Resistors in Practice
The humble striped cylinder: why circuits are full of them, and how to read their colour code at a glance.
Builds on: 1.2 Ohm's Law
Why deliberately resist?
It sounds odd to buy a component whose whole job is to impede current — but resistors are how you tell a circuit how much current to use. They protect delicate parts (an LED without its resistor dies in a blink — you’ll prove this in Unit 3), set voltage levels, control timing, and convert current into measurable voltages. A typical circuit board carries dozens.
The colour code
Common resistors are too small to print numbers on, so since the 1920s they’ve worn coloured bands. On a 4-band resistor: the first two bands are digits, the third multiplies by a power of ten, and the fourth states the tolerance.
| Colour | Digit | Multiplier |
|---|---|---|
| Black | 0 | ×1 |
| Brown | 1 | ×10 |
| Red | 2 | ×100 |
| Orange | 3 | ×1 k |
| Yellow | 4 | ×10 k |
| Green | 5 | ×100 k |
| Blue | 6 | ×1 M |
| Violet | 7 | — |
| Grey | 8 | — |
| White | 9 | — |
Tolerance band: gold = ±5%, silver = ±10%, brown = ±1%. Example: yellow–violet–red–gold reads 4, 7, ×100 → 4.7 kΩ ±5%. That exact resistor appears in your final build.
Why the weird values?
Resistors come in standard series like E12: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 (then ×10, ×100, …). The numbers look random but are spaced evenly in ratio (each ≈ 1.21× the last), so that with ±10% tolerance the ranges tile the number line with no gaps. You never need exactly 500 Ω — the 470 Ω or 560 Ω bin has you covered.
Besides its resistance, a resistor has a maximum power it can dissipate without cooking — the common little ones are rated ¼ W. What that means and how to check it is exactly the next lesson.
⚡ Lab — Colour-Code Decoder & Trainer
Build any resistor from its bands — or decode a random one.
- Set yellow–violet–red–gold and confirm you get 4.7 kΩ ±5%.
- Make 1 kΩ, 330 Ω and 47 kΩ — the three other values in your final build.
- Hit 🎲 and decode the mystery resistor before revealing.