Understanding Capacitors

A capacitor is an electronic component that stores electrical energy in an electric field. Unlike a battery that stores energy through chemical reactions, a capacitor stores energy by holding opposite electric charges on two conductive plates separated by an insulating material called a dielectric. They are found in virtually every electronic circuit.

Objectives

• Understand how capacitors store energy in electric fields
• Calculate capacitance, charge, and energy storage
• Read capacitor value codes (3-digit, 4-digit, and direct marking)
• Identify different types of capacitors and their applications
• Understand series and parallel capacitor combinations
• Work with RC time constants for timing circuits
• Safely handle and discharge capacitors

Key Takeaways

• Capacitors store energy in electric fields (E = ½CV²)
• Q = CV relates charge, capacitance, and voltage
• Series capacitors decrease total; parallel capacitors add
• RC time constant (τ = RC) controls charging/discharging speed
• Capacitors block DC but pass AC (reactance decreases with frequency)
• Polarized capacitors must be connected correctly or they can explode
• Always respect voltage ratings and derate appropriately
• Charged capacitors are dangerous - always discharge safely!

Theory

Capacitors in Series

Total capacitance DECREASES when capacitors are in series (opposite of resistors).

1/C_total = 1/C1 + 1/C2 + 1/C3 + ... + 1/Cn

Capacitors in Parallel

Parallel capacitors simply add up (opposite of parallel resistors).

C_total = C1 + C2 + C3 + ... + Cn

Applications

• Power Supply Filtering: Large electrolytics smooth rectified AC; 0.1µF ceramics decouple IC power pins
• Signal Coupling: Pass AC signals while blocking DC bias voltages between amplifier stages
• Timing Circuits: RC time constant controls delays, oscillators, and waveform generators
• Filtering: Low-pass, high-pass, and band-pass filters for signal processing
• Energy Storage: Camera flash, defibrillators, pulsed lasers, backup power
• Motor Applications: Single-phase motor starting and power factor correction

Practice Problems

Problem 1: A 470µF capacitor is charged to 12V. How much charge does it hold?

Problem 2: How much energy is stored in a 1000µF capacitor charged to 50V?

Problem 3: Three capacitors (10µF, 22µF, 47µF) are in parallel. What is total capacitance?

Problem 4: Two 100µF capacitors are in series. What is the equivalent capacitance?

Problem 5: A 100µF capacitor charges through 47kΩ. What is the time constant?

Problem 6: Decode capacitor marking "224K". What is capacitance and tolerance?