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Real Resistors: Tolerance, Power Rating, and Selection

Resistors are not ideal components. Real resistors vary in value, dissipate heat, and change behavior with temperature. Understanding these realities is essential for reliable circuit design.

Tolerance​

note

Tolerance defines how much a resistor’s actual value can vary from its nominal value.

When you buy a 1kΩ resistor, it is rarely exactly 1000Ω.

Common tolerances:

  • ±10%
  • ±5%
  • ±1%
  • ±0.1% (precision)

Example:

  • 1kΩ ±5% → 950Ω to 1050Ω
tip
  • Audio, LEDs, general circuits → 5% or 10%
  • Measurement, analog, timing → 1% or better
  • High precision → precision resistors or resistor networks

Resistor Tolerance

Power Rating​

warning

Exceeding a resistor’s power rating causes overheating, value drift, and eventual failure.

Resistors are also rated by how much power they can safely dissipate.

Common ratings:

  • 1/8 W
  • 1/4 W
  • 1/2 W
  • 1 W and above

Power equations:

  • P = V² / R
  • P = I² × R

Example:

  • 10kΩ at 10V → 10mW (1/4W is safe)
  • 1Ω at 10V → 100W (very large resistor required)
tip

Always select a resistor rated for at least 2× the calculated power.

Resistor Power Dissipation

Temperature Coefficient​

note

Resistance changes with temperature. This behavior is defined by the temperature coefficient (ppm/°C).

Typical values:

  • Metal film: ~100 ppm/°C
  • Precision resistors: <25 ppm/°C
  • Wirewound: application-dependent
important

Precision and outdoor circuits must account for temperature effects.

Resistor Temperature Coefficient

Through-Hole Resistors​

note

Through-hole resistors use leads that pass through PCB holes.

Features:

  • Easy to handle and replace
  • Higher power capability
  • Color bands indicate value and tolerance

Common types:

  • Carbon film
  • Metal film
  • Wirewound (high power)

Through-Hole Resistor Color Bands

SMD (Surface-Mount) Resistors​

note

SMD resistors mount directly on the PCB surface and are used in modern electronics.

Common sizes:

  • 0402, 0603, 0805, 1206

Features:

  • Small size
  • Automated assembly
  • Lower power ratings
  • Numeric value codes (e.g., 103 = 10kΩ)

Typical power ratings:

  • 0402 → 1/16 W
  • 0603 → 1/10 W
  • 0805 → 1/8 W
  • 1206 → 1/4 W

SMD Resistor Package Sizes

Selecting Resistors​

tip

When selecting a resistor, ensure:

  • Correct resistance value
  • Suitable tolerance
  • Adequate power rating with margin
  • Acceptable temperature coefficient
  • Proper package (through-hole or SMD)

Standard Resistor Values (E12 – 5%)​

note

Standard values reduce cost and improve availability.

10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82
(Multiply by 10, 100, 1k, etc.)

Key Takeaway​

important

Real resistors are defined by value, tolerance, power rating, temperature behavior, and package.
Designing with these constraints separates reliable circuits from field failures.