Resistors Explained
Resistors are one of the most fundamental components in electronics. They control current, divide voltage, protect sensitive components, and help set operating points in circuits. In this guide, we’ll cover both modern SMD resistors and classic through-hole resistors, how to read their values, and how to choose the right resistor for your application.
What Does a Resistor Do?
A resistor opposes the flow of electric current. When current flows through a resistor, electrical energy is converted into heat. This behavior is described by Ohm’s Law:
By choosing the right resistance value, engineers can control how much current flows through a circuit and how voltage is distributed.
Modern SMD Resistors
Surface-Mount Device (SMD) resistors are the most common type used in modern electronics. They are small, efficient, and designed for automated assembly.
Common SMD Package Sizes
| Package | Size (mm) | Typical Power Rating |
|---|---|---|
| 0402 | 1.0 × 0.5 | 1/16 W |
| 0603 | 1.6 × 0.8 | 1/10 W |
| 0805 | 2.0 × 1.25 | 1/8 W |
| 1206 | 3.2 × 1.6 | 1/4 W |
Larger packages can dissipate more heat, which is why power rating is closely tied to physical size.
Choosing a Resistor: Key Specifications
1. Resistance Value
Measured in ohms (Ω), this determines how much current the resistor limits. Standard values follow the E-series (E12, E24, E96), ensuring predictable spacing between values.
2. Tolerance
Tolerance indicates how much the actual resistance can vary from its stated value.
| Tolerance | Typical Use |
|---|---|
| ±5% | General-purpose electronics |
| ±1% | Signal processing, amplifiers |
| ±0.1% | Precision measurement |
3. Power Rating
Resistors dissipate heat according to:
Always choose a resistor with a power rating at least 2× higher than the calculated dissipation for reliability.
4. Temperature Coefficient (TCR)
The temperature coefficient (ppm/°C) describes how much the resistance changes with temperature.
- ±200 ppm/°C: General electronics
- ±50 ppm/°C: Analog and signal circuits
- ±10 ppm/°C: Precision instrumentation
Current Sense Resistors
Current sense resistors are low-value resistors used to measure current by monitoring voltage drop across them.
Key Characteristics
- Very low resistance (1 mΩ – 100 mΩ)
- Extremely low tolerance (±1% to ±0.1%)
- Low temperature coefficient
- High power rating for their size
These resistors are commonly found in:
- Battery management systems
- Motor drivers
- Power supplies
- Current monitoring circuits
Because their resistance is so small, even a few millivolts of drop can represent significant current.
Through-Hole Resistors and Color Codes
Through-hole resistors are older but still widely used in education, prototyping, and high-power applications. Their resistance value is indicated by colored bands.
4-Band Resistor Color Code
| Band | Meaning |
|---|---|
| 1st | First digit |
| 2nd | Second digit |
| 3rd | Multiplier |
| 4th | Tolerance |
Example: Red-Red-Brown-Gold = 22 × 10¹ = 220Ω ±5%
Common Color Values
- Black: 0
- Brown: 1
- Red: 2
- Orange: 3
- Yellow: 4
- Green: 5
- Blue: 6
- Violet: 7
- Gray: 8
- White: 9
Practical Examples
LED Current Limiting
A 5V supply powers an LED with a forward voltage of 2V at 20mA. A resistor is required to limit the current and protect the LED.
This type of calculation ensures the LED operates safely without exceeding its rated current.
Parallel Resistors
When resistors are connected in parallel, the total resistance decreases. Parallel resistors are commonly used in current sharing and load distribution.
Instead of calculating manually, you can use our Parallel Resistor Calculator
Simply enter the resistor values connected in parallel, and the calculator will instantly compute the equivalent resistance.
Voltage Divider
A voltage divider uses two resistors in series to produce a lower output voltage from a higher input voltage. This technique is widely used in sensor circuits and ADC inputs.
You can quickly design voltage dividers using our Voltage Divider Calculator
Enter the input voltage and resistor values to calculate the output voltage, or solve for resistor values based on a desired output.
Practice Problems
Problem 1: A resistor of 220Ω is connected to a 12V supply. What current flows through the resistor?
Problem 2: A circuit requires a current of 10mA from a 9V source. What resistor value is needed?
Problem 3: Two 1kΩ resistors are connected in parallel. What is the equivalent resistance?
Conclusion
Resistors may seem simple, but choosing the right one requires understanding resistance, tolerance, power rating, and temperature effects. From tiny SMD resistors to classic color-banded through-hole parts, resistors remain a cornerstone of electronics design.