What Is a Capacitor? Types, Markings, and Selection Guide

A capacitor is a passive two-terminal electronic component that stores electrical energy in an electric field. At its core, it consists of two conductive plates separated by an insulating material called a dielectric. When voltage is applied, positive charge accumulates on one plate and negative charge on the other. The fundamental relationship is defined by C = Q / V, where capacitance (C) in Farads equals the charge (Q) in Coulombs divided by the voltage (V) across the plates. The energy stored is calculated as E = ½CV². For example, a 470µF bulk capacitor charged to 50V stores roughly 0.58 Joules of energy—enough to visibly spark if shorted with a screwdriver.

Think of a capacitor like a pressurized water accumulator tank in a plumbing system: it absorbs pressure spikes (voltage transients) and releases stored fluid (charge) when the main pump (power supply) temporarily dips. This makes them indispensable for decoupling, filtering, timing, and energy storage.

Capacitor Type Comparison: Which Dielectric for Which Job?

Choosing the right capacitor is rarely about just picking the correct microfarad value. The dielectric material dictates the component's physical size, temperature stability, Equivalent Series Resistance (ESR), and failure mode. Here is how the four major families compare on the bench.

Decoding Capacitor Markings and Codes

Reading a capacitor's value depends heavily on its physical package. Through-hole ceramic and film capacitors, as well as surface-mount device (SMD) MLCCs, often use a cryptic 3-digit code because there is no room for printed text.

The 3-Digit pF Code: The first two digits are the significant figures, and the third digit is the multiplier (number of zeros) in picofarads (pF).
Example: A marking of 104 means 10 followed by 4 zeros = 100,000 pF.
Converting up the scale: 100,000 pF = 100 nF = 0.1 µF.
Another example: 473 means 47,000 pF = 47 nF.

Tolerance Letters: You will often see a letter trailing the numeric code.
• J = ±5%
• K = ±10%
• M = ±20%
• Z = +80% / -20% (common on older electrolytics)

EIA Temperature Codes (Ceramics): Class 2 ceramics use a three-character code. For X7R: 'X' means it operates down to -55°C, '7' means it operates up to +125°C, and 'R' means the capacitance will not vary more than ±15% across that entire temperature range. X5R caps top out at 85°C, while Y5V caps can lose up to 80% of their capacitance at temperature extremes—avoid Y5V for any critical decoupling.

Electrolytic and film capacitors are generally large enough to print the exact capacitance (e.g., 470µF), voltage rating (e.g., 25V), and a stripe indicating the negative lead.

Failure Modes and Visual Symptoms

Capacitors do not last forever, and their failure modes range from silent degradation to catastrophic destruction. Recognizing these symptoms is a core troubleshooting skill.

• Aluminum Electrolytic (Drying Out / Venting): The liquid electrolyte slowly evaporates over time, accelerated by heat (rule of thumb: lifespan halves for every 10°C rise above rated temp). Visual symptom: The top vent bulges outward, or the rubber bottom plug is pushed out. Electrical symptom: Massive spike in ESR and loss of capacitance, causing power supply ripple to increase until the system resets.
• MLCC (Flex Cracking): Ceramic capacitors are brittle. If the PCB bends during assembly, connector insertion, or thermal cycling, the ceramic body can crack. Visual symptom: Often invisible to the naked eye; requires a microscope. Sometimes a faint hairline crack appears near the solder fillet. Electrical symptom: Dead short circuit, which can burn a hole through the PCB trace.
• Film (Metallization Clearing): Film caps are 'self-healing.' If a voltage spike causes a localized dielectric breakdown, the thin metal layer vaporizes, isolating the fault. Visual symptom: None externally, but the part slowly loses capacitance over years of operation.

How to Safely Substitute a Missing Capacitor

When you are debugging a board at 11 PM and the exact BOM part is out of stock, you can substitute components, but you must respect the physics of the dielectric.

1. Voltage Rating: You can always substitute a higher voltage rating for a lower one (e.g., using a 25V cap instead of a 16V cap). However, beware of DC Bias Derating in Class 2 MLCCs. A 10µF X5R ceramic rated for 16V might only provide 2µF of actual capacitance when 12V DC is applied across it. Always check the manufacturer's DC bias curve (available on tools like Kemet's KSIM) before stepping up voltage in tight-tolerance circuits.
2. Capacitance Value: For bulk decoupling on a digital IC's VCC pin, going higher (e.g., 0.1µF instead of 0.01µF) is generally safe and often beneficial. For RC timing circuits, active filters, or switching regulator compensation networks, you must match the exact value or the circuit will oscillate or miss its timing window.
3. ESR (Equivalent Series Resistance): Generally, lower ESR is better because it reduces heat generation ($I^2R$ losses) and output ripple. The major exception is older Low-Dropout (LDO) voltage regulators. Many legacy LDOs rely on the specific ESR of an aluminum electrolytic output capacitor to maintain control loop stability. Swapping it for a ultra-low ESR ceramic can cause the LDO to oscillate violently. Always check the datasheet's ESR requirements before substituting LDO output caps.
4. Temperature Rating: Never substitute a lower temperature rating. If the BOM calls for an X7R (125°C), do not use an X5R (85°C) in an automotive or high-ambient industrial environment.

Frequently Asked Questions

What is a capacitor used for in a DC power supply?

In a DC power supply, capacitors serve two main roles: bulk storage and high-frequency decoupling. Large aluminum electrolytic capacitors (e.g., 1000µF) smooth out the low-frequency 100/120Hz ripple left over from the AC rectification process. Smaller ceramic MLCCs (e.g., 100nF) are placed physically close to the load ICs to provide instantaneous high-frequency current during logic switching events, preventing localized voltage brownouts that the power supply traces are too inductive to deliver.

What is the difference between a run capacitor and a start capacitor?

These terms apply to single-phase AC induction motors (like those in HVAC compressors or well pumps). A start capacitor is typically an electrolytic type with high capacitance (100-800µF) designed to provide a massive phase-shifted torque boost for just a few seconds during motor startup; it is disconnected by a centrifugal switch once the motor reaches speed. A run capacitor is an oil-filled metallized polypropylene film capacitor with lower capacitance (5-100µF) designed to stay in the circuit continuously, improving the motor's power factor and efficiency during normal operation. They are not interchangeable.

What is a capacitor's ESR and why does it matter?

ESR stands for Equivalent Series Resistance. It represents the sum of all internal resistive losses (leads, plates, dielectric losses) modeled as a single resistor in series with an ideal capacitor. ESR matters because it limits how fast the capacitor can deliver current and generates internal heat when ripple current flows through it. In switching power supplies, high ESR directly translates to higher output voltage ripple ($V_{ripple} = I_{ripple} \times ESR$). For high-ripple applications, designers specifically select 'low-ESR' polymer or specialized electrolytic capacitors.

What is a capacitor's microfarad (µF) rating?

The microfarad (µF) is a unit of capacitance, equal to one-millionth of a Farad ($10^{-6}$ F). It quantifies the component's ability to store charge per volt applied. Because one Farad is an impractically large amount of storage for standard electronics, most circuit board capacitors are rated in microfarads (µF), nanofarads (nF, $10^{-9}$ F), or picofarads (pF, $10^{-12}$ F). Supercapacitors (EDLCs), used for memory backup or regenerative braking, are the exception, routinely offering values from 1F to over 3000F.