Capacitors
Ceramic, electrolytic, film and tantalum types, ratings, ESR, decoupling, and RC timing and filters.
What a capacitor does
A capacitor is two conducting plates separated by an insulator. It stores energy in an electric field. Charge builds up as voltage is applied, and the stored charge gives the capacitor a voltage of its own. It blocks steady (DC) current once charged, and passes changing (AC) signals, which is why capacitors are used for smoothing, filtering, timing, coupling and storing energy.
Q = C × V I = C × dV/dt E = ½ × C × V²
- Capacitance C is in farads (F). A farad is huge, so parts are marked in pF (10−12), nF (10−9), µF (10−6) and, for supercapacitors, farads.
- The current is proportional to how quickly the voltage is changing. A steady voltage has no current; a sudden change draws a large one. That is why a capacitor can supply the sharp current spikes a chip draws (see decoupling below).
- Stored energy is small for most parts: 7.2 mJ in a 100 µF capacitor at 12 V.
Types of capacitor
| Type | Strengths | Weaknesses | Typical use |
|---|---|---|---|
| Ceramic (MLCC) | Tiny, cheap, low ESR, fast, non-polar | Class 2 types (X7R, X5R, Y5V) lose capacitance with voltage and temperature; can crack if the board flexes; can be microphonic | Decoupling, filtering, timing (use C0G/NP0 for precision) |
| Aluminum electrolytic | Large capacitance per cost | Polarized, wide tolerance (±20 %), high ESR, wears out when hot, leaks | Power-rail bulk storage and smoothing |
| Film (polyester, polypropylene) | Stable, low loss, tolerant of surges, self-healing | Large for the value | Audio, timing, mains filtering, motor run |
| Tantalum / polymer | Compact, stable, low ESR (polymer) | Tantalum can fail short and ignite if overstressed; polarized | Compact bulk capacitance |
| Supercapacitor | Enormous capacitance, many cycles | Low voltage (about 2.7 V per cell), leaks, high ESR | Backup power, short high-current bursts |
Ratings that matter
- Voltage rating. The maximum continuous voltage. Choose a rating of at least 1.5 to 2 times the working voltage, and more for tantalum (2×) and for circuits with spikes. An overvoltaged capacitor can short, vent or burst.
- Tolerance. 1 to 5 % for film and C0G ceramic; ±10 to 20 % for X7R and electrolytic. An “80 %-20 %” rating (Y5V) means it may be 20 % below to 80 % above the marked value.
- Dielectric class (ceramic). C0G/NP0 is stable and low-loss but small in capacitance. X7R and X5R are compact but their capacitance falls with applied DC voltage: a 10 µF X5R part at its rated voltage may offer only 30–50 % of its capacitance. Check the manufacturer’s DC-bias curve, or choose a higher voltage rating or larger case size.
- ESR and ESL (equivalent series resistance and inductance) limit how well a real capacitor supplies fast currents. Low-ESR ceramics and polymers are best next to switching regulators; plain electrolytics are poor at high frequency.
- Ripple current and temperature. Electrolytics have a maximum ripple current. Their life roughly doubles for every 10 °C cooler than the rated temperature (a 2 000 h at 105 °C part lasts about 32 000 h at 65 °C, a common rule of thumb), so keep them away from hot parts.
- Polarity. Electrolytic, tantalum and supercapacitors have a positive and a negative lead. The negative is marked by a stripe on electrolytics; the positive is marked on tantalums. Wrong polarity can cause failure, venting or fire.
Series and parallel
| Combination | Total capacitance | Note |
|---|---|---|
| Parallel | C1 + C2 + … | Capacitances add; the voltage rating is the lowest of the parts |
| Series | 1 ÷ (1 ÷ C1 + 1 ÷ C2 + …) | Less than the smallest; the voltage rating rises but does not share equally without balancing resistors |
Capacitors combine opposite to resistors. Parallel is common (adding a 100 nF ceramic beside a 10 µF electrolytic); series is rare. Example: 10 µF in series with 22 µF = 6.88 µF. Putting electrolytics in series to double the voltage rating needs equalizing resistors across each, because leakage current differences make the voltages share unevenly.
Decoupling
A digital chip draws sharp pulses of current every time its outputs switch. The supply and its wiring cannot respond that quickly, so the supply voltage near the chip dips and rings, and the chip misbehaves. A decoupling (bypass) capacitor placed right at the supply pins acts as a local reservoir for these spikes and shorts high-frequency noise to ground.
- 100 nF (0.1 µF) ceramic across every supply pin pair of every IC, as close as you can place it, with short, wide tracks to the pin and to ground.
- Bulk capacitor (10–100 µF) where power enters the board and near big loads such as motor drivers, radios and LED strings.
- A microcontroller with several supply pins needs a capacitor at each. Analog supply pins often add a ferrite bead and extra capacitors.
- The usual symptom of poor decoupling is a chip that resets, runs erratically or gives noisy readings, especially when something else switches. See Test Tools and Fault Finding.
Timing with a resistor
A capacitor charging through a resistor follows an exponential curve. The time constant τ = R × C is the time to reach about 63 % of the supply. After five time constants it is within 1 % of its final value.
τ = R × C V(t) = Vsupply × (1 − e−t/τ)
Worked example: R = 10 kΩ, C = 100 µF
- τ = 10 000 × 0.0001 = 1.0 s. The capacitor reaches 63 % after 1 s and 99 % after 5 s.
- Discharge follows the same curve downwards: the voltage falls to 37 % after one time constant.
- Choosing C for a delay: for τ = 1 ms with R = 10 kΩ, C = τ ÷ R = 100.0 nF, so use 100 nF.
- Real capacitors have tolerances of 10–20 % and electrolytics leak, so RC timing is approximate. Use a timer chip (see Op-amps and Integrated Circuits) or a microcontroller for accuracy.
Filters and coupling
cut-off frequency fc = 1 ÷ (2π × R × C)
- Low-pass filter: a resistor in series with the signal and a capacitor to ground passes low frequencies and removes high ones. With R = 1.6 kΩ and C = 1 µF, fc = 1 ÷ (2π × 1 600 × 0.000001) ≈ 99.5 Hz. Use it to smooth a noisy sensor or as an anti-alias filter before an ADC.
- High-pass filter: swap the two, with the capacitor in series and the resistor to ground. It passes changes and blocks DC.
- AC coupling. A series capacitor passes the signal and blocks the DC level. The capacitor with the input resistance forms a high-pass filter, so size it so that fc is well below the lowest frequency you need.
- Smoothing (supply). A large capacitor after a rectifier fills in the gaps between the peaks. The ripple is ΔV ≈ I ÷ (2 × f × C) for a full-wave rectifier (see Diodes and LEDs).
Markings
Small ceramic and film capacitors use a three-digit code, like SMD resistors, in picofarads: the first two digits are the value and the third is the number of zeros.
| Code | Value | Code | Value |
|---|---|---|---|
| 101 | 100 pF | 473 | 47 nF |
| 102 | 1 nF | 104 | 100 nF (0.1 µF) |
| 103 | 10 nF | 105 | 1 µF |
| 22 | 22 pF | 4n7 | 4.7 nF |
A letter after the code gives the tolerance (J = 5 %, K = 10 %, M = 20 %). Large electrolytic capacitors print the value, voltage rating and a polarity stripe directly on the can.
Stored energy and safety
- A charged capacitor can hold its charge for minutes to days after the power is removed. A 1 000 µF capacitor charged to 400 V holds ½ × 0.001 × 400² = 80 J, enough to hurt badly and to weld a screwdriver.
- Discharge before touching. Use a resistor (for example 1 kΩ, 5 W) across the terminals, never a screwdriver. Check with a meter that the voltage is gone.
- Mains equipment (power supplies, camera flashes, microwave ovens) can store dangerous charge. Do not open them unless you are trained.
- Electrolytic capacitors that are reversed or overvolted can vent hot gas or burst. Wear safety glasses when testing circuits with large ones.
Related
- Electronics Fundamentals: the overview of this guide.
- Resistors: resistor values and power ratings.
- Inductors, Transformers and Relays: the other energy-storing component.
- Batteries and Power Supplies: smoothing and decoupling in power supplies.
FAQ
Which way round does an electrolytic capacitor go?
The lead marked with a stripe (a minus sign on the sleeve) is the negative lead and goes toward the lower voltage. On a new part the positive lead is also the longer one. The board silkscreen usually shows a stripe or shading on the negative side.
Can I use a capacitor with a higher voltage rating?
Yes. A higher voltage rating is always safe. It is usually physically larger, and for a ceramic part it also loses less capacitance at the working voltage.
Is more capacitance always better?
No. Large values draw a big inrush current at power-up, can upset a regulator’s stability, and slow a circuit. Match the value to the need, and follow the regulator datasheet’s recommended output capacitance and ESR.
Why use both 100 nF and 10 µF together?
They work at different speeds. The 100 nF ceramic responds to very fast current spikes; the larger electrolytic supplies slower changes. Together they cover a wide frequency range.
What does a capacitor do in a DC circuit?
After it charges, it blocks DC. Before that, it draws current as it fills. That charging behavior is what gives timing delays and smooths ripple.
References
- Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press.
- IEC 60062, Marking codes for resistors and capacitors.
- Capacitor manufacturers’ application notes on MLCC DC-bias behavior and aluminum-electrolytic life (for example Murata, KEMET, Nichicon, Panasonic).
- Platt, C., Make: Electronics, 3rd ed., Make Community.
Last reviewed: October 2026. Values are typical: check the datasheets of the parts you use. Mains and high-voltage capacitors can be lethal; follow safe practice.