5 min read #hardware #passive-components

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Resistor

Basic Concepts

A resistor is a passive component that limits current and converts electrical energy into heat.

Symbol:
  Fixed: ──┤├──   or   ──□□□──  (IEC)
  Variable: ──┤├──   (Potentiometer/Variable Resistor)
         ↗

Physical Form: Axial lead (through-hole) or SMD (Surface Mount Device)

Parameters

Parameter	Symbol	Unit	Description
Resistance	R	Ω	Nominal resistance value
Power Rating	P	W	Maximum power dissipation
Tolerance	-	%	Deviation from nominal value
Temperature Coefficient	TCR	ppm/°C	Rate of resistance change with temperature
Voltage Rating	V	V	Maximum operating voltage

Ohm's Law Review

V = I × R
P = I² × R = V² / R

Resistors convert electrical energy into heat: every resistor is a small heater

Color Code (4 Bands)

    Band1  Band2  Band3  Band4
    ┌─┐  ┌─┐  ┌─┐      ┌─┐
    │ │  │ │  │ │      │ │
────┘ └──┘ └──┘ └──────┘ └────
  Digit1 Digit2 Multiplier  Tolerance

Color Code:
  Black=0, Brown=1, Red=2, Orange=3, Yellow=4
  Green=5, Blue=6, Purple=7, Gray=8, White=9

  Gold=×0.1 (±5%), Silver=×0.01 (±10%)

Example: Brown-Black-Red-Gold = 1 → 0 → ×100 → ±5% = 1kΩ ±5%
         Yellow-Violet-Orange-Gold = 4 → 7 → ×1k  → ±5% = 47kΩ ±5%

5-Band: First three bands are digits + fourth band is multiplier + fifth band is tolerance (higher precision)

Common Types

Type	Resistance Range	Tolerance	TCR	Characteristics	Application
Carbon Film	1Ω~10MΩ	±5%	-250~-1000	Cheap, high noise	General purpose
Metal Film	0.1Ω~10MΩ	±1%	±50	Low noise, stable	Precision
Wirewound	0.01Ω~100kΩ	±0.1%	±20	High power, low inductance	Power/Current sensing
Thick Film SMD	0Ω~100MΩ	±1%~5%	±200	Mainstream SMD	Almost all SMD boards
Resistor Network	-	-	-	Multiple resistors in one package	Bus pull-up/pull-down

Special Resistors

Potentiometer

     ┌──────┐
Vin ─┤ Variable ├── Vout
     │ Divider │
GND ─┤      │
     └──────┘

- Three terminals: Two fixed ends, sliding wiper in the middle
- Applications: Volume control, brightness adjustment, sensors

Thermistor

NTC (Negative Temperature Coefficient): Temp↑ → Resistance↓  (Most common)
PTC (Positive Temperature Coefficient): Temp↑ → Resistance↑  (Overcurrent protection)

NTC Applications: Temperature measurement, Inrush current limiting
B Parameter: R(T) = R₂₅ × exp(B(1/T - 1/298))

Photoresistor (LDR)

Light↑ → Resistance↓ (CdS material)
Dark Resistance: MΩ range
Light Resistance: Hundreds of Ω range

Applications: Automatic night lights, light sensors

Varistor (MOV)

When voltage exceeds threshold → Resistance drops sharply
Used for surge protection, absorbing transient overvoltage

Typical: Lightning surge protection at power input

Resistor Networks

Pull-up / Pull-down

Pull-up:
        Vcc
        │
        Rpu
        │
GPIO ───┼─── Signal Line
        │
      Switch to GND

Pull-down:
      Signal Line
        │
        Rpd
        │
       GND

Function: Prevent floating inputs, establish default logic level
Typical Values: 4.7kΩ ~ 100kΩ

Current Sensing (Shunt)

    ┌── Rs ──┐
    │  (mΩ)  │
  Iin        Load
    └────────┘

Vsense = I × Rs
→ Amplify Vsense to get current value

Smaller Rs is better (reduces loss)
Typical: 1mΩ ~ 100mΩ

Selection Guidelines

Resistance — Select from E-series standard values (E12/E24/E96)

E12: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 (×10ⁿ)
E24: Finer values inserted between E12 values

Power Rating — Choose ≥ 2x actual power dissipation

P = I²R → 100mA through 100Ω = 1mW → 1/8W (125mW) is sufficient
1A through 1Ω = 1W → Use at least 2W rating

Tolerance — ±5% is sufficient for LED current limiting; use ±1% for voltage dividers
TCR — Precision measurements require low temperature drift (e.g., ±25ppm)
Package — Through-hole: AXIAL-0.4; SMD: 0402/0603/0805/1206

Keywords: Resistor, Color Code, Voltage Divider, Pull-up, Pull-down, Thermistor, Current Sensing, E-Series