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Capacitor: A Detailed Educational Resource

Capacitor, Condenser, Passive Electronic Component, Capacitance

Explore the history, theory, and applications of capacitors, fundamental passive electronic components that store electrical energy in an electric field.

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Introduction

In electrical engineering, a capacitor (formerly known as a condenser) is a fundamental passive electronic component that stores electrical energy in an electric field. It is a two-terminal device designed to add capacitance to an electrical circuit.

Capacitor (Condenser): An electronic component that stores electrical energy by accumulating electric charges on two closely spaced conductors (plates) separated by an insulating material called a dielectric. The older term “condenser” is still found in some contexts, like condenser microphones.

Passive Electronic Component: An electronic component that does not require external power to operate and cannot amplify or oscillate an electrical signal. Examples include resistors, capacitors, and inductors.

While capacitance inherently exists between any two conductors in proximity, a capacitor is specifically engineered to maximize this property for circuit applications. Capacitors are ubiquitous in modern electronics, serving diverse functions from energy storage and signal filtering to timing and sensing. Unlike resistors which dissipate energy, ideal capacitors store energy, although real-world capacitors exhibit some energy dissipation due to non-ideal behaviors.

The utility of a capacitor is quantified by its capacitance, measured in Farads (F). A higher capacitance value indicates a greater ability to store charge at a given voltage.

1. History: From Leyden Jars to Modern Devices

The concept of capacitance, in its rudimentary form, has been observed in nature for millennia.

1.1 Natural Capacitance: Lightning

Natural Capacitance: Capacitance occurring in nature without human intervention.

A prime example of natural capacitance is lightning. Clouds and the Earth’s surface act as conductors, with the air between them serving as a dielectric. As charge accumulates, the potential difference rises until it exceeds the breakdown voltage of air, resulting in a dramatic discharge – lightning.

1.2 The Dawn of Artificial Capacitors: Leyden Jar

The controlled study of capacitance began in the 1740s with the accidental discovery of charge storage.

Leyden Jar: An early form of capacitor consisting of a glass jar with metal foil coatings on the inside and outside, used to store static electricity.

1.3 Evolution and Refinement

Battery (Early Capacitor Context): A group of Leyden jars connected in parallel to increase total charge storage, analogous to a battery of cannons increasing firepower.

For over a century, Leyden jars and similar devices using flat glass plates and foil were the primary capacitors.

1.4 The Rise of Modern Capacitors (1900s Onward)

The advent of wireless radio around 1900 spurred the need for standardized capacitors with lower inductance, especially for higher frequencies.

2. Theory of Operation: How Capacitors Store Charge

2.1 Overview: Conductors, Dielectrics, and Charge Accumulation

A capacitor fundamentally consists of two electrical conductors, often called plates, separated by a non-conducting region, known as a dielectric.

Conductor: A material that allows electric charge to flow easily. In capacitors, conductors are typically metallic plates or films.

Dielectric: An electrically insulating material placed between the conductors of a capacitor to increase its capacitance. Common dielectrics include air, paper, ceramic, plastic, and oxide layers.

When a voltage is applied across the capacitor’s terminals (e.g., by connecting it to a battery), an electric field develops within the dielectric. This electric field exerts a force on the charge carriers in the conductors.

Electric Field: A region around an electrically charged object in which a force is exerted on other electrically charged objects.

2.2 Capacitance (C): Quantifying Charge Storage

The capacitance (C) of an ideal capacitor is a constant value, measured in Farads (F) in the SI system. It represents the capacitor’s ability to store charge for a given voltage.

Capacitance (C): The measure of a capacitor’s ability to store electric charge. It is defined as the ratio of the charge (Q) on each conductor to the voltage (V) between them. Measured in Farads (F).

Farad (F): The SI unit of capacitance. One Farad is defined as the capacitance that stores one Coulomb of charge when a voltage of one Volt is applied across its plates. Farad is a large unit, so capacitance is often expressed in microfarads (µF), nanofarads (nF), or picofarads (pF).

Mathematically, capacitance is defined by the equation:

C = \frac{Q}{V}

Where:

Example: A capacitor with a capacitance of 1 Farad (1F) will store 1 Coulomb of charge when a voltage of 1 Volt is applied across its terminals.

2.3 Incremental Capacitance in Practical Devices

In real-world capacitors, the capacitance may not be perfectly constant and can vary slightly with factors like applied voltage or mechanical stress. In such cases, capacitance is defined in terms of incremental changes:

C = \frac{\mathrm{d}Q}{\mathrm{d}V}

This equation describes the capacitance as the ratio of a small change in charge (dQ) to a small change in voltage (dV).

2.4 Hydraulic Analogy: Visualizing Capacitance

A helpful analogy for understanding capacitor behavior is the hydraulic analogy, comparing electrical circuits to water flow systems:

Electrical ComponentHydraulic Analogy
Voltage (V)Water Pressure
Current (I)Water Flow Rate
CapacitorElastic Diaphragm in a Pipe
Capacitance (C)Diaphragm Elasticity

2.5 Circuit Equivalence: Short-Time and Long-Time Limits

Capacitors behave differently in circuits depending on the time scale. Analyzing short-time and long-time limits simplifies circuit analysis:

2.6 Parallel-Plate Capacitor: A Simple Model

The simplest capacitor model is the parallel-plate capacitor, consisting of two parallel conductive plates separated by a uniform dielectric.

Parallel-Plate Capacitor: A basic capacitor configuration consisting of two parallel conductive plates separated by a dielectric material.

The capacitance of a parallel-plate capacitor is determined by:

C = \varepsilon \frac{A}{d}

Where:

Permittivity (ε): A measure of how easily an electric field can form in a material. Higher permittivity materials allow for stronger electric fields and thus greater capacitance. Permittivity is often expressed as ε = εrε0, where εr is the relative permittivity (dielectric constant) of the material, and ε0 is the permittivity of free space (vacuum).

Key Insights from the Parallel-Plate Capacitor Formula:

Example: To increase the capacitance of a parallel-plate capacitor, one can:

  1. Increase the area of the plates.
  2. Decrease the distance between the plates.
  3. Use a dielectric material with a higher permittivity (dielectric constant).

2.7 Interleaved Capacitor: Increasing Capacitance with Multiple Plates

To further increase capacitance, practical capacitors often use multiple interleaved plates. Imagine stacking multiple parallel-plate capacitors on top of each other and connecting them in parallel.

For n number of plates in an interleaved capacitor, the total capacitance is:

C = \varepsilon_0 \frac{A}{d} (n-1)

Where:

Each pair of adjacent plates effectively forms a capacitor, and with n plates, there are (n-1) such pairs connected in parallel, thus increasing the total capacitance.

2.8 Energy Stored in a Capacitor

Charging a capacitor requires work to move charge against the electric field. This work is stored as potential energy in the electric field within the dielectric.

The energy W stored in a capacitor is given by:

W = \frac{1}{2} \frac{Q^2}{C} = \frac{1}{2} VQ = \frac{1}{2} CV^2

Where:

This stored energy can be released when the capacitor discharges, doing work in an external circuit.

Energy Density and Dielectric Strength

The maximum energy a capacitor can store is limited by the dielectric strength of the dielectric material.

Dielectric Strength (Ud): The maximum electric field strength that a dielectric material can withstand before it breaks down and becomes conductive. Measured in Volts per meter (V/m) or Volts per millimeter (V/mm).

The breakdown voltage (Vbd) of a capacitor is the voltage at which dielectric breakdown occurs:

V_{bd} = U_d d

Where d is the dielectric thickness.

The maximum energy a parallel-plate capacitor can store before breakdown is:

E_{max} = \frac{1}{2} \varepsilon Ad U_d^2

This equation shows that the maximum energy storage is proportional to the dielectric volume (Ad), permittivity (ε), and the square of the dielectric strength (Ud2).

2.9 Current-Voltage Relation: Capacitor Behavior in Circuits

The fundamental relationship between current and voltage for a capacitor is described by its current-voltage relation.

The integral form of the capacitor equation relates voltage to the integral of current:

V(t) = \frac{Q(t)}{C} = V(t_0) + \frac{1}{C} \int_{t_0}^{t} I(\tau) \, \mathrm{d}\tau

This equation states that the voltage across a capacitor at time t is determined by its initial voltage V(t0) plus the accumulation of charge due to the current I(τ) flowing into it over time.

The derivative form relates current to the rate of change of voltage:

I(t) = \frac{\mathrm{d}Q(t)}{\mathrm{d}t} = C \frac{\mathrm{d}V(t)}{\mathrm{d}t}

This equation indicates that the current through a capacitor is proportional to the rate of change of voltage across it. A rapidly changing voltage results in a larger current.

2.10 RC Circuits: Charging and Discharging a Capacitor

An RC circuit is a basic circuit consisting of a resistor (R) and a capacitor (C) connected in series, often with a voltage source and a switch. RC circuits are fundamental for understanding capacitor behavior in dynamic circuits.

RC Circuit: A circuit containing a resistor (R) and a capacitor (C), used for timing, filtering, and other applications that rely on the capacitor’s charging and discharging characteristics.

Charging Circuit:

When an initially uncharged capacitor in series with a resistor is connected to a DC voltage source V0 at time t=0, the capacitor starts to charge. The voltage across the capacitor V(t), the current I(t), and the charge Q(t) evolve over time as follows:

I(t) = \frac{V_0}{R} e^{-t/\tau_0}

V(t) = V_0 (1 - e^{-t/\tau_0})

Q(t) = CV_0 (1 - e^{-t/\tau_0})

Where τ0 = RC is the time constant of the RC circuit.

Time Constant (τ0 = RC): In an RC circuit, the time constant represents the time it takes for the voltage across the capacitor to reach approximately 63.2% of its final value during charging or to decrease to approximately 36.8% of its initial value during discharging. It is a measure of how quickly the capacitor charges or discharges.

Discharging Circuit:

If a charged capacitor with initial voltage VCi is discharged through a resistor, the equations become:

I(t) = \frac{V_{Ci}}{R} e^{-t/\tau_0}

V(t) = V_{Ci} e^{-t/\tau_0}

Q(t) = C V_{Ci} e^{-t/\tau_0}

Applications of RC Circuits:

2.11 AC Circuits: Impedance and Reactance

In AC circuits (circuits with alternating current), capacitors exhibit a frequency-dependent behavior described by reactance and impedance.

Reactance (X): The opposition to the flow of alternating current (AC) offered by a capacitor or inductor. Capacitive reactance (XC) is inversely proportional to frequency, while inductive reactance (XL) is directly proportional. Measured in Ohms (Ω).

Impedance (Z): The total opposition to the flow of alternating current (AC) in a circuit, encompassing both resistance and reactance. It is a complex quantity representing both the magnitude of opposition and the phase shift between voltage and current. Measured in Ohms (Ω).

Capacitive Reactance (XC):

The capacitive reactance (XC) is the opposition a capacitor offers to AC current. It is inversely proportional to the frequency (f) of the AC signal and the capacitance (C):

X_C = - \frac{1}{\omega C} = - \frac{1}{2\pi fC}

Where ω = 2πf is the angular frequency, and the negative sign indicates a phase lag.

Capacitive Impedance (Z):

The impedance (Z) of a capacitor in AC circuits is purely reactive (for an ideal capacitor):

Z = \frac{1}{j\omega C} = - \frac{j}{\omega C} = - \frac{j}{2\pi fC}

Where j is the imaginary unit. The impedance is a complex number with only an imaginary component, representing the capacitive reactance.

Displacement Current:

In an AC circuit, even though electrons do not flow through the dielectric, there is an “effective current” called displacement current due to the changing electric field within the capacitor. This displacement current is essential for AC current to “pass through” a capacitor.

2.12 Laplace Circuit Analysis (s-domain)

For advanced circuit analysis, especially in transient analysis and control systems, the Laplace transform is a powerful tool. In Laplace domain (s-domain), the impedance of an ideal capacitor with no initial charge is represented as:

Z(s) = \frac{1}{sC}

Where:

This representation simplifies circuit analysis by transforming differential equations into algebraic equations in the s-domain.

2.13 Circuit Analysis: Series and Parallel Capacitor Combinations

Capacitors can be connected in series and parallel configurations to achieve desired equivalent capacitance values in circuits.

Capacitors in Parallel:

When capacitors are connected in parallel, they share the same voltage across their terminals. The total equivalent capacitance Ceq is the sum of individual capacitances:

C_{eq} = \sum_{i=1}^{n} C_i = C_1 + C_2 + \cdots + C_n

Capacitors in Series:

When capacitors are connected in series, they carry the same charge. The reciprocal of the total equivalent capacitance Ceq is the sum of the reciprocals of individual capacitances:

C_{eq} = \left( \sum_{i=1}^{n} \frac{1}{C_i} \right)^{-1} = \left( \frac{1}{C_1} + \frac{1}{C_2} + \cdots + \frac{1}{C_n} \right)^{-1}

Voltage Distribution in Series Capacitors:

In a series connection, the total voltage is divided across the capacitors inversely proportional to their capacitances. The capacitor with the smallest capacitance will have the largest voltage drop across it. This is crucial for high-voltage applications where capacitors are connected in series to increase the overall voltage rating.

3. Non-Ideal Behavior: Real-World Capacitor Limitations

Ideal capacitor theory provides a simplified model. Real capacitors deviate from ideal behavior due to various imperfections and parasitic effects. These non-idealities are important to consider in practical circuit design, especially at higher frequencies or in precision applications.

3.1 Breakdown Voltage: Dielectric Failure

Every dielectric material has a dielectric strength (Eds), which is the maximum electric field it can withstand before it becomes conductive and fails (dielectric breakdown).

Breakdown Voltage (Vbd): The maximum voltage that can be applied across a capacitor before the dielectric material breaks down and becomes conductive, leading to a short circuit.

The breakdown voltage (Vbd) is given by:

V_{bd} = E_{ds} d

Exceeding the breakdown voltage can cause permanent damage to the capacitor, often resulting in a short circuit. The maximum energy a capacitor can safely store is limited by its breakdown voltage.

Factors Affecting Breakdown Voltage:

3.2 Equivalent Circuit: Modeling Non-Ideal Capacitors

To account for non-ideal behaviors, real capacitors can be modeled by an equivalent circuit that includes parasitic components in addition to the ideal capacitance.

A common equivalent circuit model includes:

Equivalent Series Resistance (ESR): The total series resistance in a capacitor’s equivalent circuit, representing energy losses due to lead resistance, plate resistance, and dielectric losses.

Equivalent Series Inductance (ESL): The total series inductance in a capacitor’s equivalent circuit, primarily due to the capacitor’s leads and internal construction.

Simplified RLC Series Model:

At higher frequencies, a simplified model is often used, consisting of an ideal capacitor in series with ESR and ESL. This RLC series model is valid over a broad frequency range but is not accurate at DC or very low frequencies where leakage resistance is significant.

3.3 Q Factor: Capacitor Efficiency

The Quality Factor (Q factor) of a capacitor is a measure of its efficiency or “ideality.” It is defined as the ratio of the capacitive reactance to the ESR at a given frequency:

Q(\omega) = \frac{|X_C(\omega)|}{\text{ESR}} = \frac{1}{\omega C \cdot \text{ESR}}

A higher Q factor indicates a more ideal capacitor with lower losses. A low Q factor indicates higher losses and deviation from ideal capacitor behavior. The reciprocal of the Q factor is the Dissipation Factor (DF).

3.4 Ripple Current: AC Current Effects

Ripple current is the AC component of current flowing through a capacitor, often encountered in power supply filtering and other applications where capacitors handle AC superimposed on DC.

Ripple Current: The AC component of current flowing through a capacitor, typically in power supply filtering applications.

Ripple current causes heat generation within the capacitor due to ESR and dielectric losses. Exceeding the capacitor’s ripple current rating can lead to overheating, performance degradation, and premature failure, especially in electrolytic capacitors (tantalum and aluminum).

3.5 Capacitance Instability: Aging and Temperature Dependence

Capacitance is not always constant and can be affected by factors like aging, temperature, voltage, and frequency.

Curie Point: The temperature above which certain materials, like ferroelectric ceramics, lose their ferroelectric properties, affecting their dielectric constant and capacitance.

Microphonic Effect: The phenomenon where mechanical vibrations or sound waves cause a change in capacitance, generating unwanted electrical signals, especially in capacitors with flexible dielectrics or plate structures.

3.6 Current and Voltage Reversal: Polarity Effects

Current reversal and voltage reversal refer to changes in the direction of current and polarity of voltage across a capacitor.

Reversal can stress the dielectric, cause heating, and shorten capacitor lifespan. Capacitor designs and voltage ratings must consider the expected level of reversal in the application.

3.7 Dielectric Absorption (Soakage): Residual Charge

Dielectric absorption, also called soakage, is a phenomenon where a capacitor, after being discharged, can spontaneously regain a small voltage over time due to hysteresis in the dielectric material.

Dielectric Absorption (Soakage): The phenomenon where a capacitor, after being discharged, can appear to “recharge” itself to a small voltage over time due to charge remaining trapped within the dielectric material.

This effect can be problematic in precision circuits like sample-and-hold circuits or timing circuits. The level of dielectric absorption varies significantly with dielectric material. Polystyrene and Teflon dielectrics exhibit very low absorption, while tantalum electrolytic and polysulfone film capacitors have higher absorption.

3.8 Leakage: Imperfect Insulation

No dielectric is a perfect insulator. A small leakage current always flows through the dielectric, represented by the parallel leakage resistance in the equivalent circuit.

Leakage Current: A small, undesirable current that flows through the dielectric of a capacitor due to the dielectric not being a perfect insulator.

Excessive leakage can be caused by dielectric deterioration due to heat, stress, humidity, or aging. Leakage can significantly affect circuit performance, especially in high-impedance circuits or timing circuits.

3.9 Electrolytic Failure from Disuse: Conditioning Loss

Aluminum electrolytic capacitors require “conditioning” during manufacturing, where a voltage is applied to form the oxide dielectric layer properly. If electrolytic capacitors are unused for extended periods, they can lose this conditioning, increasing the risk of short circuits upon subsequent operation. Re-conditioning can sometimes be achieved by slowly applying voltage.

3.10 Lifespan: Factors Affecting Longevity

Capacitor lifespan is finite and depends on various factors:

Electrolytic Capacitor Lifespan:

Electrolytic capacitor lifespan is primarily determined by electrolyte evaporation, accelerated by higher temperatures and ripple currents. The Arrhenius equation and “10-degree rule” are often used to estimate lifespan based on temperature. For every 10°C increase in operating temperature, the lifespan of an electrolytic capacitor roughly halves.

4. Capacitor Types: A Diverse Range of Technologies

Practical capacitors are available in a wide variety of types, categorized by their dielectric material, construction style, and electrical characteristics. The choice of capacitor type depends on the specific application requirements, including capacitance value, voltage rating, frequency range, temperature stability, size, and cost.

4.1 Dielectric Materials: The Heart of Capacitance

The dielectric material is the most critical factor determining a capacitor’s characteristics. Different dielectrics offer different combinations of permittivity, breakdown voltage, temperature stability, frequency performance, and loss characteristics.

Common Dielectric Materials and Capacitor Types:

4.2 Voltage-Dependent Capacitors (Varicaps)

Some dielectrics, particularly ferroelectric materials, exhibit a dielectric constant that changes with the applied electric field (and thus voltage). This results in voltage-dependent capacitance.

Varicap (Varactor Diode): A semiconductor diode specifically designed to exhibit voltage-dependent capacitance. The capacitance changes as the reverse bias voltage applied to the diode is varied. Used in electronic tuning, voltage-controlled oscillators, and frequency multipliers.

Semiconductor Diodes as Voltage-Dependent Capacitors:

Semiconductor diodes, especially varicap diodes, also exhibit voltage-dependent capacitance. In diodes, the voltage dependence arises from the change in the width of the depletion region with applied reverse bias voltage, rather than changes in the dielectric constant itself.

4.3 Frequency-Dependent Capacitors

At sufficiently high frequencies, the polarization of the dielectric may not be able to keep up with the rapidly changing electric field. This leads to frequency-dependent capacitance.

Dielectric Dispersion: The phenomenon where the dielectric constant (and thus capacitance) of a material changes with frequency, particularly at higher frequencies.

Dielectric Relaxation: The process by which the polarization of a dielectric material lags behind changes in the applied electric field, causing frequency dependence of the dielectric constant and losses. Debye relaxation is a common model for dielectric relaxation.

Dielectric Dispersion and Relaxation:

The dielectric constant and capacitance become complex functions of frequency, with both real and imaginary parts. The imaginary part relates to energy absorption and dielectric losses at higher frequencies.

MOS Capacitors and Frequency Dependence:

MOS capacitors (Metal-Oxide-Semiconductor capacitors) in semiconductor devices also exhibit frequency-dependent capacitance due to the slow generation of minority carriers. At high frequencies, only majority carriers respond, while at low frequencies, both majority and minority carriers contribute to capacitance.

5. Styles and Packages: Physical Forms of Capacitors

Capacitors come in various physical styles and packages, determined by their application, capacitance value, voltage rating, and mounting method.

Common Capacitor Styles:

6. Capacitor Markings: Decoding Capacitor Values

Capacitors are marked with codes to indicate their capacitance value, tolerance, voltage rating, and other characteristics.

6.1 Marking Codes for Larger Parts

Larger capacitors, especially electrolytic types, typically display the capacitance value directly with the unit (e.g., “220 μF”). “MF” is sometimes used as an abbreviation for microfarads (μF).

6.2 Three/Four-Character Marking Code for Small Capacitors

Small ceramic capacitors often use a three-digit or four-character code to indicate capacitance in picofarads (pF):

Example: “473K” means 47 × 103 pF = 47 nF, ±10% tolerance.

6.3 Two-Character Marking Code for Small Capacitors

For very small capacitors, a two-character code may be used, consisting of an uppercase letter (representing significant digits) and a digit (multiplier). This code is defined in ANSI/EIA-198 and IEC 60062 standards.

6.4 RKM Code

The RKM code (IEC 60062, BS 1852) is used in circuit diagrams and component markings, replacing the decimal separator with the SI prefix symbol (R for decimal point, k for kilo, M for mega, n for nano, p for pico) and “F” for Farads unit.

Examples:

6.5 Historical Units (Obsolete)

Older texts and some capacitor packages may use obsolete units:

7. Applications: Versatile Roles in Electronics

Capacitors are essential components in a vast array of electronic circuits and systems, performing diverse functions.

7.1 Energy Storage: Temporary Power and Backup

Capacitors can store electrical energy and release it quickly, acting as temporary energy storage devices.

7.2 Digital Memory: DRAM

In Dynamic Random Access Memory (DRAM), capacitors are used as fundamental storage elements to represent binary data (bits). Each memory cell in DRAM typically consists of a capacitor and a transistor. The presence or absence of charge on the capacitor represents a “1” or “0” bit.

7.3 Pulsed Power and Weapons: High-Power Pulses

Capacitors are crucial in pulsed power applications where large amounts of energy need to be released in very short pulses.

7.4 Power Conditioning: Smoothing and Filtering

Capacitors are essential for power conditioning in electronic circuits to improve power quality and reduce noise.

Decoupling Capacitor (Bypass Capacitor): A capacitor used to reduce noise and voltage fluctuations on power supply lines by providing a low-impedance path for AC noise currents to ground.

Power Factor Correction: The process of improving the power factor in an AC electrical system, typically by adding capacitors to counteract inductive loads and reduce reactive power, thereby improving energy efficiency and reducing electricity costs.

7.5 Suppression and Coupling: Signal Manipulation

Capacitors are used for signal suppression and signal coupling in electronic circuits.

AC Coupling (Capacitive Coupling): Using a capacitor to transmit AC signals while blocking DC signals, commonly used to isolate DC bias levels between different circuit stages.

Snubber Circuit: A circuit, often consisting of a capacitor and resistor in series, used to suppress voltage transients and oscillations when switching inductive loads, protecting components and reducing EMI.

7.6 Motor Starters: Phase Shifting for AC Motors

7.7 Signal Processing: Filtering and Integration

Capacitors are essential components in signal processing circuits.

7.8 Tuned Circuits: Resonance and Frequency Selection

Capacitors and inductors (L) combined in tuned circuits (resonant circuits) are used to select specific frequencies.

Tuned Circuit (Resonant Circuit): A circuit containing an inductor (L) and a capacitor (C) that resonates at a specific frequency, allowing it to selectively pass or reject signals at that frequency. Used in radio receivers, oscillators, and filters.

The resonant frequency (f) of a tuned circuit is given by:

f = \frac{1}{2\pi \sqrt{LC}}

7.9 Sensing: Capacitive Sensors

Capacitance changes can be used to sense various physical parameters, forming the basis of capacitive sensors.

7.10 Oscillators: Generating Oscillating Signals

Capacitors, in combination with resistors, transistors, or operational amplifiers, are used in oscillator circuits to generate periodic waveforms (e.g., sine waves, square waves). The RC time constant often determines the oscillation frequency.

7.11 Producing Light: Light-Emitting Capacitors (Electroluminescent Panels)

Light-emitting capacitors (LECs) use phosphorescent dielectrics to generate light when an AC voltage is applied. If one plate is transparent, light is visible. Used in electroluminescent panels for backlighting displays and signage.

8. Hazards and Safety: Handling Capacitors Responsibly

Capacitors, especially high-voltage and high-energy types, can pose hazards if mishandled.

8.1 Energy Hazards: Electric Shock and Burns

Capacitors store energy, and even after power is removed, they can retain a dangerous charge.

Safety Precautions:

8.2 PCB Contamination (Historical)

Older oil-filled paper or plastic film capacitors may contain polychlorinated biphenyls (PCBs), a hazardous environmental pollutant. Capacitors containing PCBs were often labeled “Askarel” or other trade names. PCB-filled capacitors were used in older fluorescent lamp ballasts and other applications. Proper disposal of old capacitors is essential to prevent PCB contamination.

8.3 Catastrophic Failure Modes

Capacitors can fail catastrophically if overstressed by voltage, current, or reverse polarity.

Safety Measures:

9. See Also

10. Notes (Original Wikipedia Notes Section)

11. References (Original Wikipedia References Section)

12. Further Reading (Original Wikipedia Further Reading Section)

13. External Links (Original Wikipedia External Links Section)