Last week, we dove into the fundamentals of resistance, exploring what it means and how it works. Continuing on that, folks starting out on electronics need to know how to read through resistors by appearance.
For through-hole resistors, a color code band system was introduced a century ago by the Radio Manufacturers Association (RMA) in the U.S. It became widely standardized as part of military requirements during World War 2. The first two (or three, in precision resistors) bands denote significant digits, the next one is a multiplier, and the final band represents tolerance. For example, a resistor with bands of brown, black, and red corresponds to 1, 0, and a multiplier of 100, making it a 1,000-ohm resistor (or 1 kΩ)
To make this easier to memorize, we use the classic mnemonic like: BB ROY of Great Britain had a Very Good Wife, which stands for Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray, White, representing the digits 0 through 9. The tolerance band is typically gold (±5%), silver (±10%), or none (±20%). Precision resistors can include additional bands for tighter tolerances.
SMD resistors use alphanumeric codes instead of color bands due to their compact size. There are different types here. In the Three-Digit Code type, the first two digits represent the significant figures, and the third digit is the multiplier. E.g, a resistor marked 472 means 47 × 10² = 4,700Ω. For a four-digit code, you get more range in resistors with the first three digits representing the significant figures, and the fourth digit being the multiplier. Eg. 1634 means 163 × 10⁴ Ω. Another type is EIA Codes(E6, E12, E24, E48, E96, E192) Which indicate value by 2 digits and letter as tolerance(Check images). The most common one is the E96 Code which defines 96 distinct resistance values within each decade(10-100 Ω, 100- 1 kΩ etc). 96 values in a decade are derived via a formula such that any resistor in the series is within ±1% of its nominal value.
Understanding these details isn’t just academic, it can be a lifesaver in the field, especially during troubleshooting. Take the time to learn them!
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