---
title: Resistor
url: https://doc.liz6.com/en/hardware/02-passive-components/01-resistor
locale: en
area: hardware
tags:
- hardware
- passive-components
date: 2026-06-30
modified: 2026-07-16
description: Resistor Basic Concepts A resistor is a passive component that limits current and converts electrical energy into heat. Parameters | Parameter | Symbol | Unit |…
---

# 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

1. **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
   ```

2. **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
   ```

3. **Tolerance** — ±5% is sufficient for LED current limiting; use ±1% for voltage dividers

4. **TCR** — Precision measurements require low temperature drift (e.g., ±25ppm)

5. **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*