---
title: Decoupling
lang: en
type: notion
tags:
  - eee
  - notion
  - puissance
prerequis:
  - alimentation-electronique-en
aa:
  - RA-EEE-C03-2/EEE/3
phases:
  - dossier-technique
draft: false
source_fr: embarque/decouplage.md
source_sha256: 300bdf95a33a61b38b3bc5ad6b181f0a75ba4cb94c2046ea671a002b310327b7
---

**Decoupling** means placing capacitors as close as possible to each component, to serve its **current peaks** locally, those peaks being too brief for the power supply to catch up with them. Without it, every peak digs into the voltage at the component's pin — enough to spoil a reading or to reset a [[microcontroleur-en|microcontroller]] — and the fault is all the nastier for being invisible on a [[multimetre-en|multimeter]].

![Two panels compared. Without decoupling: a regulator feeds a microcontroller at the end of a long track; when current is drawn, the voltage at the pin dips below the microcontroller's minimum voltage and it resets. With decoupling: a bulk capacitor and a 100 nF ceramic placed as close as possible to the pin serve the peak locally, and the voltage stays above the threshold.](/ressources/img/decouplage/generique.svg)

## What is it for?

A regulator holds the voltage of its rail, but it does not react instantly. And above all, the **track or the wire** between it and the component has a resistance and an **inductance** that oppose rapid changes in current. When a component draws a brief peak — an output switching, a radio module transmitting, a motor starting —, the current cannot build up quickly enough through the track: the voltage **at the pin** collapses locally for a few microseconds, then everything settles again.

A few microseconds are all it takes, though: a microcontroller resets, an [[adc-en|ADC]] conversion returns a nonsense value, a radio module drops its packet. These are the "unexplained" intermittent failures par excellence. The multimeter, far too slow, shows a perfectly healthy rail. Only the [[oscilloscope-en|oscilloscope]] shows the dip.

The decoupling capacitor is the answer: a **local reservoir**, charged to the rail voltage, which supplies the peak on the spot for as long as the current takes to build up from the supply, then recharges.

## How to decouple

**Two stages that complete each other.** A **bulk capacitor** (an electrolytic of 100 to 470 µF) takes the slow, large variations. It goes at the input of a stage or across a heavy consumer (a motor driver). A **100 nF ceramic capacitor** answers the fastest variations. It goes right against **every supply pin of every integrated circuit**. Neither replaces the other: the bulk is too "slow" for fast edges, the ceramic too small for large demands.

**As close as possible, always.** How well decoupling works is measured in millimetres: on a [[pcb-en|printed circuit board]], the 100 nF goes against the pin; on a breadboard, in the holes immediately next to the component. Ten centimetres away at the end of a wire, the inductance of the wire cancels its effect, so you may as well fit nothing at all.

**One 100 nF per chip, not one for all of them.** Decoupling is local by nature: every integrated circuit (microcontroller, driver, sensor, memory) gets its own. The application circuits in datasheets say so every time. And commercial boards, such as an [[arduino-alimentation-en|Arduino board]], already come covered.

> [!tip] Tip
> **Tried and tested parts** — **100 nF X7R** ceramic (the absolute standard, worth buying by the bag), aluminium electrolytic **100 to 470 µF** for the reservoirs, 10 µF ceramic as a compact addition. Take a **voltage rating of at least twice the rail** (25 V on a 12 V rail), and read the application circuit in the datasheet of the component you are decoupling: it gives the values expected.

## Example — the 3-axis arm

On the running example arm, each **A4988 driver** gets its own bulk capacitor (100 µF, required by the driver's datasheet): the three stepper motors draw their current in peaks, at every step. The microcontroller and each encoder get their **100 nF** at the supply pin. Without the bulk capacitors at the drivers, starting the three axes at once digs into the power rail, the dip travels, and the microcontroller resets mid-movement, a textbook intermittent failure, invisible on a multimeter.

## Pitfalls

**Decoupling from a distance.** A capacitor at the end of a long track or a flying wire decouples nothing: the inductance of the path cancels its effect. The distance is counted in millimetres.

**One big capacitor for the whole board.** The central reservoir does not answer the fast peaks of each chip. The two stages — shared reservoir, ceramic per pin — complete each other, neither replaces the other.

**Fitting an electrolytic the wrong way round.** Unlike a ceramic, an electrolytic is **polarised**: reversed, it destroys itself, sometimes by bursting. Find the − stripe before you solder.

**Adding capacitors at random while debugging.** If a capacitor "fixes" some erratic behaviour, work out which peak and which component were at fault. Otherwise the problem will come back in another form.

**Choosing a voltage rating right at the rail voltage.** A capacitor ages badly at its limit. Twice the rail is the safe reflex.

## Going further

- **A capacitor's impedance depends on frequency** — which is why several values go in parallel (bulk + ceramic) rather than one large one: each covers its own band.
- **Power planes** — on a dense PCB, a pair of supply and ground planes behaves as a distributed capacitor in its own right, a natural complement to local decoupling.

## See also

- [[alimentation-electronique-en|Designing a power supply]] — the architecture page, of which decoupling is one of the moves
- [[pcb-en|Designing a board (PCB)]] — where the capacitors go is settled during layout
- [[oscilloscope-en|Oscilloscope]] — the only instrument that shows the peak and the dip
- [[arduino-alimentation-en|Powering an Arduino board]] · [[micropython-alimentation-en|a board running MicroPython]] · [[xiao-alimentation-en|a XIAO ESP32-S3]] — the decoupling already fitted on a commercial board