When makers and students ask what does capacitor store, the strict physics answer is electrostatic energy in an electric field. Unlike a battery, which stores energy chemically and releases it through ion transfer, a capacitor stores separated electrical charge ($Q = CV$) across a dielectric insulator. It does not 'store current,' nor does it generate electrons. It simply hoards potential energy ($E = \frac{1}{2}CV^2$) and releases it the moment the circuit demands a change in voltage.

Understanding this distinction between chemical storage and electrostatic storage is the difference between a reliable power supply and a workbench covered in electrolyte fluid. Below, we break down the physics, dissect a real-world bench failure, and build a practical framework for selecting, reading, and substituting capacitors.

The Physics of the Store: Charge vs. Energy

To understand component selection, you must separate capacitance from energy storage. Capacitance ($C$) is merely the physical capacity to hold separated charge. Energy ($E$) is the actual work the capacitor can perform, which scales with the square of the voltage.

Bench Example: A 10,000µF capacitor charged to 16V stores 1.28 Joules of energy. If you charge that exact same physical capacitor to 50V (assuming its voltage rating allows it), it stores 12.5 Joules—nearly ten times the energy, despite the capacitance value remaining identical.

This squared relationship is why high-voltage snubber circuits and camera flashes use capacitors: they need to dump massive amounts of energy in microseconds. However, because the energy is stored in an electric field rather than a chemical reaction, the internal resistance (ESR) and the physical construction of the dielectric dictate how fast that energy can be extracted without destroying the component.

Bench Scenario: The 10,000µF Filter Capacitor Disaster

Theory only matters when it keeps your PCB from catching fire. Here is a classic scenario where misunderstanding how a capacitor stores and releases charge led to a catastrophic failure.

The Setup

A hobbyist was repairing a 12V-to-5V buck converter (based on the LM2596) that powered a 3A servo array. The original output filter capacitor—a 1000µF 16V low-ESR aluminum electrolytic—had vented. The builder replaced it with a standard, off-the-shelf 1000µF 16V general-purpose electrolytic capacitor, assuming that since the µF and voltage ratings matched, the circuit would work perfectly.

The Numbers

The LM2596 switches at 150kHz. At a 3A load, the inductor ripple current pushed 1.5A RMS through the output capacitor. The original low-ESR cap had an Equivalent Series Resistance of 0.05Ω. The replacement general-purpose cap had an ESR of 0.25Ω and a maximum ripple current rating of just 0.5A.

The Outcome

Within four minutes of operation, the replacement capacitor hissed, bulged, and popped its pressure vent, spraying hot electrolyte across the bench and taking the LM2596 switching node out with it.

What Went Wrong

The builder focused only on what the capacitor stores (bulk charge) and ignored how fast it had to absorb and release that charge. The power dissipated as heat inside the capacitor is calculated by $P = I_{rms}^2 \times ESR$. For the replacement cap, $1.5^2 \times 0.25 = 0.56W$. A standard 10x10mm radial capacitor can only dissipate about 0.1W before its internal temperature exceeds the 105°C rating. The electrolyte boiled, pressure spiked, and the mechanical vent failed. Capacitor theory dictates that high-frequency filtering requires low ESR, not just high capacitance.

Choosing the Right Dielectric: Type Comparison Matrix

Knowing 'what does capacitor store' is only step one. Step two is choosing the right dielectric material to store it. Here is how the major types compare when it comes time to select a part for your BOM.

Capacitor Type Construction / Dielectric Typical Tolerance Tempco (Temp Stability) Typical Use Case
MLCC (Ceramic) Multi-layer ceramic (Class II X7R/X5R) ±10% to ±20% Moderate (Voltage & Temp dependent) High-freq decoupling, bypass, compact filtering
Aluminum Electrolytic Etched aluminum foil / liquid or solid electrolyte ±20% High (Degrades at temp extremes) Bulk power filtering, low-freq smoothing, audio coupling
Tantalum (MnO2/Polymer) Sintered tantalum powder / manganese dioxide ±10% to ±20% Very Stable Space-constrained filtering, medical, aerospace
Film (Polypropylene) Metallized plastic film ±1% to ±5% Extremely Low (Highly linear) AC line snubbers, high-voltage, audiophile crossovers
Warning: Tantalum Polarity. Unlike aluminum electrolytics which merely vent when reverse-biased, standard MnO2 tantalum capacitors fail via exothermic thermal runaway. If reverse-biased or subjected to voltage spikes exceeding their rating, they will ignite. Always use polymer tantalums or add series resistance if surge currents are expected.

Decoding the Shell: How to Read Capacitor Markings

Capacitor markings are notoriously inconsistent across form factors. Here is how to read the physical part sitting in your component bin.

Ceramic MLCC (3-Digit Code)

Small through-hole and SMD ceramics use a three-digit EIA code. The first two digits are the significant figures, and the third is the multiplier (number of zeros) in picofarads (pF).

  • 104: 10 followed by 4 zeros = 100,000 pF = 100 nF = 0.1 µF.
  • 473: 47 followed by 3 zeros = 47,000 pF = 47 nF.
  • 221: 22 followed by 1 zero = 220 pF.

A trailing letter indicates tolerance: J = ±5%, K = ±10%, M = ±20%, Z = +80%/-20%.

Aluminum Electrolytic

Radial and axial electrolytics print values directly. You will see 1000µF, 25V, and a temperature rating (usually 85°C or 105°C). The critical marking is the polarity stripe. On modern Japanese/European caps, the stripe denotes the negative lead. On some vintage or Soviet-era caps, the stripe may denote the positive lead—always verify with a datasheet or multimeter diode mode if the part is unbranded.

Failure Modes and Visual Autopsies

Capacitors rarely fail without leaving evidence. Learning to read these visual symptoms saves hours of multimeter probing.

  1. Domed or Popped Vent (Electrolytic): The classic sign of over-voltage, reverse polarity, or excessive ripple current (as seen in our LM2596 scenario). The internal electrolyte boiled, generating gas that forced the mechanical cross-vent on the can top to rupture. Fix: Check for AC ripple on DC lines and verify ESR ratings.
  2. Cracked Terminals / Flex Cracks (MLCC): Surface-mount ceramics are brittle. If the PCB flexes during assembly or operation, the ceramic body cracks, often internally. This creates a dead short or a massive drop in capacitance. Visually, you might see a tiny hairline fracture near the solder joint, or the part may look perfectly fine while failing in-circuit. Fix: Use flexible-termination (soft-termination) MLCCs in high-stress board areas.
  3. Charred or Melted Casing (Tantalum/Film): Indicates a massive over-current event or reverse polarity. Tantalum failures often leave a distinct burnt smell and a blackened crater on the PCB. Film capacitors may melt their outer epoxy dip without shorting, simply opening the circuit. Fix: Verify inrush current limiting and absolute polarity before applying power.
  4. Shrunken Sleeve / Dried Electrolyte: If an aluminum capacitor's plastic sleeve is shrunk and brittle, or the rubber bottom plug is pushed out, the electrolyte has vaporized over years of heat exposure. The capacitance will measure correctly on a cheap meter, but the ESR will be sky-high. Fix: Replace all caps near hot components (heatsinks, power resistors) proactively.

The Substitution Playbook: Swapping Parts Safely

When your exact BOM part is out of stock, you must substitute safely. Follow this decision matrix to avoid destroying your circuit. For deeper engineering context on stability and component selection, refer to Texas Instruments application notes on LDO stability and Cornell Dubilier's technical papers.

1. Voltage Rating: Always Derate

Never substitute a lower voltage rating. However, simply matching the voltage is often not enough. For ceramic capacitors (especially X5R and X7R), DC bias effect causes the actual capacitance to drop significantly when voltage is applied. A 10µF 16V X7R ceramic might only provide 3µF of actual capacitance at 12V DC. Rule: For ceramics, select a voltage rating at least 2x to 3x your maximum operating voltage.

2. Capacitance Value: Context Matters

In power filtering (buck converters, linear regulators), substituting a value +20% to +50% higher is usually beneficial, as it lowers output ripple. In timing or oscillator circuits (555 timers, RC filters, crystal load caps), you must match the value exactly, or your frequency and phase margins will shift.

3. ESR Substitution: The LDO Trap

For switching regulators (buck/boost), substituting a lower ESR capacitor is almost always safe and improves efficiency. However, for older Low-Dropout Regulators (LDOs) like the LM1117, lower ESR can cause oscillation. These LDOs rely on the ESR zero of an aluminum electrolytic capacitor to maintain control loop stability. If you substitute a 0.01Ω ESR ceramic for a 0.5Ω ESR electrolytic, the LDO will ring and output high-frequency noise. Fix: If using ceramics on older LDOs, add a 1Ω series resistor to artificially inject ESR.

Safety Reminder: Before desoldering or probing any capacitor in a power supply, de-energize the circuit and discharge the capacitor using a high-wattage bleed resistor (e.g., 1kΩ 5W). A 400V capacitor in a PC power supply stores enough energy to be lethal, and shorting it with a screwdriver will weld the tool to the traces and destroy the PCB.

Understanding what a capacitor stores is just the beginning. By respecting the physics of the electric field, matching the dielectric to the ripple current, and reading the physical markings correctly, you transition from simply swapping parts to engineering reliable, long-lasting circuits.