Passives
Resistor
Limits how much current flows. Every volt across it pushes a fixed amount of current through it, and the energy it takes turns into heat.
Think of it as: A narrow section of pipe.
Pins
- Either end
- : A resistor has no polarity. It works the same both ways round.
Through-hole resistors show their value as coloured bands; surface-mount (SMD) chips print a 3-digit code: two digits and then the number of zeros.
What to look for inside
- Electrons (gold) drift from one end to the other as the current rises and falls.
- They keep bumping into the atoms of the material. Each bump hands over a little energy, which is why the resistor warms up.
- More current means more bumps per second, so the heat grows with the square of the current: P = I²R.
The numbers that matter
| Spec | Typical |
|---|---|
Resistance Sets the current for a given voltage. Values come in standard steps (E12: 1.0, 1.2, 1.5, 1.8, 2.2 …). | 1 Ω to 10 MΩ |
Tolerance How far the real value may be from the printed one. | ±1 % or ±5 % |
Power rating The heat it can survive. Stay under half of it; see derating. | 0.1 W (0603) to 0.25 W (through-hole) |
Where you’ll use it
- Limiting the current through an LED.
- Making a voltage divider to scale a voltage down for a sensor input.
- Pull-up and pull-down resistors that give a GPIO pin a known level.
Common mistakes
- Using a part that is too small for the power. A 0603 chip carrying 0.3 W will overheat and fail.
- Misreading the colour bands. Read from the end the bands are closest to; the gold or silver band is the tolerance and goes last.
Lessons that use it
- Lesson 0.1: Your First Circuit: Voltage, Current & Resistance
- Lesson 1.1: Ohm's Law, Power & Why Resistors Get Hot
- Lesson 0.2: Series vs Parallel Circuits Explained
- Lesson 2.1: Voltage Dividers & the Loading Effect
- Lesson 1.2: Kirchhoff's Current & Voltage Laws Explained
- Lesson 3.1: Diodes & LEDs: Forward Voltage and the I-V Curve
- Lesson 4.1: Pull-Up Resistors & Button Debounce Explained
- Lesson 4.2: Open-Drain vs Push-Pull Outputs (I2C Pull-Ups)
Go deeper
Ohm’s law: the voltage across equals current times resistance.
All the power a resistor takes becomes heat.
Longer and thinner means more resistance, which is why thin PCB traces have resistance too.