Capacitor in Electronics – What It Is and What It Does


Capacitor Definition and Symbol

A capacitor is an electrical component that stores energy in an electric field. It is a passive device that consists of two conductors separated by an insulating material known as a dielectric. When a voltage is applied across the conductors, an electric field develops across the dielectric, causing positive and negative charges to accumulate on the conductors. This stored energy is released when needed, making capacitors essential components in various electronic circuits.

How a Capacitor Works

When a capacitor is connected to a power source, electrons accumulate at one of the conductors (the negative plate), while electrons are removed from the other conductor (the positive plate). This creates a potential difference (voltage) across the plates and establishes an electric field in the dielectric material between them. The capacitor continues charging until the voltage across its plates equals the voltage of the power source.

Once the capacitor is fully charged and the voltage across its plates equals the voltage of the power source, the following occurs:

  1. Current Stops Flowing: In a direct current (DC) circuit, the current flow effectively stops because the capacitor acts like an open circuit. The electric field between the plates of the capacitor is at its maximum value, corresponding to the applied voltage. No further charge movement occurs.
  2. Stored Energy: The stored energy in the capacitor remains until it is connected to a circuit that allows it to discharge. The stored energy (๐ธ) in a capacitor is: ๐ธ = ยฝCV2, where C is the capacitance and ๐‘‰ is the voltage across the capacitor.
  3. Potential Difference Maintained: The capacitor maintains a potential difference across its plates equal to the voltage of the power source. This potential difference is accessible when the capacitor is connected to another circuit element.
  4. Discharge: If a path is available for the charges to move (for instance, by connecting a resistor across the capacitor), the capacitor starts discharging. The discharge process results in a current flowing in the circuit. The voltage across the capacitor decreases over time until it reaches zero, at which point the capacitor is fully discharged.

Types of Capacitors and Their Uses

There are several types of capacitors, each with unique properties and applications:

  • Ceramic Capacitors: Made from ceramic materials, these capacitors are useful in electronic circuits for their stability, reliability, and wide range of capacitance values. Ceramic capacitors are common in filtering and timing applications.
  • Electrolytic Capacitors: These capacitors use an electrolyte to achieve higher capacitance values. They are polarized, meaning they have a positive and negative lead. Electrolytic capacitors find use in power supply circuits for filtering and smoothing.
  • Tantalum Capacitors: Similar to electrolytic capacitors but using tantalum for the anode, these capacitors offer high capacitance in a small package. Tantalum capacitors are for situations requiring stable and reliable performance, such as in smartphones and computers.
  • Film Capacitors: Using a thin plastic film as the dielectric, these capacitors have high precision and stability. They are useful in audio and high-frequency applications.
  • Supercapacitors: Also known as ultracapacitors, these have very high capacitance values. Supercapacitors are for applications requiring rapid charge and discharge cycles, such as in regenerative braking systems and backup power supplies.

Safety and Dangers of Capacitors

Capacitors are potentially dangerous because they store a significant amount of energy. Short-circuiting or mishandling a charged capacitor results in a rapid discharge, causing sparks, burns, or even an electric shock. In extreme cases, large capacitors deliver a potentially lethal shock.

Capacitors vs. Batteries

Both capacitors and batteries store electrical energy, but they do so in fundamentally different ways:

  • Capacitors store energy in an electric field and release energy very quickly. They are useful in applications requiring rapid charge and discharge cycles.
  • Batteries store energy chemically and release it more slowly. They are useful for providing a steady supply of energy over a longer period.

Connecting a Capacitor to a Battery

Connecting a capacitor to a battery starts charging the capacitor. Electrons flow from the negative terminal of the battery to one plate of the capacitor and from the other plate to the positive terminal of the battery. This process continues until the voltage across the capacitor equals the voltage of the battery. Once fully charged, the current flow stops, and the capacitor holds the charge until it is discharged.

Capacitors with AC and DC

Capacitors behave differently depending on whether they are in direct current or alternating current situations:

  • Direct Current (DC): When connected to a DC source, a capacitor charges up to the source voltage and then acts as an open circuit. This blocks any further DC current.
  • Alternating Current (AC): With AC, the voltage across the capacitor continuously changes. The capacitor charges and discharges cyclically. This results in an AC current flowing through the capacitor, with the capacitor acting as a reactive component that impedes the flow of AC to a degree that depends on the frequency of the AC signal.

History of the Capacitor

The concept of the capacitor dates back to the 18th century. In 1745, Ewald Georg von Kleist discovered that an electric charge could be stored by connecting a high-voltage electrostatic generator to a volume of water in a hand-held glass jar. Independently, Pieter van Musschenbroek of Leyden University developed a similar device around the same time. This device became known as the Leyden jar. These early capacitors were used to conduct experiments in electricity and laid the foundation for the development of modern capacitors.

Capacitance

Capacitance is a capacitor’s ability for storing an electric charge per unit of voltage across its plates. The formula for capacitance is:

๐ถ=๐‘„ / ๐‘‰โ€‹

where:

  • ๐ถ is the capacitance in farads (F),
  • ๐‘„ is the charge in coulombs (C),
  • ๐‘‰ is the voltage in volts (V).

Units of Measuring Capacitance

The unit of capacitance is the farad (F), named after Michael Faraday. A farad is equivalent to one coulomb per volt. Due to the large size of the farad, capacitors typically have capacitance in microfarads (ยตF, 10โˆ’6 F), nanofarads (nF, 10โˆ’9 F), and picofarads (pF, 10โˆ’12 F).

Dielectric Material

A dielectric material is the insulating substance between the plates of a capacitor. It increases the capacitor’s capacitance by reducing the electric field strength for a given charge on the plates. Common dielectric materials include air, paper, plastic, ceramic, and glass.

Dielectric Constant and Permittivity

The dielectric constant (๐œ…) is a measure of a material’s ability to increase the capacitance of a capacitor compared to a vacuum. It is the ratio of the permittivity of the dielectric material (๐œ€) to the permittivity of free space (ฮต0โ€‹):

๐œ… = ๐œ€ / ๐œ€0โ€‹

The permittivity (ฮต) is a measure of how much electric field is reduced within the material.

Equation for Capacitance of a Parallel Plate Capacitor

The capacitance (๐ถ) of a parallel plate capacitor is:

๐ถ = ๐œ€๐ด / ๐‘‘โ€‹

where:

  • ๐œ€ is the permittivity of the dielectric material,
  • ๐ด is the area of one of the plates,
  • ๐‘‘ is the separation between the plates.

Example Problem

For example, calculate the capacitance.

Given:

  • Plate area (๐ด) = 0.01 mยฒ,
  • Plate separation (๐‘‘) = 0.001 m,
  • Dielectric constant (ฮบ) = 4,
  • Permittivity of free space (ฮต0โ€‹) = 8.854ร—10โˆ’12.

First, find the permittivity of the dielectric:

๐œ€ = ๐œ…๐œ€0 = 4ร—8.854ร—10โˆ’12 = 3.5416ร—10โˆ’11โ€‰F/m

Then, use the capacitance formula:

๐ถ = ๐œ€๐ด/๐‘‘ = 3.5416ร—10โˆ’11 ร— 0.01 / 0.001 = 3.5416ร—10โˆ’10โ€‰F = 354.16โ€‰pF

Thus, the capacitance of the parallel plate capacitor is 354.16 picofarads (pF).

Safety Precautions for Handling Capacitors

Capacitors store significant electrical energy and improper handling is dangerous. Here are essential safety precautions:

  • Proper Discharging: Prevent electric shock by always discharging capacitors using a resistor or a discharge tool before handling.
  • Use Safety Equipment: Wear insulated gloves and safety glasses. Use tools with insulated handles.
  • Handling Large Capacitors: Avoid direct contact with terminals and respect voltage ratings. Handle high-voltage capacitors with extra caution.
  • Prevent Short Circuits: Ensure proper wiring and use insulation around terminals to avoid accidental contact.
  • Storage and Disposal: Store in a dry, cool place away from heat and moisture. Follow local regulations for disposal, especially for capacitors containing hazardous materials.
  • Electrolytic Capacitors: Connect with the correct polarity to prevent failure. Protect from voltage surges.
  • High-Voltage Capacitors: Clearly label and isolate high-voltage capacitors to prevent accidental contact.
  • Capacitor Failure: Look for signs of damage like bulging or leakage. Replace damaged capacitors with ones of the same or higher rating.
  • Training and Awareness: Ensure proper training and awareness of risks. Have emergency procedures in place for accidents involving capacitors.

References

  • Bird, John (2010). Electrical and Electronic Principles and Technology. Routledge. ISBN 978-0-08089056-2.
  • Floyd, Thomas (2005) [1984]. Electronic Devices (7th ed.). Upper Saddle River, New Jersey, USA: Pearson Education. ISBN 0-13-127827-4.
  • Ho, Janet; Jow, T. Richard; Boggs, Steven (2010). “Historical Introduction to Capacitor Technology”. IEEE Electrical Insulation Magazine. 26 (1): 20โ€“25. doi:10.1109/mei.2010.5383924
  • Kaplan, Daniel M.; White, Christopher G. (2003). Hands-On Electronics: A Practical Introduction to Analog and Digital Circuits. Cambridge University Press. ISBN 978-0-52189351-0.
  • Ulaby, Fawwaz Tayssir (1999). Fundamentals of Applied Electromagnetics (2nd ed.). Upper Saddle River, New Jersey, USA: Prentice Hall. ISBN 978-0-13011554-6.