeros as the multiplier's digit.

   Example

   Yellow 4, violet 7, red 2 zeros: 4700 Ω, or 4.7 kΩ.

   Why: the multiplier band is a power of ten, so its digit counts the zeros.

4. 4

   Gold ±5%, silver ±10%, brown ±1%. Work out the lowest and highest real value.

   Example

   ±5% of 4700 Ω is 235 Ω: 4465 Ω to 4935 Ω.

   Why: the band is a promise about the real part, and a measurement inside the range is not a fault.

5. 5

   Measure the resistance with the part out of the circuit.

   Why: brown, red and orange look alike under some lights, and a meter settles it.