What is a capacitor?

Hello, colleagues,

A capacitor is a passive component that stores energy in an electric field. Its basic structure consists of two conductors separated by an insulating material called a dielectric. Applying a voltage creates equal and opposite charges on the conductors. In the ideal model, charge does not travel through the insulation from one conductor to the other.

Capacitors store energy, reduce supply ripple, couple signals and provide timing functions. Reading about them alongside other components in our electrical and electronics guide makes it easier to understand how power supplies and control boards work.

Capacitance, farads and component markings

Capacitance (C) describes the charge stored per unit of voltage. Its unit is the farad (F); practical values commonly use µF, nF and pF: 1 µF = 1,000 nF = 1,000,000 pF. For an ideal constant-capacitance model, Q = C × V and stored energy is E = ½ × C × V².

A 1,000 µF capacitor stores 0.072 joules at 12 V. The same capacitance stores 80 joules at 400 V, provided the component and circuit are suitable for that voltage. Stored energy rises with the square of voltage. A high-voltage capacitor can remain hazardous after power is disconnected.

In common three-digit coding, 104 = 100,000 pF = 100 nF = 0.1 µF. On a component marked “470 µF 25 V,” the first number is capacitance and the second is its voltage rating. Voltage rating, capacitance and polarity are separate properties: a 25 V marking does not mean the capacitor produces 25 V.

What does “blocks DC and passes AC” mean?

Under constant DC voltage, the charging current of an ideal capacitor eventually approaches zero. When voltage changes, current follows i = C × dV/dt. Real capacitors have leakage current. “Blocks DC” therefore describes the ideal condition after the charging transient.

Its response to AC signals depends on frequency. Ideal capacitive reactance is X_C = 1 / (2πfC). For 100 nF, it is approximately 1.59 kΩ at 1 kHz and 15.9 Ω at 100 kHz. Parasitic inductance limits this simple model at high frequencies.

Charging, discharging and the RC time constant

A capacitor charging through a resistor changes voltage gradually. Its time constant is τ = R × C. Starting from zero volts in an ideal circuit, it reaches approximately 63.2% of the supply voltage after one time constant and 99.3% after five.

With R = 10 kΩ and C = 100 µF, τ = 1 second. On the first charge from a 5 V supply, the capacitor reaches about 3.16 V after one second and 4.97 V after five seconds. Ohm’s law gives an initial current of 5 V / 10 kΩ = 0.5 mA, which decreases as the capacitor charges.

Filtering in power supplies

After rectifier diodes, a smoothing capacitor charges near the voltage peaks and supplies energy to the load between them. This reduces ripple voltage. A simple approximation is ΔV ≈ I × Δt / C. Increasing capacitance can reduce ripple while also increasing startup current and stress on the rectifier.

Transformer-based supplies and switch-mode power supplies do not have identical capacitor requirements. Switching frequency, ripple current, ESR and regulator stability must be considered together. Installing a higher µF value is not a universal repair method.

Coupling versus decoupling

A coupling capacitor separates DC operating points while allowing a changing signal to pass. A decoupling or bypass capacitor provides local energy and reduces supply noise. Placing it close to the supply pins of an integrated circuit or microcontroller helps reduce connection inductance.

In transistor and MOSFET switching circuits, capacitance affects transition speed and driver current. In LED supplies, a capacitor may reduce ripple, but it does not replace current limiting. Decoupling alone does not provide galvanic isolation either.

Timing, resonance and sensing

RC networks create delays and filters; the exact timing depends on thresholds and tolerances. Load capacitance affects crystal oscillators. Coupling and impedance matching matter in RF modules. An ideal LC circuit containing an inductor resonates at f₀ = 1 / (2π√LC); a capacitor alone is not an LC resonant circuit.

Capacitive sensors detect changes in capacitance caused by an object or surrounding material. This illustrates how capacitance can serve as part of a measurement principle, as well as providing energy storage.

Main capacitor types

Aluminum electrolytic: Common for high capacitance and supply smoothing. Most are polarized, although special bipolar versions exist. Reverse voltage must be avoided. Tantalum and polymer types: Offer compact construction with different ESR and current characteristics; polarity, surge current and the manufacturer’s derating guidance matter.

Ceramic: Common for compact decoupling and high-frequency applications. C0G/NP0 types provide more stable capacitance, while the effective capacitance of X5R/X7R types can fall significantly under DC bias. The nominal µF value may differ from the effective value in operation.

Film: Used for low-loss circuits and, with suitable types, AC, pulse or motor applications. Variable capacitors: Adjust capacitance in tuning circuits. Supercapacitors: Provide large capacitance for short-term backup, with low cell voltage and leakage requiring consideration. Our electrolytic, ceramic and film capacitor comparison explains these selection differences.

ESR, ripple current and operating life

ESR means equivalent series resistance. Ripple current produces heat in this resistance, approximated by P ≈ I_rms² × ESR. Check temperature limits, ripple current, leakage, tolerance and the conditions behind the stated life rating alongside capacitance. A 105 °C marking does not promise the same service life in every circuit.

Series and parallel connections

Capacitances add in parallel. In series, 1/C_eq = 1/C1 + 1/C2 + …. Two equal 100 µF capacitors give 200 µF in parallel or 50 µF in series. Series voltage sharing depends on leakage and tolerances: voltage ratings cannot simply be added without checking the sharing. Balancing circuitry may be necessary.

Industrial drives and motor capacitors

In a variable frequency drive, capacitors buffer energy on the DC bus. Capacitance loss or increased ESR can increase ripple and heating, but one fault code alone does not prove capacitor failure. The bus voltage may remain present after the supply is disconnected.

A motor start capacitor and a continuous-duty run capacitor have different roles and operating duties. Match the required µF value, AC voltage rating and duty classification. Mains interference suppression also requires suitable X/Y safety classes. A fuse and a varistor perform different protective functions; a capacitor cannot replace them.

How do you measure a capacitor or identify a fault?

Before measuring capacitance with a multimeter, isolate all energy sources, follow the manufacturer’s discharge procedure and check residual voltage using DC voltage measurement. Do not measure µF on an energized capacitor or short its terminals with a screwdriver. High-energy circuits require suitable equipment and competent working practices.

When measuring a capacitor with a multimeter, parallel circuit paths can affect the reading; disconnecting the component may be necessary after safe discharge. A correct µF reading alone does not prove it is healthy. ESR, leakage and behavior at operating voltage also matter. Bulging and leakage can indicate failure, but a faulty capacitor does not always show visible damage.

You may share this article with credit to OtomasyonAVM. Use the relevant product datasheet for component selection and connections.