Introduction
Resistors are the most fundamental electronic component. Understanding how they work and how to apply Ohm's Law is essential for every electronics project.
What Is a Resistor?
A resistor limits the flow of electric current. It's measured in Ohms (Ω). Think of it like a narrow section of pipe that slows water flow.
Ohm's Law
The relationship between Voltage (V), Current (I), and Resistance (R):
V = I × R
| If you know... | Formula |
|---|---|
| V and R | I = V / R |
| V and I | R = V / I |
| I and R | V = I × R |
Example
An LED needs 20mA of current and has a 2V forward voltage. Your supply is 5V. What resistor do you need?
R = (5V - 2V) / 0.02A = 150Ω
Use the next standard value: 220Ω (safe margin).
Resistor Color Code
Most resistors have 4 colored bands:
| Band | Colors (0-9) |
|---|---|
| 1st digit | Black(0), Brown(1), Red(2), Orange(3), Yellow(4), Green(5), Blue(6), Violet(7), Grey(8), White(9) |
| 2nd digit | Same as above |
| Multiplier | Black(×1), Brown(×10), Red(×100), Orange(×1k), Yellow(×10k), Green(×100k) |
| Tolerance | Gold(±5%), Silver(±10%) |
Reading Example
Brown - Black - Red - Gold = 10 × 100 = 1kΩ ± 5%
Common Resistor Values
- 220Ω — LED current limiting
- 1kΩ — General purpose, pull-down
- 4.7kΩ — I2C pull-up
- 10kΩ — Pull-up/pull-down, voltage dividers
Series vs Parallel
- Series: R_total = R1 + R2 + R3
- Parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3