Capacitors Explained — How They Work and What They Do
Capacitors store and release electrical energy. They are the second most common component in electronics after resistors, appearing in almost every circuit — from simple filters to complex power supplies and timing circuits.
Open Capacitor SimulatorRC Circuit Diagram
How a Capacitor Works
A capacitor consists of two conductive plates separated by an insulating material (the dielectric). When voltage is applied, positive charge builds on one plate and negative charge on the other. The capacitor stores this charge and the energy associated with it.
Unlike a battery, a capacitor does not generate voltage — it stores it. A fully charged capacitor acts like a short circuit to AC signals (passing them through) and like an open circuit to DC (blocking it after charging).
RC Time Constant — Charging and Discharging
When a capacitor charges through a resistor, it does not charge instantly. The time it takes is determined by the RC time constant (τ, pronounced tau):
Example: R = 10kΩ, C = 100µF → τ = 10,000 × 0.0001 = 1 second. After 5 seconds, fully charged.
During charging, voltage across the capacitor follows: V(t) = Vfinal × (1 − e^(−t/RC)). During discharging: V(t) = Vinitial × e^(−t/RC).
Simulate RC ChargingCapacitor Types
| Type | Capacitance | Voltage Rating | Best For |
|---|---|---|---|
| Ceramic | 1pF–10µF | Up to 1kV | Decoupling, filtering, high frequency |
| Electrolytic | 1µF–100,000µF | Up to 500V | Power supply filtering, large capacitance |
| Film (polyester) | 1nF–100µF | Up to 630V | Audio circuits, precision applications |
| Tantalum | 0.1µF–1000µF | Up to 50V | Compact, SMD applications |
| Supercapacitor | 0.1F–3000F | 2.7V–5.5V | Energy storage, backup power |
Common Uses
- Decoupling / bypass: 100nF ceramic capacitor from each IC power pin to ground prevents supply voltage spikes from causing glitches. Every IC needs one.
- Power supply filtering: Large electrolytic (470µF–10,000µF) capacitors smooth rectified AC into steady DC voltage by filling in the dips between rectifier pulses.
- RC filters: Paired with a resistor to create low-pass, high-pass, or band-pass filters that let through only certain frequency ranges.
- Timing circuits: The RC time constant is used in 555 timer circuits to set oscillator frequency and pulse durations.
- Coupling: AC coupling capacitors pass AC signals while blocking DC offsets between circuit stages.
Common Beginner Mistakes
Reversed Electrolytic Capacitor
Electrolytic and tantalum capacitors are polarised. Reverse connection can cause them to fail catastrophically — sometimes explosively. Always check the stripe (negative leg) and longer lead (positive leg).
Using Electrolytic for High-Frequency Filtering
Electrolytic capacitors have high equivalent series resistance (ESR) and inductance at high frequencies. For decoupling ICs from high-frequency noise, use ceramic capacitors (100nF), not electrolytic.
Calculating Time Constant Wrong
τ = R × C in base units only: ohms × farads = seconds. Using kΩ with µF gives you milliseconds, not seconds — a 1000× error if you forget to convert. Always convert to base units first.
Assuming Capacitor is Fully Discharged
After switching power off, large capacitors (especially in power supplies) retain charge for minutes or hours. Always check and discharge before working on circuits with capacitors above 100µF.