Capacitor Glossary

Aluminum Electrolytic Capacitor

A polarized capacitor that uses an aluminum oxide film as the dielectric, formed on an etched aluminum foil anode. The cathode connection is made through a liquid or solid electrolyte. These capacitors offer high capacitance per unit volume, making them the standard choice for power supply filtering, DC bus energy storage, and coupling/decoupling in electronic circuits.

Available in radial, axial, snap-in, and screw terminal (computer grade) packages. Typical values range from 0.1µF to 100,000µF at voltages from 6.3V to 500V+. Limited lifespan due to electrolyte evaporation—typically 2,000 to 12,000 hours at rated temperature.

Axial Lead

A capacitor package style where the two wire leads exit from opposite ends of the cylindrical body, aligned along the central axis. Axial capacitors are typically mounted flat against a PCB or suspended between two terminal points.

Common in older through-hole designs and point-to-point wiring. Being replaced by radial and SMD packages in modern designs but still widely used in audio equipment, military/aerospace, and legacy equipment repairs.

Capacitance

The ability of a capacitor to store electrical charge, measured in farads (F). In practice, capacitors are rated in microfarads (µF), nanofarads (nF), or picofarads (pF). Capacitance is determined by the dielectric material, plate area, and distance between plates.

1 farad = 1,000,000 µF = 1,000,000,000 nF = 1,000,000,000,000 pF. Standard tolerances for electrolytic capacitors are ±20%, while film and ceramic types are available at ±5%, ±2%, or ±1%.

Ceramic Capacitor

A non-polarized capacitor that uses a ceramic material as the dielectric. Class I ceramics (C0G/NP0) offer stable capacitance across temperature and voltage. Class II ceramics (X7R, X5R) provide higher capacitance but with less stability. Class III ceramics (Y5V, Z5U) offer the highest capacitance with the least stability.

Widely used for bypassing, decoupling, and high-frequency filtering. MLCC (multi-layer ceramic capacitor) construction allows very small packages. Susceptible to DC bias effect—actual capacitance can drop 50-80% under applied voltage for Class II/III types.

Computer Grade Capacitor

A large-format aluminum electrolytic capacitor with screw terminals, designed for industrial applications requiring high capacitance, high ripple current handling, and extended life. Named for their original use in mainframe computer power supplies.

Typical specifications: 1,000µF to 100,000µF, 200V to 500V, 10,000+ hour life at 85°C or 105°C. Used in UPS systems, variable frequency drives (VFDs), welding equipment, and industrial power supplies. Also called large can capacitors or screw terminal capacitors.

DC Bias Effect

The reduction in effective capacitance that occurs when a DC voltage is applied across a capacitor. This effect is most pronounced in Class II and Class III ceramic capacitors (X7R, X5R, Y5V), where capacitance can drop by 50-80% at rated voltage.

A 10µF X5R capacitor rated at 25V may only provide 3-5µF when 25V is actually applied. This must be accounted for in circuit design. Film and electrolytic capacitors are minimally affected by DC bias.

DC Bus

The internal DC power rail in a variable frequency drive (VFD), UPS, or power converter. The DC bus is created by rectifying incoming AC power and is filtered by large electrolytic capacitors. These DC bus capacitors are the components that most commonly fail in industrial drives.

Typical DC bus voltages: 325V DC for 230V AC input systems, 650V DC for 460V AC input systems. DC bus capacitors must handle high ripple currents from both the input rectifier and output inverter switching.

Derating

The practice of operating a capacitor below its maximum rated voltage or temperature to extend its operational life. For aluminum electrolytic capacitors, operating at 80% of rated voltage can approximately double the expected lifespan.

Common derating guidelines: operate at 60-80% of rated voltage for extended life. For temperature, every 10°C reduction below rated temperature approximately doubles electrolytic capacitor life (Arrhenius equation).

Dielectric

The insulating material between a capacitor’s conductive plates that stores electrical energy in an electric field. The dielectric type determines a capacitor’s key characteristics including capacitance per volume, voltage rating, temperature stability, frequency response, and lifespan.

Common dielectrics: aluminum oxide (electrolytic), tantalum pentoxide (tantalum), polypropylene and polyester (film), ceramic compounds (ceramic), mica (silver mica). The dielectric constant (k) determines how much charge can be stored—higher k means more capacitance in less space.

Dielectric Absorption

The tendency of a capacitor to recover a portion of its voltage after being fully discharged. When a charged capacitor is briefly shorted and then opened, the voltage will partially return over seconds to minutes. Also called voltage recovery or battery effect.

Dielectric absorption is highest in electrolytic capacitors (10-15%) and lowest in polystyrene and polypropylene film capacitors (0.01-0.05%). Critical in sample-and-hold circuits, precision timing, and integrator applications. This is also why discharged high-voltage capacitors can become dangerous again if left open-circuit.

Dissipation Factor (DF)

A measure of energy loss in a capacitor, expressed as the ratio of the equivalent series resistance (ESR) to the capacitive reactance at a given frequency. Also called the loss tangent (tan δ). A lower dissipation factor indicates less energy wasted as heat.

Measured at standard frequencies: 120Hz for electrolytic, 1kHz for film and ceramic. Typical values: electrolytic 5-20%, film 0.01-0.1%, ceramic C0G <0.1%, ceramic X7R 1-2.5%. High DF causes self-heating under AC conditions.

Dry Capacitor

An electrolytic capacitor that has lost a significant portion of its liquid electrolyte through evaporation, resulting in increased ESR, reduced capacitance, and degraded performance. Drying is the primary failure mechanism for aluminum electrolytic capacitors.

Signs of a dry capacitor: elevated ESR on testing, reduced capacitance (below -20% tolerance), drive or power supply fault codes related to DC bus voltage, visible venting or residue around the capacitor’s rubber seal.

Dual Run Capacitor

A motor capacitor with three terminals that combines two capacitors in a single can—one section for the compressor motor and one for the condenser fan motor in an HVAC system. Rated as two capacitance values, such as 35+5 µF at 370V or 440V AC.

The three terminals are labeled C (Common), HERM (Compressor/Hermetic), and FAN. The larger capacitance value serves the compressor motor; the smaller value serves the fan. Always replace with the same dual values. Common residential AC sizes: 25+5, 30+5, 35+5, 40+5, 45+5 µF.

Electrolyte

The conductive liquid or gel inside an aluminum electrolytic capacitor that serves as the true cathode. The electrolyte makes contact with the dielectric oxide layer on the etched anode foil, enabling high capacitance in a compact package.

Electrolyte formulation determines key performance: low-impedance types use different solvents than long-life types. The electrolyte slowly evaporates through the rubber seal over the capacitor’s life, which is why electrolytic capacitors have a finite rated lifespan.

End of Life (EOL)

The point at which a capacitor manufacturer discontinues production of a specific part number, typically announced with a last-time buy notification. After EOL, the component must be sourced from remaining inventory or cross-referenced to an equivalent.

EOL is different from capacitor failure—it refers to the product’s manufacturing status, not its electrical condition. Equipment with EOL capacitors can continue operating until the capacitors actually fail, but proactive replacement planning is recommended.

ESL (Equivalent Series Inductance)

The parasitic inductance inherent in a capacitor’s construction, primarily from its leads and internal connections. ESL limits a capacitor’s effectiveness at high frequencies by creating a series resonant frequency above which the capacitor behaves as an inductor.

Self-resonant frequency = 1 / (2π√(LC)). Above this frequency, the capacitor’s impedance increases instead of decreasing. Typical ESL: radial electrolytic 10-50nH, MLCC ceramic 0.5-2nH, film 5-20nH. Critical for high-frequency decoupling and switching power supply design.

ESR (Equivalent Series Resistance)

The total effective resistance of a capacitor, including the resistance of the leads, electrodes, electrolyte (in electrolytic types), and dielectric losses. ESR causes a capacitor to dissipate energy as heat when AC current flows through it. Lower ESR is generally better, especially in power applications.

Measured in milliohms (mΩ) at a standard frequency (typically 100kHz for electrolytic, 1MHz for ceramic/film). ESR increases as electrolytic capacitors age, making it the primary diagnostic parameter for capacitor health. Low-ESR capacitors are specifically designed for switching power supply output filtering.

Film Capacitor

A capacitor that uses a thin plastic film as the dielectric. Common film materials include polypropylene (PP), polyester (PET/Mylar), polyphenylene sulfide (PPS), and polycarbonate. Film capacitors are non-polarized, self-healing, and offer excellent stability and long life.

Advantages over electrolytic: no polarity, longer lifespan (effectively unlimited), lower ESR, better frequency response, no electrolyte to dry out. Disadvantages: lower capacitance per volume, higher cost per microfarad. Used in motor run circuits, power factor correction, snubbers, audio crossovers, and high-reliability applications.

Impedance

The total opposition to AC current flow through a capacitor at a given frequency, combining the effects of capacitive reactance (Xc), ESR, and ESL. Impedance is frequency-dependent: it decreases with frequency due to capacitive reactance, reaches a minimum at the self-resonant frequency, then increases due to ESL.

Impedance (Z) = √(ESR² + (Xc - XL)²), where Xc = 1/(2πfC) and XL = 2πfL. Capacitor datasheets often include impedance vs. frequency plots. For bypass/decoupling applications, select a capacitor whose impedance is low at the frequencies of interest.

Leakage Current

The small DC current that flows through a capacitor when a DC voltage is applied. Ideally zero, leakage current represents an imperfection in the dielectric. For aluminum electrolytic capacitors, some leakage is normal and expected—specified in the datasheet as a maximum value.

Typical formula for electrolytic capacitor leakage: I = 0.01 × C × V (where C is in µF, V is rated voltage, and I is in µA) after a stabilization period. Elevated leakage current in a used capacitor indicates dielectric degradation. Capacitors stored for extended periods may need reforming to reduce leakage.

Life Rating

The guaranteed minimum operational life of an electrolytic capacitor at its maximum rated temperature and rated voltage, expressed in hours. Common ratings are 2,000, 5,000, 10,000, and 12,000 hours. Life increases exponentially as operating temperature decreases below the rated maximum.

Arrhenius equation for electrolytic life estimation: L = L₀ × 2^((T₀ - T)/10), where L₀ is rated life, T₀ is rated temperature, and T is actual operating temperature. Example: a 5,000-hour/105°C capacitor operating at 65°C has an estimated life of 5,000 × 2⁴ = 80,000 hours (≈9 years continuous).

MLCC (Multi-Layer Ceramic Capacitor)

A ceramic capacitor constructed from many alternating layers of ceramic dielectric and metal electrodes, fired into a monolithic block. The multi-layer structure provides high capacitance in a very small surface-mount package.

Available from 0201 (0.6mm × 0.3mm) to 2220 (5.7mm × 5.0mm) package sizes. Values up to 100µF are possible in the largest sizes. Subject to cracking from board flex, microphonic noise (piezoelectric effect), and DC bias derating.

Motor Run Capacitor

A capacitor designed to remain in circuit continuously during motor operation, creating a phase shift in the auxiliary winding to improve efficiency and power factor. Motor run capacitors are oil-filled or dry film types rated for continuous AC duty.

Typical ratings: 1.5µF to 60µF at 370V AC or 440V AC. Must be a film/oil type—never use an electrolytic capacitor as a motor run capacitor. Common in HVAC compressors, fan motors, pumps, and single-phase induction motors.

Motor Start Capacitor

A capacitor used only during motor startup (1-3 seconds) to provide additional starting torque. Motor start capacitors are electrolytic types with much higher capacitance than run capacitors, connected in circuit only during the starting phase by a centrifugal switch or start relay.

Typical ratings: 70µF to 800µF at 110V, 165V, 220V, or 330V AC. Not designed for continuous duty—will overheat and fail if left in circuit. Failure of the start relay or centrifugal switch can cause the start capacitor to explode.

NOS (New Old Stock)

Genuine, unused components that were manufactured some time ago and have been stored since. NOS capacitors are original-specification parts that were never installed but may have been in storage for years or decades.

Important considerations for NOS electrolytic capacitors: verify date codes, inspect for physical damage, check that storage conditions were appropriate, and reform before use if stored more than 2-3 years. Film, ceramic, and mica capacitors store indefinitely with minimal degradation.

Polarity

The requirement that a polarized capacitor (electrolytic, tantalum) be connected with the correct orientation—positive terminal to higher voltage. Reversing polarity causes the dielectric oxide layer to break down, resulting in excessive current flow, heating, gas generation, and potentially violent failure.

Marked on the component: electrolytic capacitors mark the negative (-) terminal with a stripe; tantalum capacitors mark the positive (+) terminal. Non-polarized capacitors (film, ceramic, mica) have no polarity requirement.

Power Factor Correction (PFC)

The use of capacitors to offset the inductive reactance of loads (motors, transformers) and bring the power factor closer to unity (1.0). PFC capacitors reduce reactive power drawn from the utility, lowering electricity costs and freeing up transformer capacity.

PFC capacitors are typically metallized polypropylene film types rated for continuous AC duty at line voltage (240V-690V AC). They must be designed for the specific harmonic environment and may require detuning reactors if significant harmonic distortion is present.

Radial Lead

A capacitor package style where both wire leads exit from the same end (bottom) of the cylindrical or rectangular body. Radial capacitors are designed to stand upright on a PCB, saving board space compared to axial packages.

The most common through-hole package for modern electrolytic capacitors. Lead spacing (pitch) is standardized: 2mm, 2.5mm, 3.5mm, 5mm, 7.5mm, and 10mm are common. Radial packages range from tiny (4mm diameter) to large (35mm+ diameter).

Rated Voltage (WVDC)

The maximum DC voltage that can be continuously applied to a capacitor at or below its rated temperature. Abbreviated as WVDC (Working Voltage DC) for DC-rated capacitors or WVAC for AC-rated types. Exceeding the rated voltage accelerates dielectric degradation and risks catastrophic failure.

For aluminum electrolytic capacitors, derating to 80% of rated voltage is recommended for extended life. For ceramic capacitors, the effective capacitance decreases under applied DC voltage (DC bias effect). AC-rated capacitors (motor types) must use the AC voltage rating—do not convert from DC ratings.

Reforming

The process of gradually applying voltage to an aluminum electrolytic capacitor that has been stored without power for an extended period (typically 2+ years). Reforming rebuilds the aluminum oxide dielectric layer, which partially dissolves during unpowered storage.

Procedure: apply 10% of rated voltage through a current-limiting resistor, hold until leakage current stabilizes, then increase in 10-25% increments. Monitor leakage current—it should decrease at each voltage step. Full reforming typically takes 1-4 hours. Skip this step for new capacitors from recent production.

Ripple Current

The AC component of current flowing through a capacitor in a circuit with both DC and AC present. In power supply and drive applications, ripple current generates heat inside the capacitor through ESR losses. Exceeding the rated ripple current causes overheating and accelerated degradation.

Rated in amps RMS at a specified frequency (typically 100/120Hz for power supply applications). Higher-frequency ripple current may require a correction factor from the datasheet. Critical parameter for DC bus capacitors in VFDs, UPS systems, and switching power supplies.

RoHS (Restriction of Hazardous Substances)

An EU directive restricting the use of certain hazardous materials (lead, mercury, cadmium, hexavalent chromium, PBB, PBDE) in electronic components including capacitors. RoHS-compliant capacitors use lead-free terminals and solder.

Most modern capacitors are RoHS compliant. Some legacy and military-spec capacitors predate RoHS and use leaded solder. When replacing in RoHS-required applications, verify the replacement is RoHS certified.

Self-Healing

A property of metallized film capacitors where a localized dielectric breakdown vaporizes the thin metal electrode around the fault, isolating it and allowing the capacitor to continue functioning. This makes film capacitors inherently more reliable than other types for overvoltage events.

Only metallized film capacitors self-heal—foil-and-film types do not. Each self-healing event slightly reduces total capacitance. Motor run capacitors and power factor correction capacitors rely on this property for safe operation under transient overvoltage conditions.

Silver Mica Capacitor

A precision capacitor using natural mica sheets as the dielectric with silver electrodes deposited directly on the mica surface. Silver mica capacitors offer extremely tight tolerances (±1%), low temperature coefficient, high Q factor, and excellent long-term stability.

Typical values: 1pF to 10,000pF. Used in RF circuits, oscillators, filters, and precision timing. Becoming increasingly difficult to source as fewer manufacturers produce them. Among the most dimensionally stable capacitor types available.

Snap-In Mount

A mounting style for medium to large electrolytic capacitors where short, sturdy pins snap into holes on a PCB, providing both electrical connection and mechanical support. Snap-in capacitors bridge the gap between small radial types and large screw terminal (computer grade) types.

Common in power supplies, VFDs, and UPS systems for capacitors in the 100µF to 10,000µF range. Pin configurations vary by manufacturer—verify pin diameter, spacing, and pattern when cross-referencing replacements.

Snubber Capacitor

A capacitor used in a snubber circuit to limit voltage transients and rate of voltage change (dV/dt) across power semiconductors such as IGBTs, MOSFETs, thyristors, and diodes. Snubber capacitors protect switching devices from voltage spikes that could cause failure.

Typically film capacitors (polypropylene) rated for high peak current and high dV/dt. Values usually 0.01µF to 1µF at 600V to 2000V. Found in VFDs, inverters, motor drives, and power converters. Must have very low ESR and ESL for effective snubbing.

Surge Voltage

The maximum voltage that can be applied to a capacitor for brief periods without permanent damage, typically 10-15% above the rated (WVDC) voltage. Surge voltage specifications include a maximum duration and number of occurrences.

For aluminum electrolytic capacitors, typical surge rating is 1.15× rated voltage for up to 30 seconds, no more than 5 times in the capacitor’s life. Repeatedly operating at surge voltage dramatically shortens life. Not a substitute for proper voltage derating.

Tantalum Capacitor

A polarized capacitor using tantalum metal as the anode, with tantalum pentoxide as the dielectric. Available in solid (manganese dioxide or polymer cathode) and wet (liquid electrolyte) types. Tantalum capacitors offer higher capacitance per volume than aluminum electrolytic in small packages.

Advantages: stable capacitance, low ESR (polymer types), long shelf life, good temperature range. Disadvantages: limited voltage range (typically ≤50V), failure mode can be short circuit (fire risk without current limiting), sensitive to voltage spikes, higher cost than aluminum electrolytic.

Temperature Coefficient

The rate at which a capacitor’s value changes with temperature, expressed in ppm/°C (parts per million per degree Celsius) or as a percentage over a temperature range. Capacitors with low temperature coefficients maintain more stable values across operating conditions.

C0G/NP0 ceramic: ±30 ppm/°C (best stability). Polypropylene film: -200 ppm/°C. X7R ceramic: ±15% over -55°C to 125°C. Aluminum electrolytic: capacitance varies +20% to -40% over temperature range. Silver mica: ±50 ppm/°C.

Temperature Rating

The maximum ambient temperature at which a capacitor can operate continuously at rated voltage while meeting its life specification. For aluminum electrolytic capacitors, the two standard ratings are 85°C and 105°C.

The 20°C difference between 85°C and 105°C rated capacitors translates to approximately 4× the life at the same operating temperature (per the Arrhenius equation: 2^(20/10) = 4). Always replace with the same or higher temperature rating.

THB (Temperature, Humidity, Bias)

An accelerated reliability test for capacitors where the component is subjected to high temperature, high humidity, and applied voltage simultaneously. THB testing evaluates resistance to moisture ingress and electrochemical migration under harsh conditions.

Standard conditions: 85°C, 85% relative humidity, rated voltage applied. Duration: typically 1,000 hours. Critical for automotive (AEC-Q200), industrial, and outdoor applications where moisture exposure is expected.

Through-Hole

A component mounting technology where the capacitor’s leads are inserted through holes drilled in a PCB and soldered on the opposite side. Through-hole components are larger than surface-mount equivalents but offer stronger mechanical connections.

Includes radial, axial, snap-in, and screw terminal package styles. Still widely used for large capacitors, high-power applications, and equipment designed for serviceability. Being replaced by SMD in high-volume consumer electronics manufacturing.

Tolerance

The allowable deviation from the nominal capacitance value, expressed as a percentage. Tighter tolerances are available at higher cost and are required for precision applications such as filters, timing circuits, and resonant circuits.

Standard tolerance codes: M = ±20% (electrolytic), K = ±10% (film, ceramic), J = ±5% (precision film, C0G ceramic), G = ±2%, F = ±1%, C = ±0.25%, B = ±0.1%. Most electrolytic capacitors are ±20%; most motor capacitors are ±5% or ±10%.

Vacuum Impregnation

A manufacturing process where capacitor windings are impregnated with oil or resin under vacuum to fill air voids, improve dielectric strength, and enhance heat dissipation. Used in high-quality motor capacitors, power film capacitors, and paper/oil types.

Oil-filled motor capacitors use vacuum impregnation to ensure complete saturation of the film winding. This eliminates air pockets that could cause partial discharge (corona) and premature failure under AC voltage stress.

Varistor (MOV)

A voltage-dependent resistor (not technically a capacitor) often used alongside capacitors in power circuits for transient voltage suppression. Metal Oxide Varistors (MOVs) clamp voltage spikes to protect sensitive components including capacitors.

Mentioned here because varistors are frequently confused with capacitors in circuit boards, and failed varistors can cause symptoms similar to capacitor failure (overvoltage damage to other components). Varistors degrade after absorbing transients and should be inspected during capacitor replacement.

VFD (Variable Frequency Drive)

An electronic device that controls the speed and torque of an AC motor by varying the frequency and voltage of the power supplied to the motor. VFDs contain large DC bus capacitors that are the most failure-prone component in the drive.

Also called: adjustable frequency drive (AFD), adjustable speed drive (ASD), variable speed drive (VSD), AC drive, or inverter. Major manufacturers include Allen-Bradley (Rockwell), Siemens, ABB, Danfoss, and Yaskawa. DC bus capacitor replacement is the most common VFD repair.

Voltage Coefficient

The change in capacitance as a function of applied DC voltage. Most significant in Class II and Class III ceramic capacitors (X7R, X5R, Y5V) where capacitance can decrease by 50-80% at rated voltage. Related to but distinct from the DC bias effect.

Voltage Recovery

See Dielectric Absorption. The phenomenon where a discharged capacitor regains a portion of its charge over time due to slow polarization relaxation in the dielectric material.

Safety implication: high-voltage capacitors (in drives, UPS, power supplies) that appear to be discharged can develop dangerous voltages if left open-circuit. Always verify zero voltage with a meter before handling, even after discharge.

Wet Electrolytic

An electrolytic capacitor that uses a liquid electrolyte as the cathode connection, as opposed to solid polymer or manganese dioxide types. Most aluminum electrolytic capacitors are wet electrolytic. The liquid electrolyte enables higher capacitance but limits operational life.

The electrolyte slowly evaporates through the end seal, which is the primary aging mechanism. Wet electrolytic capacitors should not be stored in high-temperature environments. Extended storage without applied voltage can allow the oxide dielectric to deteriorate, requiring reforming before use.

X and Y Safety Capacitors

Safety-rated capacitors designed for use in AC mains circuits where failure could create a shock or fire hazard. X-class capacitors connect line-to-line (across the mains). Y-class capacitors connect line-to-ground. Both are designed to fail safely (open circuit) rather than short circuit.

X1: rated for ≤2.5kV pulse, X2: ≤1.2kV pulse. Y1: rated for peak voltage ≤8kV (double insulation), Y2: rated for peak voltage ≤5kV (basic insulation). Must be replaced with equivalent safety-rated types—never substitute a standard film capacitor for a safety-rated X or Y capacitor.