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Learn Electronics: Capacitors: What They Are and How They Work


In the vast world of electronics, capacitors stand out as one of the fundamental components, akin to resistors and inductors. At its core, a capacitor is akin to a miniature energy reservoir. It has the capacity (hence the name) to collect and store energy, only to release it later when needed. Imagine a dam: it holds back water (analogous to electrical energy) and can release it in a controlled manner to generate electricity. Similarly, capacitors store electrical energy but on a much smaller scale.

Visually, capacitors come in various shapes and sizes, from tiny SMDs hidden in our smartphones to large metal canisters seen in heavy-duty machinery or older tube radios. Regardless of their physical appearance, the fundamental principle remains the same: they store energy in an electric field created between two metal plates separated by an insulating material.

But why is this ability to store and release energy so crucial? Think of a camera’s flash, the quick burst of energy required to light up a dark scene for a brief moment. Or consider the stability required in many electronic circuits, where capacitors play a pivotal role in smoothing out voltage fluctuations, ensuring consistent performance.

In essence, capacitors are silent, passive workers in the realm of electronics, seamlessly doing their job and ensuring that our devices function correctly.

How common are capacitors in electronic devices?

They are in practically every electronic device, I would be tempted to say every device but I am sure there must be something out there to prove me wrong. Open up any device and you will see them in their many types.

Basic definition of a capacitor

A capacitor is a two-terminal passive electronic component that stores electrical energy in an electric field. It consists of two conductive plates separated by an insulating material known as the dielectric.

How Capacitors Work

When voltage is applied across a capacitor, positive charges build up on one plate and negative charges on the other, creating an electric field between them.

This stored energy can be released when the capacitor is connected in a circuit, which allows the charges to flow.

Capacitance is measured in farads(F) and this signifies the capacitors ability to store energy. The higher the farads the more energy it can store.

Although super capacitors can get to multiple farads (F), most practical capacitors use microfarads (µF), nanofarads (nF), or picofarads (pF). The capacitance value depends on the surface area of the plates, the distance between them, and the dielectric material’s properties.

Types of Capacitors

  • Electrolytic Capacitors: Polarised capacitors with high capacitance values.
    Some typical electrolytic capacitors
    Some typical electrolytic capacitors
  • Ceramic Capacitors: non-polarised capacitors often used for frequency tuning.
  • Tantalum Capacitors: Polarised capacitors with stable capacitance.
  • Film Capacitors: Made from thin plastic films and used in high-frequency applications.
  • Super-capacitors: Can store large amounts of energy, bridging the gap between capacitors and batteries.

Common Uses and Applications

  • Filtering: Smoothing out voltage in power supplies.
  • Timing: In combination with resistors in RC circuits to create time delays.
  • Coupling and Decoupling: Transferring AC signals between stages while blocking DC.
  • Energy Storage: Providing bursts of energy when needed.

Important Characteristics and Specifications

  • Voltage Rating: The maximum voltage a capacitor can withstand. When replacing a capacitor in a device, you can use a replacement with the same voltage rating or higher. However, not lower.
  • Tolerance: How close the actual capacitance value is to the stated value. This is usually quite a high number such as 20%.
  • Equivalent Series Resistance (ESR): A measure of resistance in real-world capacitors. Circuits can often fail when the ESR of a capacitor becomes too high. Get an ESR meter!
  • Leakage Current: A tiny amount of current that flows through the dielectric.

How do you calculate capacitors in a circuit?

Capacitors in a circuit can be arranged in series or in parallel. Calculating the total capacitance for either is quite straightforward.

Capacitors in series

When capacitors are connected in series, the total capacitance.$$C_{total}$$ is found using the reciprocal formula:



$$C_1,C_2,C_3,…$$ are the capacitances of the individual capacitors.

For instance, if you have just two capacitors in series:

$$\frac{1}{C_{\text{total}}} = \frac{1}{C_1} + \frac{1}{C_2}$$

To find

$$C_{total} $$

take the reciprocal of the sum:

$$C_{\text{total}} = \frac{1}{\left( \frac{1}{C_1} + \frac{1}{C_2} \right)}$$

Note: The total capacitance of capacitors in series is always less than the smallest capacitor’s capacitance.

Capacitors in Parallel

For capacitors in a parallel configuration, the total capacitance is the sum of the individual capacitances:


$$C_{\text{total}} = C_1 + C_2 + C_3 + \dots$$

So, if you have two capacitors in parallel:

$$C_{\text{total}} = C_1 + C_2$$​

Note: The total capacitance of capacitors in parallel is always greater than the largest capacitor’s capacitance.


  1. Series Configuration

Suppose you have two capacitors in series, one with a capacitance of 4µF and the other 6µF:


To get $$C_{total}$$

$$C_{total}=\frac1{{\displaystyle\frac14}+{\displaystyle\frac16}}=2.4\mu f$$

  1. Parallel Configuration

Using the same capacitors in a parallel configuration:

$$C_{total}=4+6=10\mu f$$

In Summary:

  • For capacitors in series, the combined capacitance decreases.
  • For capacitors in parallel, the combined capacitance increases.

Understanding these basic principles allows you to calculate the effective capacitance for various configurations in more complex circuits.


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