What Is a Resistor?
A resistor is a passive component that presents a specific opposition to current in a circuit. Its unit is the ohm (Ω), and it's used to limit current, divide voltage and set signal levels. It's the most common component in electronics.
What Does a Resistor Do?
- Protecting components like LEDs from overcurrent (current limiting)
- Lowering a voltage with voltage divider circuits
- Providing a stable level on digital inputs with pull-up / pull-down
- Setting values in signal and timing (RC) circuits
Relationship With Ohm's Law
The foundation of a resistor is the relation V = I × R: voltage equals current times resistance. For example, to pass 10 mA through an LED that drops 2 V from a 5 V source, the resistor needed is R = (5 − 2) / 0.01 = 300 Ω. For details, see What Is Ohm's Law?
Resistor Types
| Type | Property | Use |
|---|---|---|
| Fixed resistor | Value doesn't change | General purpose |
| Adjustable (potentiometer) | Set by hand | Volume/brightness control |
| Thermistor (NTC/PTC) | Changes with temperature | Temperature sensing |
| LDR (photoresistor) | Changes with light | Light sensor |
There are also types by physical build: the SMD/chip resistor for surface mount, the resistor network (array) for multiple resistors in one package, and the trimpot for precise manual adjustment.
What to Look At When Choosing a Resistor
There are three basic parameters: resistance value (Ω), power rating (W) and tolerance (%). The power a resistor will dissipate is found with P = V × I; a resistor that can't handle that value heats up and burns. You can compare suitable resistors in the catalog for your need.
Common Mistakes
- Neglecting the power rating: choosing a resistor with the right ohm value but insufficient wattage leads to burnout.
- Misreading the color code: reading the bands from the wrong end changes the value completely.
- Forgetting tolerance: precise circuits may need 1% instead of 5% tolerance.
Frequently Asked Questions
Does a resistor have a direction?+
How do you read a resistor's value?+
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