At the most fundamental physics level, capacitors store electrical energy in an electric field between two conductive plates separated by an insulating dielectric. But on the workbench, the answer to "what do capacitors do" is highly dependent on where you place them. In a DC power supply, they smooth voltage ripple. In an AC audio path, they block DC offset while passing the signal. In a motor circuit, they create a phase shift to generate starting torque. In high-speed digital logic, they act as localized energy reservoirs to prevent brownouts during nanosecond switching events.
Choosing the wrong capacitor chemistry or misreading a three-digit code can result in anything from a noisy audio amplifier to a catastrophic short circuit that destroys your PCB. This guide breaks down the practical application, identification, and substitution rules for the most common passive components in your parts bin.
The Core Function: What Do Capacitors Actually Do?
To understand what capacitance achieves in a real circuit, consider the water tower analogy. A well pump (your rectifier or switching regulator) delivers water in pulsed, uneven bursts. The water tower (the capacitor) absorbs those bursts and releases a steady, smooth flow of water to the town (your load) on demand.
Let us look at a concrete numeric example. Suppose you are building a linear power supply and need to smooth a full-wave rectified 60 Hz AC signal (which results in a 120 Hz ripple frequency) to power a 1A load. The formula for peak-to-peak ripple voltage is:
ΔV = I / (f × C)
If you use a standard 1000µF (0.001F) electrolytic capacitor, your ripple will be: 1A / (120Hz × 0.001F) = 8.33V of ripple. If your nominal DC is 12V, the voltage will sag down to 3.67V every half-cycle, causing your linear regulator to drop out. To achieve a ripple of less than 1V, you must solve for C: C = 1 / (120 × 1) = 0.0083F, meaning you need an 8300µF capacitor.
Capacitor Types and Selection Criteria
Not all capacitors are interchangeable. The dielectric material inside the component dictates its equivalent series resistance (ESR), frequency response, and physical size. Here is how to select the right type for the job.
| Type | Construction / Dielectric | Typical Tolerance | Tempco (Temp. Coefficient) | Typical Use & Selection Criteria |
|---|---|---|---|---|
| Ceramic (MLCC) | Multi-layer ceramic (X7R, C0G/NP0) | ±5% to ±20% | C0G: ±30ppm/°C X7R: ±15% |
High-frequency decoupling, RF filters. Choose for ultra-low ESL (Equivalent Series Inductance) and fast transient response. Avoid X7R in precision timing due to voltage coefficient. |
| Aluminum Electrolytic | Etched aluminum foil with liquid/polymer electrolyte | ±20% | High (Capacitance drops significantly below -20°C) | Bulk energy storage, power supply smoothing. Choose when you need high capacitance (10µF to 10,000µF) at low cost. Watch out for high ESR and limited lifespan at high temperatures. |
| Tantalum | Tantalum metal anode with manganese dioxide/polymer cathode | ±10% to ±20% | Stable over wide range | Compact bulk decoupling, medical/military. Choose for high volumetric efficiency and stable capacitance over temperature. Never use where high inrush current or reverse voltage is possible. |
| Film (Polyester/Polypropylene) | Metallized plastic film wound or stacked | ±1% to ±10% | Very stable, predictable | Audio crossovers, AC snubbers, motor run. Choose for high voltage handling, AC line applications, and audio paths because they exhibit near-zero dielectric absorption (the tendency to retain residual charge after discharge). |
When designing with Multi-Layer Ceramic Capacitors (MLCCs), remember that Class II dielectrics (like X5R and X7R) lose massive amounts of capacitance when DC voltage is applied. A 10µF, 16V, 0805 X5R capacitor might only provide 2.5µF of actual capacitance when biased at 12V. Always check the manufacturer's DC bias graph or step up to a larger physical package (e.g., 1206) or higher voltage rating (e.g., 25V or 50V) to mitigate this drop.
Decoding Capacitor Markings and Codes
Reading the markings on a physical part is a mandatory bench skill, as manufacturers use different coding systems depending on the component's physical size.
- Ceramic Disc and MLCC (3-Digit Code): The first two digits are the significant figures, and the third digit is the multiplier in picofarads (pF). For example, a marking of 104 means 10 × 104 pF = 100,000 pF = 100nF = 0.1µF. A marking of 222 is 22 × 102 pF = 2200 pF = 2.2nF.
- Electrolytic (Direct Print): Values are usually printed plainly (e.g., 470µF 25V). The critical marking here is the negative stripe on the can, which indicates the cathode. Installing these backward causes rapid internal gas generation and venting.
- Film Capacitors (Alphanumeric): Often use a combination of letters and numbers. A marking like 2J104J breaks down as: 2J (630V DC rating), 104 (100nF capacitance), and the trailing J (±5% tolerance). Other tolerance codes include K (±10%) and M (±20%).
- Surface Mount Tantalum: Usually feature a bar or line on one end indicating the positive anode (opposite of electrolytic stripes). Values might be printed as 107, meaning 10 × 107 pF = 100µF.
Failure Modes and Visual Symptoms
Capacitors do not always fail as an open circuit; they often degrade in ways that cause secondary failures in your surrounding circuitry. Recognizing these visual and electrical symptoms is critical for troubleshooting.
1. Aluminum Electrolytic: Venting and ESR Degradation
Visual Symptom: The top of the can bulges upward, or the rubber plug at the bottom extrudes, leaving a crusty brown or black residue on the PCB. Sometimes, they look perfectly normal externally.
Electrical Reality: The internal electrolyte dries out over time, especially near heat sinks. This causes the Equivalent Series Resistance (ESR) to skyrocket. In a switching power supply, a capacitor with high ESR will fail to filter high-frequency ripple, causing the PWM controller to starve and the output voltage to bounce erratically. Always test electrolytic capacitors with an ESR meter, not just a standard capacitance meter.
2. MLCC (Ceramic): Flex Cracking
Visual Symptom: Often invisible to the naked eye. Under magnification, you may see a hairline fracture near the terminal ends, or a dark scorch mark on the PCB beneath the component.
Electrical Reality: MLCCs are brittle. If the PCB flexes during depaneling, connector insertion, or mechanical mounting, the ceramic body cracks. This allows moisture to enter, eventually creating a low-resistance short circuit between the internal layers, which can pull a power rail directly to ground.
3. Tantalum: Thermal Runaway
Visual Symptom: The component is charred, cracked open, or completely missing, leaving a scorched crater on the board. It smells distinctly of burning plastic.
Electrical Reality: Tantalum capacitors are highly intolerant of reverse voltage, voltage spikes, and high ripple current. If subjected to these, the internal manganese dioxide cathode undergoes an exothermic chemical reaction, literally catching fire. Always derate tantalum voltage by at least 50% (e.g., use a 16V part on a 5V rail).
Safe Substitution Rules When the Exact Part is Missing
When you are repairing a board or prototyping and lack the exact BOM part, you can substitute components, but you must follow strict electrical boundaries to avoid creating a fire hazard or a non-functional circuit.
- Voltage Rating Can Go UP, Never DOWN: You can safely replace a 25V capacitor with a 50V capacitor. However, be aware that higher voltage electrolytics often have higher ESR and larger physical footprints, which might prevent them from fitting in the designated PCB space or cause issues in high-frequency switching nodes.
- Capacitance Tolerance Depends on the Circuit: For bulk power supply filtering, substituting a 1000µF with a 1200µF or even 820µF is usually fine. However, in timing circuits (like a 555 timer oscillator), active filters, or resonant LC tanks, you must match the exact capacitance value and use a tight-tolerance part (±1% or ±5%), or the frequency response will shift entirely.
- Never Cross Polarities in AC Paths: You cannot substitute a polarized electrolytic capacitor for a non-polarized film or ceramic capacitor in an AC signal path or motor run circuit. The reverse voltage during the negative half-cycle will destroy the electrolytic. If you must use electrolytics in a non-polar application, you must wire two identical polarized capacitors in series, back-to-back (negative-to-negative), and double the voltage rating.
- Watch the Temperature Rating: If replacing an 85°C electrolytic with a 105°C part, that is an upgrade. Replacing a 105°C with an 85°C part in a switching power supply will result in premature failure due to the high ambient heat generated by nearby MOSFETs and inductors.
Frequently Asked Questions
What do capacitors do in an AC motor?
In single-phase AC induction motors, capacitors are used to create a phase shift in the current of a secondary "start" or "run" winding. Because single-phase power does not inherently create a rotating magnetic field, the capacitor delays the current in the auxiliary winding by up to 90 degrees. This creates the necessary phase difference to generate starting torque (start capacitors, which are switched out via a centrifugal switch) or to improve running efficiency and power factor (run capacitors, which remain in the circuit continuously).
What do capacitors do on a PC motherboard?
On a motherboard, capacitors primarily serve two roles: bulk energy storage and high-frequency decoupling. Around the CPU Voltage Regulator Module (VRM), large polymer or electrolytic capacitors store bulk energy to handle massive, sudden current draws when the CPU transitions from idle to full load. Smaller MLCCs (often 0.1µF or 0.01µF) are placed as close to the IC power pins as possible to provide instantaneous charge for nanosecond switching events, preventing localized voltage droops (brownouts) that would cause logic errors or system crashes.
What do capacitors do in an audio crossover?
In passive speaker crossovers, capacitors act as high-pass filters. Because a capacitor's impedance (reactance) decreases as frequency increases ($X_c = 1 / (2\pi fC)$), it blocks low-frequency bass signals while allowing high-frequency treble signals to pass through to the tweeter. They also block any DC offset from the amplifier from reaching the voice coils, protecting the drivers from thermal damage. Film capacitors (like polypropylene) are strictly preferred here over electrolytics to avoid introducing distortion via dielectric absorption.
What do capacitors do when they fail in a switching power supply?
When the output filter capacitors in a switching power supply (like a laptop brick or LED driver) degrade and lose capacitance or gain ESR, the power supply loses its ability to smooth the high-frequency switching ripple. The immediate symptom is often an audible high-pitched whining or squealing from the transformer, erratic output voltages, or the power supply entering a continuous "hiccup" protection mode where it turns on and off repeatedly. In severe cases, the increased ripple current overheats the switching MOSFET, causing a catastrophic short circuit.
