Table of Contents
Capacitor Fundamentals
A capacitor is a passive electronic component that stores electrical energy in an electric field between two conducting plates separated by a dielectric material. Capacitors are one of the three fundamental passive components (alongside resistors and inductors) and are found in virtually every electronic circuit. They serve roles ranging from energy storage and power filtering to timing, coupling, and tuning.
When a voltage is applied to a capacitor, charge accumulates on the plates until the voltage across the capacitor equals the applied voltage. The amount of charge stored is proportional to the voltage: Q = CV. The energy stored is E = 1/2 CV squared. Unlike a battery, which stores energy chemically, a capacitor stores energy directly in the electric field and can charge and discharge almost instantaneously.
Key Formulas
The time constant tau = RC determines charging and discharging speed. After one time constant, the capacitor reaches 63.2% of its final value. After 5 time constants (5RC), it reaches 99.3% and is considered fully charged for practical purposes.
Capacitor Types
| Type | Capacitance Range | Voltage Range | Best For |
|---|---|---|---|
| Ceramic (MLCC) | 1 pF - 100 μF | 6V - 3kV | Decoupling, RF |
| Electrolytic (Al) | 0.1 μF - 1 F | 6V - 500V | Power filtering |
| Film | 100 pF - 100 μF | 50V - 2kV | Audio, timing |
| Tantalum | 0.1 μF - 1 mF | 4V - 50V | Compact filtering |
| Supercapacitor | 0.1 F - 3000 F | 2.5V - 5.5V | Energy storage |
Frequently Asked Questions
How do capacitors differ from batteries?
Capacitors store energy in electric fields and can charge/discharge in microseconds to seconds, while batteries store energy chemically and discharge over minutes to hours. Capacitors have much higher power density (can deliver energy quickly) but much lower energy density (store less total energy). Supercapacitors bridge the gap, offering more energy than regular capacitors but more power than batteries, useful for regenerative braking and short-term backup power.
What is ESR and why does it matter?
Equivalent Series Resistance (ESR) is the internal resistance of a real capacitor, caused by plate resistance, dielectric losses, and lead resistance. High ESR limits the capacitor's ability to deliver current quickly, generates heat under ripple current, and reduces filtering effectiveness. Ceramic capacitors have the lowest ESR (milliohms), followed by film capacitors, with electrolytic capacitors having the highest ESR (typically 0.01-1 ohm). Low ESR is critical in switch-mode power supply output filters.
What happens if you exceed the voltage rating?
Exceeding a capacitor's voltage rating causes the dielectric to break down, creating a conducting path between the plates. In electrolytic capacitors, this can cause rapid heating, gas generation, and potentially violent rupture or explosion. Ceramic capacitors may crack or develop short circuits. Film capacitors often self-heal from minor overvoltage events. Always select a voltage rating at least 50% above the maximum expected working voltage for reliability.