Capacitors: what they are, how they work and the main types
What a capacitor is and how it stores charge, the main types and what each is used for, how to read the value printed on one, and how to combine capacitors in series and parallel.
A capacitor stores electric charge and gives it back very quickly. That one property makes it one of the most useful parts in electronics: capacitors steady the supply to chips, smooth the output of power supplies, let audio signals through while blocking DC, and set the timing in circuits such as LED flashers. This guide explains how they work, the main types and what each is used for, how to read the value printed on one, and how to combine them.
How a capacitor works
A capacitor is two metal plates with an insulator between them, called the dielectric. Connect it to a battery and charge flows onto the plates: one becomes negative and the other positive. The insulator stops the charges from meeting, so the capacitor holds them, with the energy stored in the electric field between the plates.
How much charge it holds is its capacitance, measured in farads (F). The charge is the capacitance multiplied by the voltage: Q = C × V. A 100 µF capacitor charged to 12 V holds 0.0012 coulombs. Larger plates hold more, and plates closer together hold more; the type of insulator also makes a big difference.
Once a capacitor is charged, a steady DC voltage pushes no more current through it. A changing voltage, such as an audio signal, keeps charging and discharging it, so the change gets through. That is why a capacitor can block DC but pass a signal.
Units: pF, nF and µF
A farad is a very large amount, so most capacitors are measured in much smaller units:
| Unit | Fraction of a farad | Same as |
|---|---|---|
| Picofarad (pF) | one millionth of a millionth | 0.001 nF |
| Nanofarad (nF) | one thousand millionth | 1,000 pF |
| Microfarad (µF) | one millionth | 1,000 nF = 1,000,000 pF |
Parts lists mix these freely, so the same capacitor can appear as 100,000 pF, 100 nF or 0.1 µF.
How to read the value on a capacitor
Large capacitors have the value printed in full, such as 100µF 25V. Small ones carry a three-digit code instead: the first two digits are the value and the third is the number of zeros, in picofarads. So 104 is 100,000 pF, which is 100 nF or 0.1 µF. A letter after the code gives the tolerance (J is ±5%, K is ±10%) and a code in front, such as 2A, gives the voltage. The capacitor code calculator reads any of these codes for you, with tables of the common ones.
The main types
| Type | Typical values | Polarised? | Used for |
|---|---|---|---|
| Ceramic | pF up to about 10 µF | No | The most common type: small and cheap. Decoupling chips and high-frequency circuits. |
| Film | From about 100 pF to microfarads | No | Stable values and low losses: timing circuits, audio, loudspeaker crossovers and filters. |
| Aluminium electrolytic | About 1 µF to 1,000 µF | Yes | A lot of capacitance for the size: smoothing power supplies. |
| Tantalum | Microfarads | Yes | Decoupling, alongside ceramic capacitors. |
| Supercapacitor | Farads | Yes | Storing energy: faster to charge and discharge than a battery, but holding less. Low voltage ratings, for example 2.5 V. |
Ceramic and film capacitors can go into a circuit either way round. Electrolytic, tantalum and supercapacitors must go in the right way: the stripe on an electrolytic marks the negative lead.
What capacitors do in a circuit
Decoupling
Chips draw current in short bursts, which makes the supply voltage dip and adds noise. A capacitor between the supply and ground, as close to the chip as possible, supplies those bursts locally and absorbs the noise. The usual value is 0.1 µF (100 nF, code 104), sometimes with a 1 µF or 10 µF capacitor beside it.
Smoothing a power supply
The output of a rectifier is a series of bumps, not steady DC. A large capacitor across the output charges up on each peak and feeds the circuit between them, which leaves a nearly steady voltage.
Blocking DC and filtering
Because a capacitor blocks steady DC but passes changes, it is used to pass a signal from one stage to the next without its DC level, and in filters. A loudspeaker crossover uses this: a capacitor in series with the tweeter (the small speaker for high notes) blocks the low frequencies and lets the high ones through.
Timing
A capacitor charging through a resistor takes a predictable time. The time constant is τ = R × C (ohms times farads gives seconds). After one time constant the capacitor reaches about 63% of the supply voltage, and after five it is about 99% charged. With 10 kΩ and 100 µF, τ = 10,000 × 0.0001 = 1 second. This is how the 555 timer in flashing an LED with a 555 timer sets its flash rate.
Bursts of energy
A capacitor stores far less energy than a battery of the same size, but it can release it much faster. A camera flash charges a capacitor slowly from its battery, then empties it into the flash tube in a moment.
Capacitors in parallel and in series
In parallel, the values add up: C = C1 + C2 + C3 and so on. Two 100 nF capacitors in parallel make 200 nF, and 10 µF, 1 µF and 0.1 µF in parallel make 11.1 µF.
In series, the total is smaller than the smallest one: 1/C = 1/C1 + 1/C2 and so on. For two capacitors, C = (C1 × C2) ÷ (C1 + C2). Two 100 nF capacitors in series make 50 nF, and 10 µF in series with 22 µF makes about 6.9 µF. This is the opposite of resistors, which add up in series. (For resistors, see what resistors are and how they work.)
Two equal capacitors in series also share the voltage, which is how two 10 F, 2.5 V supercapacitors in series make a 5 F capacitor that can take 5 V.
Voltage ratings and safety
- Stay under the voltage rating. Every capacitor has a maximum voltage, and going over it usually destroys it. Choose one rated well above the highest voltage in your circuit, including spikes.
- Fit polarised capacitors the right way round. An electrolytic capacitor connected backwards can fail with a bang, swell or burst open, and then act as a short circuit.
- Large capacitors can hold a charge after the power is off. The capacitors in mains power supplies can keep enough charge to give a dangerous, even lethal, shock long after the equipment is switched off. Do not open mains-powered equipment unless you know how to discharge its capacitors safely. Many power supplies include a bleeder resistor to drain them.
Questions people ask
What does a capacitor do?
It stores electric charge and releases it quickly. In practice that means steadying the supply to chips, smoothing power supplies, blocking DC while passing signals, filtering, and setting the timing in circuits.
What is the difference between a capacitor and a battery?
A battery stores far more energy and releases it steadily over a long time. A capacitor stores much less, but can release it much faster and can be charged and discharged many more times.
Can a capacitor give you a shock after it is unplugged?
Yes. Large capacitors, especially in mains power supplies, can stay charged for a long time after the power is switched off.
More for electronics: the capacitor code calculator, the LED resistor calculator, and the rest of the Electronics section.
Sources: SparkFun, Capacitors (how they work, Q = CV, types and their ranges, decoupling, smoothing, crossovers, series and parallel, voltage ratings); SparkFun, Polarity (ceramic and electrolytic capacitors); Film capacitor (Wikipedia); RC time constant (Wikipedia); Bleeder resistor (Wikipedia). Worked examples calculated by Braindumps.