A capacitor is a passive electronic component that stores electrical energy in an electric field. Capacitors are widely used in electronic circuits for blocking direct current, bypassing signals of particular frequencies, storing energy, smoothing power supply outputs, coupling signals, filtering and many other applications.
This comprehensive guide provides an in-depth overview of capacitors including:
- What is a capacitor and how does it work?
- Capacitor theory and key parameters
- Different types of capacitors and their properties
- Reading capacitor values and markings
- Equivalent series resistance (ESR)
- Leakage current specifications
- Capacitance tolerance and stability
- Temperature characteristics
- Capacitor reliability factors
- Dielectric materials used in capacitors
- Applications of the various capacitor types
- Capacitor testing methods
- Innovations in capacitor technology
By understanding capacitor fundamentals, functions and characteristics, engineers can choose the right capacitor type for their specific application and circuit needs.
What is a Capacitor?
A capacitor is a two-terminal passive electronic component that stores energy in the form of an electrostatic field between a pair of conductive plates separated by an insulating dielectric material. When voltage is applied across the plates, opposite electric charges accumulate on each plate to create an electric field through the dielectric.
The basic operating principle involves:
- Positive charge collected on one plate when voltage applied
- Negative charge collected on the other plate
- The charges are separated by the dielectric that prevents flow
- Charges create an electric field proportional to voltage
- Field stores energy that can be discharged back
Some key properties that define a capacitor:
- Capable of storing electrical potential energy
- Constructed of conductive surfaces surrounding dielectric
- Allows passage of AC signals while blocking DC
- Charges almost instantly to applied voltage
Capacitors are fundamental components that enable modern electronics. Next, we look at how the various parameters characterize capacitor behavior and performance.
Capacitor Theory and Parameters
The most important capacitor parameters to understand are:
Capacitance
The capacitance C determines how much charge Q can be stored per unit voltage V applied and is measured in Farads:
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C = Q/V
Typical capacitors have capacitance ranging from picofarads (pF) to microfarads (μF) or more. Higher capacitance allows more charge storage.
Voltage Rating
Maximum DC voltage or peak AC voltage that can be applied without damaging the capacitor. Rating depends on dielectric strength.
Tolerance
Specifies maximum allowable deviation from stated capacitance, typically ±1%, ±5% or ±10%. Tighter tolerance increases cost.
Temperature Coefficient
Indicates the degree to which capacitance changes with temperature, expressed in parts-per-million per degree Celsius (ppm/°C).
Dielectric Material
The insulating material separating the capacitor plates. Determines key characteristics like voltage rating, frequency response, temperature dependence and energy density.
Equivalent Series Resistance (ESR)
The resistive component of the capacitor that causes power loss and heating. Lower ESR extends capacitor life.
Dissipation Factor (DF)
Ratio of ESR to capacitive reactance, indicates inefficiency. Lower values preferred.
Leakage Current
Unwanted current that leaks through the dielectric due to imperfect insulation. Should be minimal for reliable operation.
Proper selection requires matching these specifications to circuit application requirements as we will see later. Next, let’s look attypes of capacitors.
Types of Capacitors
There are many types of capacitors differentiated by their dielectric material. Key capacitor classes include:
Ceramic Capacitors
One of the most widely used. Made from sintered ceramic compounds like titanium dioxide with noble metal electrodes. Values range from picofarads to microfarads.
Key features:
- Inexpensive and compact
- Low inductance makes them suitable for high frequency circuits
- Capacitance values not highly stable with temperature/voltage
- ceramic dielectric provides high insulation but lower capacitance density than other materials
Applications: bypassing, coupling, filtering, timing. Preferred for high frequency and SMD use.
Ceramic capacitors. Image credit: Arrow Electronics
Electrolytic Capacitors
Uses anodic aluminum or tantalum oxide dielectric with a liquid electrolyte or conductive polymer. High volumetric efficiency provides large capacitance values.
Features:
- Available up to 1Farad capacitance
- Compact size for given capacity
- High leakage current compared to other types
- Polarized with defined positive and negative terminals
Applications: power supply filtering, buffering and energy storage. Mainly used where large capacitance is required.
Electrolytic capacitors. Image credit: Arrow Electronics
Tantalum Capacitors
Similar construction to electrolytic but uses tantalum pentoxide dielectric layer on porous anode slug. More stable, higher capacitance density but higher cost.
Features:
- Values up to 600uF volt in compact sizes
- More stable and reliable than aluminum electrolytic
- Lower ESR, tolerates more ripple current
- Still polarized, though non-polar tantalum exists
Applications: buffering, bypassing, filtering and coupling in small designs. Preferred over electrolytic if stability needed.
Plastic Film Capacitors
Uses insulating plastic film like polyester, polypropylene or polystyrene sandwiched between metal foil plates. Not polarized.
Features:
- Extremely low leakage current
- Stable over time and temperature
- Lower inductance than electrolytic
- Withstand high current pulses
Applications: timing, coupling, active crossovers. Valued in audio and RF circuits.
Mica Capacitors
Employs thin sheets of mica as dielectric with metal foil or evaporated film electrodes. Operates over wide temperature range.
Features:
- Low loss, excellent stability
- Tolerates high temperatures up to 200°C
- Large size for given capacitance
- High voltage rating
Applications: resonant circuits, transmitters, ultrasonic equipment. Suited for high power, frequency stability and reliability.
Silver Mica Capacitors
Similar construction to mica capacitors but with higher capacitance density. Silver electrode deposition gives capacitance up to 1000pF.
Features:
- Extremely precise and stable capacitance
- Low dielectric losses
- Withstands high voltages up to 25kV
- Large temperature coefficient of capacitance
Applications: radio frequency tuning circuits requiring precision and stability.
Supercapacitors
Uses double-layer or hybrid capacitance to achieve extremely high capacitances. Can store much more energy than other capacitor types.
Features:
- Capacitance from 1F to over 10,000F
- Very high charge/discharge rates
- Lower voltage rating usually under 3V
- High energy density
Applications: energy storage/harvesting, burst power delivery, short term backup supplies.
This overview summarizes the vast range of capacitor technologies available to designers along with their key characteristics and typical usage scenarios.
How to Read Capacitor Values
Capacitor values are commonly expressed in pico-farads (pF) to micro-farads (μF) units along with voltage rating and tolerance. Tiny surface mount types may use coded markings while through-hole capacitors usually have labeled values. Some key aspects:
Unit Prefixes
- pF (picofarad)
- nF (nanofarad)
- μF (microfarad)
- Common values range from pF to thousands of μF
Larger values may use ‘mF’ for millifarads or kilo prefixes.
Voltage Rating
- Maximum working DC or peak AC volts
- Typical ratings from 4V to over 1kV
Tolerance
- Allowed deviation from nominal capacitance
- Expressed as percentage or ppm
- 1%, 2%, 5%, 10%, 20% are common
Polarity Marking
- Band indicates polarity on polarized types
- Helps identify positive and negative terminals
Package Size
- Code gives case dimensions for SMD capacitors
- 0201, 0402, 0603 are common surface mount sizes
Date Code
- Often printed with lot and plant codes
- Identifies manufacturing date and batch
Understanding capacitor labeling helps confirm correct component values are used in circuit design and production.
Equivalent Series Resistance (ESR)
The equivalent series resistance (ESR) is the resistive component exhibited by non-ideal capacitors under AC excitation. It combines the dielectric loss with the ohmic contact resistance between terminals and electrodes. Key considerations for ESR:
- Creates unwanted power heating within capacitor
- Causes voltage drop and power loss at higher frequencies
- Limits usable ripple current in output filters
- Degrades capacitor performance over time
- Specified on datasheets, lower value preferred
- Affects maximum usable frequency range
- Varies over temperature range
- Electrolytic capacitors higher ESR than film or ceramic
ESR directly impacts capacitor performance in power electronics applications. Proper derating for ESR extends capacitor lifetime and improves circuit efficiency.
Leakage Current Specification
All capacitors exhibit some small leakage current that flows through insulation resistance when voltage is applied. Key factors around leakage current:
- Caused by imperfect dielectric insulation properties
- Specified on datasheets as leakage current or insulation resistance
- Higher temperature increases leakage
- Will drain stored charge over time
- Leakage resistance must be sized appropriately by design
- Electrolytic capacitors have higher leakage than other types
- Tantalum wet electrolytics are lowest leakage electrolytic
- Look for low leakage ceramic and film caps where needed
Low leakage is critical for long-duration energy storage and high impedance circuits.
Capacitance Tolerance and Stability
Tolerance indicates the maximum deviation of actual capacitance from the labelled nominal value. Tighter tolerance capacitors have more stable and precise capacitance but increase cost.
Key considerations around tolerance and stability:
- Laser trimmed film capacitors attain tolerances down to ±1%
- Cost increases dramatically below 5% tolerance
- Tolerance can change over temperature and age
- Matching critical for ratioed capacitors in precision circuits
- Stability important for oscillators, timers and filters
Applications requiring timing accuracy or frequency stability rely on highly stable capacitance.
Temperature Characteristics
Nearly all capacitors exhibit some change in capacitance over operating temperature range. Factors related to temperature behavior:
- Temperature coefficient specified in parts-per-million (ppm) per °C
- May increase or decrease with rising temperatures
- Stability over temperature range important for many circuits
- Ceramic class 2, film, and mica most stable with temperature
- Electrolytic capacitance drops significantly at lower temperatures
- C0G/NP0 ceramics offer near zero temperature coefficient
- Lower temperature dependence preferred for precision
Datasheets provide capacitance vs. temperature curves showing the capacitance change over the full operating range, which must be accounted for in designs.
Capacitor Reliability Considerations
Capacitor reliability strongly influences operational lifetime. The main failure factors include:
Voltage Breakdown
- Dielectric break down when voltage rating exceeded. Fails short.
Wear-out
- Gradual performance degradation over operational life
Destructive Internal Heating
- Heat damage where ripple current rating is exceeded
Charge Leakage
- Insufficient leakage resistance drains stored charge
Mechanical Damage
- Damaged casing or connections cause open circuit
Parametric Drift
- Capacitance value shifts over time and use
Dendrite Shorting
- Conductive filaments grow internally leading to shorts
Infant Mortality
- Early failures due to manufacturing defects
Environmental factors like humidity, temperature cycles, shock, and vibration exacerbate these failure mechanisms.
Dielectric Materials Overview
Different dielectric materials are used in capacitor construction, impacting performance:
Polypropylene (PP)
- Almost universal dielectric for film caps
- High insulation resistance and self-healing properties
- Low dielectric losses suit critical filter and timing circuits
Polyester (PET)
- Low cost film frequently used in circuits not needing tight tolerance
- More susceptible to humidity than PP
Polystyrene (PS)
- Stable, precise values for exacting applications
- Large tolerances above 1000pF values
Polyphenylene Sulfide (PPS)
- Can operate up to 200°C allowing replacement of tantalum electrolytic caps
- Interest for automotive and down-hole environments
Ceramic Compounds
- Titanium dioxide with noble metal electrodes common
- Tight tolerance, temperature stable “C0G/NP0” dielectric for precision timing applications
Mica
- Potassium chloride mica split into thin sheets
- Natural mineral with high dielectric strength
Tantalum Oxide
- Thin insulating layer on sintered porous tantalum slugs
- Used in tantalum electrolytic capacitors
Aluminum Oxide
- Anodic aluminum oxide layer with solid or liquid electrolyte
- Dielectric in aluminum electrolytic capacitors
Multiple materials technology options enable tradeoffs across cost, performance and reliability metrics.
Capacitor Applications and Usage
The different capacitor types fulfill distinct circuit applications based on their characteristics:
Signal Coupling and Decoupling
Reactively couple or decouple AC signals of particular frequencies. Ceramic and film types work well with negligible effect on desired frequency bands.
Power Supply Bypassing
Shunt high frequency noise on DC supply lines to ground. Ceramic capacitors with low ESR and inductance ideal for bypassing up to several hundred MHz.
Power Supply Filtering
Smooth pulsating DC from rectified AC. Electrolytic capacitors provide large capacitance for output filtering up to 100 Hz.
Energy Storage
Store and discharge energy as needed. Electrochemical supercapacitors optimal for capturing regenerative energy.
Tuned Circuits
Resonance with inductors in radio or oscillator tank circuits. Silver mica and COG/NPO ceramic highly stable for tuned frequency generation.
Snubbers
Suppress voltage transients across inductive loads. Film capacitors withstand pulse discharging.
Timing Circuits
Set precise time constants in oscillator and timer circuits. Tight tolerance film and ceramic suited for timing accuracy.
Voltage Multiplication
Voltage multiplier circuits to generate higher DC voltages. High voltage ceramic, film or mica capacitors.
AC Power Factor Correction
Correction of phase lag between voltage and current. Higher value film capacitors.
Capacitor selection comes down to matching dielectric material properties and specifications to application requirements.
Capacitor Testing and Measurement
To confirm capacitors meet expected characteristics, various lab testing procedures apply:
Capacitance Testing
- LCR meters measure capacitance and equivalent series resistance
- Verify capacitance value and tolerance match ratings
Voltage Proof Testing
- Apply overvoltage stress between the terminals
- Confirms dielectric withstands required voltage
Insulation Resistance Testing
- Measure leakage resistance or current
- High resistance meter or DC source with current measurement
Temperature Characterization
- Measure capacitance at temperature extremes
- Altitude chambers used to simulate operating environments
Lifetime Testing
- Apply accelerated test conditions including voltage, ripple current, temperature
- Monitor parameters over time to assess wear-out mechanisms
Destructive Physical Analysis
- Dissect failed capacitors to determine internal failure modes
- Scanning electron microscopy used to image defects
Careful testing validates expected capacitor functionality and lifetime before committing to production use.
Recent Innovations in Capacitors
Capacitor technology continues advancing with new materials and designs:
Lithium-Ion Capacitors – Offer energy density approaching batteries with charge/recharge cycles and power delivery of capacitors. Useful for regenerative energy capture.
Nanolaminate Dielectrics – Thin film deposition creates dielectric layers with enhanced properties. Allows higher voltage ratings.
High Aspect Ratio Tantalum – Etched porous structures increase surface area and capacitance density while reducing ESR.
Niobium Oxide Dielectrics – Used in next-gen high CV density ceramic capacitors to replace expensive palladium barrier layers.
Flexible and Wearable Capacitors – Novel materials enable thin, skin-like capacitive laminates for conformal energy storage applications.
Integrated Capacitive Layers – Back-end integrated capacitance layers directly in multi-layer IC packages help supply on-die power integrity.
Quantum Metamaterial Capacitors – Exotic metamaterials exhibiting negative permittivity enable resonant effects leading to order-of-magnitude higher capacitances.
These and other innovations will unlock new applications for capacitors ranging from electric vehicles to mobile devices to high energy physics.
Conclusion
Capacitors are one of the most ubiquitous passive components found in nearly every electronic system and available in a broad range of types, sizes and specifications tailored to the needs of diverse modern applications.
