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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
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
-
Capacitance — Select based on filtering frequency
f = 1/(2πRC) → Filter cutoff frequency -
Voltage Rating — Select ≥ 1.5x operating voltage
-
Type
- Power supply filtering → Electrolytic/Tantalum capacitors
- High-frequency decoupling → Ceramic capacitors (MLCC)
- Precision circuits → Film/Tantalum capacitors
-
ESR — Low ESR required for switch-mode power supplies
Keywords: Capacitor, Capacitive Reactance, Blocks DC Passes AC, Filtering, Energy Storage