What Is a Capacitor? Types, Markings, and Failure Modes Explained

The Physics and Function: What Is a Capacitor?

A capacitor is a passive two-terminal electronic component that stores electrical energy in an electric field. Physically, it consists of two conductive plates separated by an insulating material called a dielectric. When a voltage is applied, electrons accumulate on one plate and are depleted from the other, creating an electrostatic charge.

To put a number on it: if you charge a 100µF capacitor to 12V DC, the energy stored is calculated using the formula E = ½CV². That yields 0.5 × 0.0001F × 144V² = 7.2 millijoules. While that sounds small, delivering those 7.2mJ in a microsecond results in a massive instantaneous current spike, which is exactly why we use them for decoupling and flash circuits.

The Water Analogy: Think of a capacitor as a flexible rubber membrane sealed inside a water pipe. If you apply steady water pressure (DC voltage), the membrane stretches until it pushes back with equal force, and the flow stops. But if you rapidly push and pull the water (AC signal), the membrane flexes back and forth, effectively passing the alternating motion down the pipe without any water actually crossing the barrier.

Decoding Capacitor Types: Which Dielectric for Which Job?

The dielectric material dictates almost everything about a capacitor's behavior: its size, temperature stability, equivalent series resistance (ESR), and microphonic noise. Here is the selection matrix for the four most common families you will encounter on a bench or PCB.

Reading the Code: How to Identify Value, Voltage, and Tolerance

Unlike resistors with their color bands, capacitor markings are notoriously fragmented across manufacturers and form factors. According to the Vishay Capacitor Marking Guide and IEC 60062 standards, here is how to decode what is printed on the physical part.

1. The 3-Digit Ceramic Code (pF base)

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

• 104 = 10 × 10⁴ pF = 100,000 pF = 100 nF = 0.1 µF.
• 472 = 47 × 10² pF = 4,700 pF = 4.7 nF.
• 220 = 22 × 10⁰ pF = 22 pF (Note: if it says 221, that is 220 pF).

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

2. Aluminum Electrolytic Printing

These are large enough to print directly. You will see the capacitance in µF, the voltage rating (WV or VDC), and a temperature rating (usually 85°C or 105°C). Crucial: Look for the negative stripe with minus signs (-) running down the side. The short leg is also typically the negative terminal on new, unclipped parts.

3. SMD Tantalum Voltage Letters

Tantalums print a letter for voltage and a number for capacitance. For example, a marking of C106 means:

• C = 16V (A=10V, C=16V, D=20V, E=25V).
• 106 = 10 × 10⁶ pF = 10,000,000 pF = 10 µF.
• The thick colored band on the top case always marks the Positive (+) anode.

The Substitution Matrix: Rules for Swapping Parts Safely

When your exact BOM part is out of stock or you are repairing a legacy board, you cannot just grab any capacitor with the same µF rating. Follow these bench-tested substitution rules to avoid creating new problems.

• Voltage Rating: You can always go UP in voltage (e.g., replace a 16V cap with a 25V or 50V cap). Never go down. However, higher voltage electrolytics are physically larger and may have higher ESR.
• Capacitance Value: For power supply bypass/decoupling, going up 20-50% is usually fine. For timing circuits (like a 555 timer) or active filters, the value must be exact; use a 1% or 5% tolerance replacement.
• Temperature Coefficient: Never substitute a Class II ceramic (X7R/Y5V) for a Class I (C0G/NP0) in an RF or precision analog circuit. X7R capacitors exhibit piezoelectric microphonics and lose massive capacitance under DC bias. A 10µF X7R cap at 12V bias might actually be functioning as a 2µF cap. For deep data on this, refer to KEMET's technical resources on MLCC DC bias characteristics.
• Polarity: Never replace a non-polarized film or ceramic cap with a polarized electrolytic or tantalum unless you have verified with an oscilloscope that there is absolutely zero reverse DC bias or AC swing across the node.

Autopsy of a Dead Cap: Visual and Measured Failure Modes

Capacitors are the most common point of failure in power electronics. Recognizing how they die saves hours of debugging. For comprehensive failure analysis data, the Cornell Dubilier Technical Papers library is an industry-standard reference.

Aluminum Electrolytic: The Bulge and the Leak

• Visual Symptom: The top cross-vent is domed or popped. Brown, crusty electrolyte has leaked onto the PCB. It smells distinctly like rotten fish or burnt sugar.
• Cause: Overvoltage, excessive ripple current causing internal heating, or reverse polarity. The liquid electrolyte boils, generating gas that triggers the mechanical vent.
• Measured Symptom: Capacitance drops significantly (e.g., a 1000µF cap reads 200µF), and ESR spikes above 1 ohm. Your multimeter's capacitance mode might show a value, but an ESR meter will reveal it is effectively dead at high frequencies.

MLCC (Ceramic): The Invisible Short

• Visual Symptom: Often looks perfectly fine to the naked eye. Under a microscope, you may see a hairline crack near the end termination or a tiny scorch mark.
• Cause: Mechanical board flex (e.g., snapping apart PCB panels without a proper router) or thermal shock from an unpreheated wave solder bath. The ceramic cracks, and internal electrodes short together.
• Measured Symptom: Dead short (0.0 ohms) across the pads. This will drag down the entire voltage rail and cause your power supply to fold back or trip its overcurrent protection.

Tantalum: Thermal Runaway

• Visual Symptom: The component is charred black, cracked open, or entirely missing, leaving a scorched crater on the PCB.
• Cause: Tantalum capacitors fail short-circuit. If the power supply can deliver enough current, the short turns into a thermal runaway fire. Voltage spikes exceeding 20% of the rated voltage or momentary reverse polarity will trigger this.
• Prevention: Derate tantalum voltage by at least 50% (use a 20V rated part on a 10V rail) and always use a current-limiting resistor or fuse upstream if the supply is capable of high fault currents.

Frequently Asked Questions

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

In a DC power supply, capacitors perform two main jobs: bulk smoothing and high-frequency decoupling. A large aluminum electrolytic capacitor (e.g., 2200µF) acts as a reservoir, filling in the voltage valleys left by the rectified AC waveform to reduce 120Hz ripple. Smaller ceramic capacitors (e.g., 100nF) are placed physically close to IC power pins to provide instantaneous current during logic switching, acting as a localized, low-impedance micro-battery that prevents voltage brownouts.

Can I use a higher voltage capacitor instead of the original?

Yes, substituting a higher voltage capacitor is generally safe and often improves reliability because the dielectric is under less electrical stress. However, there are two physical trade-offs. First, higher voltage capacitors are physically larger, which may cause clearance issues on a crowded PCB. Second, in the case of electrolytic capacitors, a much higher voltage rating can result in a higher Equivalent Series Resistance (ESR) and lower ripple current rating for the same physical can size, which could negatively impact switching regulator stability or heat dissipation.

What is the difference between a capacitor and a battery?

The fundamental difference lies in how they store energy and their power density. A battery stores energy chemically through electrochemical reactions, giving it high energy density (it can run a device for hours) but relatively low power density (it cannot dump all its energy in a millisecond without damage). A capacitor stores energy electrostatically in an electric field. It has low energy density but massive power density, capable of charging and discharging in microseconds millions of times without chemical degradation.

Why do ceramic capacitors lose capacitance under DC bias?

This phenomenon, known as DC bias effect, primarily affects Class II and Class III ceramic dielectrics (like X7R, X5R, and Y5V). These materials are ferroelectric. When a strong DC electric field is applied, the internal dipole domains align and become 'locked' in place, reducing the material's ability to polarize further in response to AC signals. Consequently, a 10µF X5R capacitor rated for 6.3V might only exhibit 2µF of effective capacitance when 5V DC is applied across it. Class I dielectrics (C0G/NP0) do not exhibit this effect because they use non-ferroelectric, linear paraelectric materials.