---
title: Capacitor
url: https://doc.liz6.com/en/hardware/02-passive-components/02-capacitor
locale: en
area: hardware
tags:
- hardware
- passive-components
date: 2026-06-30
modified: 2026-07-16
description: Capacitor Basic Concepts A capacitor is a passive component that stores electric charge, consisting of two conductors separated by an insulator. Parameters | Pa…
---

# Capacitor

## Basic Concepts

A **capacitor** is a passive component that stores electric charge, consisting of two conductors separated by an insulator.

```
    +Q         -Q
  ┌─────┐   ┌─────┐
  │  ┌──┴───┴──┐  │
  │  │        │  │
  │  │  Dielectric  │  │  ← Insulator
  │  │        │  │
  │  └────────┘  │
  └──────┬───────┘
         │
        GND
```

### Parameters

| Parameter | Symbol | Unit | Description |
|------|------|------|------|
| Capacitance | C | Farad (F) | Ability to store charge |
| Voltage Rating | V | Volt (V) | Maximum operating voltage |
| Temperature Coefficient | TC | ppm/°C | Variation with temperature |
| ESR | - | Ω | Equivalent Series Resistance |

### Common Units

- 1 F (Farad) = 10³ mF = 10⁶ μF = 10⁹ nF = 10¹² pF

## Core Formulas

### Capacitance Definition
```
C = Q / V

Q: Charge (Coulomb)
V: Voltage (Volt)
```

### Capacitive Reactance (AC Impedance)
```
Xc = 1 / (2πfC)

f: Frequency (Hz)
C: Capacitance (F)

Characteristics: Higher frequency results in lower reactance → "Blocks DC, passes AC"
```

### Energy Storage
```
W = ½CV²

C: Capacitance (F)
V: Voltage (V)
```

## Important Characteristics

### 1. Blocks DC, Passes AC
- **DC**: Acts as an open circuit once stabilized (after charging is complete)
- **AC**: Impedance decreases as frequency increases

### 2. Voltage Cannot Change Instantaneously
- Voltage rises gradually during charging
- Voltage falls gradually during discharging
- **Time Constant**: τ = RC

### 3. Frequency Characteristics

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  </defs>
  <text x="360" y="28" text-anchor="middle" font-size="17" font-weight="700" fill="#1f2933">Frequency Characteristics: High Reactance at Low Frequencies, Low Reactance at High Frequencies</text>
  <text x="70" y="71" font-size="13" font-weight="700" fill="#475569">Low Frequency</text>
  <line x1="110" y1="66" x2="610" y2="66" stroke="#475569" stroke-width="1.7" marker-end="url(#cap-freq-arrow)"/>
  <text x="650" y="71" font-size="13" font-weight="700" fill="#475569">High Frequency</text>
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  <text x="200" y="128" text-anchor="middle" font-size="13" font-weight="700" fill="#3730a3">High Reactance (Open Circuit)</text>
  <text x="200" y="150" text-anchor="middle" font-size="11" fill="#4f46e5">Low-frequency signals → Act as open circuit</text>
  <rect x="380" y="100" width="280" height="68" rx="8" fill="#f0fdfa" stroke="#99f6e4"/>
  <text x="520" y="128" text-anchor="middle" font-size="13" font-weight="700" fill="#0f766e">Low Reactance (Short Circuit)</text>
  <text x="520" y="150" text-anchor="middle" font-size="11" fill="#0f766e">High-frequency signals → Impedance approaches zero, acts as short circuit</text>
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  <text x="76" y="206" font-size="12.5" fill="#334155">The higher the frequency, the lower the reactance — this is the origin of the capacitor's "blocks DC, passes AC" characteristic.</text>
</svg>

## Common Types

| Type | Capacitance Range | Voltage Rating | Characteristics | Applications |
|------|----------|------|------|------|
| Ceramic Capacitor | pF ~ μF | Low~Medium | Inexpensive, small | Decoupling, filtering |
| Electrolytic Capacitor | μF ~ F | Medium~High | Polarized, large capacity | Power supply filtering |
| Tantalum Capacitor | μF ~ mF | Low~Medium | Expensive, stable | Precision filtering |
| Film Capacitor | nF ~ μF | High | Stable, low ESR | Audio, power |

## Circuit Applications

### 1. Filtering
```
     ┌──┐
Vin ─┤C ├─── Vout
     └──┘

- Ripple smoothing
- Power supply decoupling
```

### 2. Coupling
- Blocks DC, allows only AC signals to pass

### 3. Timing/Oscillation
- RC time constant circuits
- Oscillator circuits with inductors

## Selection Guidelines

1. **Capacitance** — Select based on filtering frequency
   ```
   f = 1/(2πRC)  → Filter cutoff frequency
   ```

2. **Voltage Rating** — Select ≥ 1.5x operating voltage

3. **Type**
   - Power supply filtering → Electrolytic/Tantalum capacitors
   - High-frequency decoupling → Ceramic capacitors (MLCC)
   - Precision circuits → Film/Tantalum capacitors

4. **ESR** — Low ESR required for switch-mode power supplies

---
*Keywords: Capacitor, Capacitive Reactance, Blocks DC Passes AC, Filtering, Energy Storage*