How Does a Capacitor Charge and Filter Noise?
- Basic
- Article
- Ages 13+
- 4 min read
A capacitor stores energy and has two metal plates separated by an insulating layer. It charges as charge gathers on the plates and discharges when charge has another path. Decoupling capacitors are often placed close to an IC to reduce noise on its power supply.
Contents
- Two metal plates
- Insulating layer
What you will learn
- Describe how a capacitor is built.
- Explain how a capacitor charges and discharges.
- See why a capacitor near a chip steadies the supply.
Before you start
It helps to know what voltage, current and resistance are.
How is a capacitor built?
A capacitor is made from two metal plates and an insulating material called a dielectric. What makes it special is its ability to store energy.
Capacitance describes how much charge is stored for a given voltage: Q = C × V. Its standard unit is the farad. Each capacitor also has a maximum voltage rating; exceeding it can destroy the part.
- Metal plate
- Dielectric
- Lead
How does a capacitor charge and discharge?
When positive and negative charges gather on the capacitor plates, the capacitor becomes charged. If the circuit gives the charges another path to each other, they leave the capacitor and it discharges.
In the ideal-capacitor model, a steady voltage across the capacitor means no charging or discharging current. A real capacitor can still have a small leakage current.
- Battery
- Capacitor
- Charging
- Lamp
- Discharging
A capacitor does not hold its energy forever. Every capacitor is prone to leaking a tiny amount of current through the dielectric, and leakage makes the stored energy drain away slowly.
How does a capacitor filter power noise?
A decoupling capacitor reduces high-frequency noise on a power supply. It connects between the power source and ground.
High-frequency ripple can pass through the capacitor to ground while DC continues to the chip. If the supply voltage drops briefly, a decoupling capacitor can also supply energy briefly; its voltage falls as it delivers charge.
- Noise on the supply line
- Decoupling capacitor
- Chip
Real-world example
With an adult, look at an image of a small circuit board. Some boards place a small capacitor between an IC’s power and ground pins; it may be a decoupling capacitor. A capacitor on a real board can retain charge after power is off, so do not touch or experiment with the board.
Common misunderstandings
A capacitor holds its charge forever. Every capacitor can leak a small current through the dielectric, so the energy drains away.
A capacitor can be wired either way round. Electrolytic capacitors are usually polarized; reverse connection can cause failure or rupture. A disconnected capacitor can retain dangerous voltage: do not touch or experiment until a qualified person has checked it is safe.
Current always passes through a capacitor. In the ideal model, there is no charging or discharging current when the voltage is steady; a real capacitor can still have a small leakage current.
Frequently asked questions
What is a capacitor used for?
A capacitor stores electric energy, and is also used as a decoupling capacitor to suppress noise on the power line.
What unit is capacitance measured in?
The standard unit of capacitance is the farad.
What is a decoupling capacitor?
A decoupling capacitor connects between the power source and ground to suppress high-frequency noise in power supply signals.
How does a capacitor charge and discharge?
It charges when charge gathers on the plates and discharges when the circuit gives the charges another path to each other.
Summary
- A capacitor has two metal plates and a dielectric, and it stores energy.
- It charges as charge gathers on the plates and discharges when there is a path for the charge to leave.
- A decoupling capacitor between supply and ground suppresses high-frequency noise on the power line.
Terms in this article
English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.
- CapacitorTụ điện
- A component of two metal plates separated by a dielectric, used to store electric energy.
- DielectricChất điện môi
- The insulating material between the two plates of a capacitor.
- CapacitanceĐiện dung
- The ratio of stored charge to the voltage across a capacitor: C = Q / V.
- Decoupling capacitorTụ tách
- A capacitor between the supply and ground that suppresses high-frequency noise on the power line.
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Sources
These sources are written for older readers. If they feel hard, read them with an adult.
- Capacitors(opens in a new tab)
SparkFun ElectronicsTier C: other sourcesRead on October 29, 2026
- Injury Caused by High Voltage Capacitor Discharge(opens in a new tab)
University of California, Berkeley, Environment Health and SafetyTier A: government, standards or research bodiesRead on October 29, 2026