What is a capacitor and how does it work?

Whether you open an old radio, a smartphone, or a 3D printer, you often see small cylinders or discs: capacitors. For many beginners, they're mysterious components. Why are there so many in a circuit? What exactly do they do?

Capacitors, along with resistors, are the most commonly used passive components in electronics. They are found in power supplies, signal processing, timing, and energy storage. Without capacitors, many circuits simply won't work.

What exactly do they do? They store electrical charge and release it in a controlled manner. This way, they dampen noise, filter signals, or create a time delay.

In this article, we explain capacitors step by step. You'll learn how they work and what they're used for. We'll discuss different types and provide practical examples. Finally, we'll offer tips for getting started with capacitors yourself.

What is a capacitor?

A capacitor is an electrical component that stores and releases charge quickly. It's similar to a mini battery, but it works differently. A battery delivers energy slowly. A capacitor charges quickly and discharges just as quickly.

The structure is simple: two metal plates with an insulator, the dielectric, between them. The plates don't touch, so no current flows directly through them. Yet, the capacitor stores energy. An electric field is created between the plates, temporarily storing the charge.

A useful analogy is with a water tank. The voltage represents the water pressure. The charge is the volume of water. The capacitance is the size of the tank. The larger the capacitance (measured in farads), the more charge the capacitor can hold at the same voltage.

This property makes capacitors important in many electronic applications. They dampen peaks, filter signals, and provide timing.

How does a capacitor work?

When you connect a capacitor to a voltage source, it charges. One plate becomes positively charged, the other negatively. The insulator between the plates prevents the charge from flowing back immediately. This creates an electric field that traps the energy.

Charging and discharging

As the capacitor charges, it gains charge. This continues until the voltage across the plates equals that of the source.

To discharge the capacitor, connect it to a load, such as a light bulb. The stored charge then flows through the circuit until the capacitor is empty.

The relationship between voltage, charge and capacitance is:

Q = C × U

Q = charge in coulombs

C = capacitance in farads

U = voltage in volts

Load curve and time constant

A capacitor doesn't charge linearly, but according to a curve. This is called an exponential curve. After one time constant (τ = R × C), the capacitor is 63% charged. After five time constants, it is practically full.

This principle is used in RC timers. Think of flashing lights or simple electronic clocks.

Example

Suppose you use a 1000 µF capacitor and a 1 kΩ resistor.

The time constant τ = R × C = 1000 × 0.001 = 1 second.

After 1 second the capacitor is at 63% of the voltage.

After 2 seconds it is at 86%.

After 5 seconds it is almost fully charged.

Important properties and types of capacitors

Capacitors come in many shapes and sizes. When choosing the right capacitor, consider a few key features:

Capacity

This indicates how much charge a capacitor stores per volt of voltage. The unit is farad (F), but you'll often find smaller values: microfarad (µF), nanofarad (nF), or picofarad (pF).

Working voltage

Every capacitor has a maximum voltage. If you exceed this voltage, it can fail or even explode.

Polarity

Some types, such as electrolytic capacitors, have positive and negative terminals. Connecting them incorrectly will destroy them. Other types, such as ceramic capacitors, have no polarity.

Commonly used types

Ceramic : Small, inexpensive, and suitable for filters or decoupling.

Electrolytic capacitor (ELC) : High-capacitance, polarized. Widely used in power supplies.

Film capacitor : Very stable. Often used in audio equipment and power filters.

Tantalum : Compact and high-capacity. Sensitive to misconnection.

Supercapacitor : Has extremely high capacity. Suitable for temporary energy storage, such as emergency power.

Each type has its own strengths and weaknesses. So choose wisely based on your application and requirements.

Practical applications of capacitors

Capacitors seem like simple components, but many devices wouldn't work without them. Here are their main applications:

1. Filter voltage

In power supplies, capacitors smooth out AC voltage ripple. Without this filtering, a DC voltage would be full of noise. Think of a Raspberry Pi : without capacitors, it runs unstable, and chips can even be damaged.

2. Decoupling

In microcontrollers and digital chips, you often see small ceramic capacitors right next to the pins. They absorb current spikes and keep the power supply stable, preventing signal distortion. This also extends the lifespan of components.

3. Linking signals

In audio equipment and amplifiers, capacitors block DC voltage but allow AC voltage to pass through. This keeps signals clear, preventing interference or distortion in audio and video signals.

4. Store energy

A camera flash uses a capacitor to quickly store energy and release it all at once. Electric motors also use them for starting. In some applications, supercapacitors even replace batteries.

5. Set time

Together with a resistor, a capacitor forms an RC circuit. This allows you to set delays or frequencies. Think of flashing lights, timers, or simple oscillators in toys and electronics.

6. Correct power factor

In factories, large capacitor banks improve the power factor. This reduces energy consumption, improves efficiency, and prevents fines from the grid operator. They also extend the lifespan of heavy machinery.

How do you recognize and use a capacitor?

Capacitors look different depending on the type:

• Ceramic : small discs or cubes.
• Elco (electrolytic) : cylindrical, with a marking for the negative terminal.
• Film capacitor : usually rectangular in shape.

Read value

Large capacitors have their value and operating voltage printed directly on the housing, such as "100µF 16V." Smaller capacitors use a numerical code. For example, "104" means 100,000 pF, or 100 nF.

Connect correctly

Pay attention to the polarity of electrolytic capacitors. If you connect them incorrectly, they can overheat or even explode. Also check that the operating voltage is high enough for your application.

Testing capacitors

Use a multimeter with a capacitance measurement function to check the value. For advanced diagnostics, measure the ESR (Equivalent Series Resistance). A high ESR indicates aging or damage.

Practical tips

Place decoupling capacitors as close as possible to the IC power pins. Always replace old capacitors with ones of the same or higher voltage. Discharge large capacitors before use: they may still hold a charge, even when the device is turned off.

Common mistakes and misunderstandings

Even experienced hobbyists sometimes make mistakes with capacitors. Here are the most common mistakes:

Confusing a capacitor with a battery: A capacitor stores energy, but discharges quickly. Therefore, it is not a replacement for a battery.

Applying too much voltage: If you exceed the maximum voltage, the capacitor may short-circuit or explode.

Reversing polarity: Electrolytic capacitors are sensitive to incorrect connection. Incorrect connection can cause immediate damage.

Underestimating the type and value: Not every capacitor is suitable. The correct type and capacitance are crucial for a properly functioning circuit.

Ignore temperature: High temperatures accelerate aging. Choose capacitors with a sufficient temperature range and place them wisely.

Conclusion

Capacitors may seem small and simple, but they're essential in almost every electronic circuit. They store charge, filter noise, couple signals, and temporarily supply power to a circuit.

Do you understand how capacitors work? Then you'll also better understand how electronics as a whole function. Whether you're creating a hobby project, trying to keep a Raspberry Pi stable, or building an amplifier, you won't get far without capacitors.

Want to get started? Start simple. Build a circuit with an LED that stays lit briefly after you disconnect the power. This way, you can immediately see how the capacitor stores and releases charge. Then, you can gradually experiment with RC timers, filters, or even a small flashlight.

Experiment with different values and types to discover their impact on speed, stability, and signal processing.

In short: the more you work with capacitors, the more you'll understand electronics. They're the silent force behind every well-functioning circuit. If you understand them, you'll understand the heart of many devices.