---
title: Electronic protections
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/protection-electronique.md
source_sha256: 349774be146704fcd6d05e84c6663a505f1a52b97cdd5532775bff8ad611966b
---

**Electronic protections** make the common faults — short circuit, reverse polarity, overvoltage, electrostatic discharge — **non-destructive**: the fault costs a fuse or a second of fallback, not a board. They are designed on the assumption that someone **will get it wrong** when plugging in, and they sit at the head of the power supply, between the input connector and the rails.

![Protection stage at the head of the power supply: the input connector feeds a reverse-polarity block (diode or P-MOSFET), then a fuse, then a node from which a TVS diode clamps to ground, before distribution to the rails. Three faults are annotated: reverse polarity is blocked by the first block, a downstream short circuit is cut by the fuse, overvoltage is clamped by the TVS.](/ressources/img/protection-electronique/generique.svg)

## What is it for?

Over the life of a project, all three faults will eventually happen: a wire that slips and shorts, a connector plugged in backwards one tired evening, a voltage spike when a motor is switched off. The question is not *whether*, but *when*, and what the fault will cost. Protected, it costs a fuse worth a few cents and five minutes. Unprotected, it costs a component, sometimes the whole chain it was feeding, and the days of [[matrice-de-risques-en|delay]] to re-order. The design rule fits in one sentence: expect the mistake, and make it a non-event.

## How do you protect?

**Against a short circuit: limit the current.** The **fuse** (5×20 mm glass in a fuse holder) cuts permanently above its rating, chosen **above the normal peak** of consumption, never below. The **polyfuse** (resettable PTC fuse) resets itself once it has cooled, handy during bring-up. And for the first tests of a new build, the electronic current limit of a **bench power supply** (constant-current mode) turns a short circuit into a simple fallback: that is the protection to set *before* plugging anything in.

![Fuse wiring: in series on the supply wire, between the source and the load. A downstream short circuit blows the fuse, which cuts the circuit. Rating chosen above the normal peak; the resettable polyfuse goes in the same place.](/ressources/img/protection-electronique/fusible.svg)

**Against reverse polarity: block or key it.** A **series diode** blocks a reversed connection: simple, but it drops ~0.7 V (~0.3 V for a Schottky) and heats up at high current. A **P-MOSFET** wired as a reverse-polarity block does the same job with a negligible drop: it is the clean solution beyond a few hundred mA. The best protection, though, remains **mechanical**: a keyed connector (XT60, polarised JST) makes the fault physically impossible, and it is chosen at design time.

![Reverse-polarity diode wiring: in series on the positive wire, oriented to let current through in the right direction. If the supply is plugged in backwards, it blocks the current. Beyond a few hundred milliamps, a P-MOSFET takes the same place with no voltage drop.](/ressources/img/protection-electronique/anti-inversion.svg)

**Against overvoltage: clamp it.** A **TVS diode** (or a Zener) fitted in parallel clamps voltage spikes — a transient from the source, the kickback when a load is switched off — to ground, before they reach the circuits. It is chosen at the voltage of the rail: transparent in normal operation, conducting as soon as the voltage runs away.

![TVS diode wiring: in parallel between the rail and ground, after the series protections. A voltage spike arriving on the rail is clamped to ground before it reaches the load.](/ressources/img/protection-electronique/tvs.svg)

**Against the switch-off overvoltage of inductive loads: the flyback diode.** A coil — relay, DC motor, solenoid valve — opposes changes in its current: when the transistor driving it opens, it generates an overvoltage of several tens of volts to keep its current flowing, and it is the transistor that takes the hit. A **flyback diode** fitted in reverse across the coil offers a recirculation path: the current dies away gently, the transistor survives. It is **mandatory** on every driven inductive load, and often already built into off-the-shelf drivers and relay modules, so check the datasheet.

![Flyback diode wiring: anti-parallel across the coil (relay or motor), cathode towards the positive. When the driving transistor switches off, the coil current closes through the diode instead of destroying the transistor.](/ressources/img/protection-electronique/roue-libre.svg)

**Against electrostatic discharge (ESD): protect the exposed lines — and the hands.** A charged human body carries several **kilovolts**: touching a pin is enough to punch through a CMOS input, either at once or by weakening it for later. On the circuit side, **ESD-specific TVS diodes** go on every line accessible from the outside (USB connector, terminal block, buttons). On the handling side, the habits matter as much as the components: touch a ground before picking up a board, hold boards by their edges, keep components in their antistatic bags.

![ESD TVS wiring: in parallel between the data line and ground, as close as possible to the exposed connector. The several-kilovolt electrostatic discharge is clamped to ground before it reaches the microcontroller pin.](/ressources/img/protection-electronique/esd.svg)

**The complementary parts.** Three protections often exist *already* in the system, and are worth knowing so as not to duplicate them needlessly: the **thermal protection** built into modern regulators (they shut down when they overheat); the ***brown-out detector*** of the microcontroller, which holds it in reset when its supply falls below a threshold instead of letting it run off the rails; and the **BMS** (*battery management system*) of lithium batteries, essential against overcharge and deep discharge. Finally, when power and control have to be firmly separated, an **optocoupler** isolates the two worlds galvanically: no electrical path, the signal travels as light.

> [!tip] Tip
> **Proven references** — 5×20 glass fuse plus fuse holder; resettable PTC polyfuse; Schottky **1N5819** or **SS34** (light reverse-polarity blocking, fast flyback); **1N4007** (relay flyback); P-MOSFET **AO3401** (small currents) or **IRF4905** (high currents); **SMBJ** series TVS chosen at the rail voltage; for ESD on data lines, **USBLC6** or the **PESD** family. These are stable, common families: start from them, check availability and read the datasheet before ordering.

## Example — the 3-axis arm

On the arm of the running example, the input stage chains the three defences: a **reverse-polarity MOSFET** guards the whole system against a reversed connection, a **fuse** rated on the peak of the three stepper motors protects the power rail, a **TVS** near the connector absorbs transients from the source. The **flyback diodes** of the motors are already built into the A4988 drivers, a frequent case: check in the [[lire-une-datasheet-en|datasheet]] what the component already includes before adding your own. The 5 V logic, behind its regulator, ends up doubly sheltered.

## Pitfalls

**Under-rating the fuse.** A fuse at the level of the normal peak blows the first time a motor starts. The rating goes above the expected peak, and below the current that destroys the wiring.

**Replacing a blown fuse with a bigger one.** A fuse that blows is reporting a fault. Over-rating it removes the defence, not the fault: the next part upstream will play the role of the fuse.

**Relying on a series diode at high current.** At 2 A, an ordinary diode dissipates ~1.4 W and steals 0.7 V from the rail. Beyond a few hundred mA, the P-MOSFET is the answer.

**Forgetting the flyback diode on an inductive load.** A relay or a motor driven by a transistor with no flyback diode: the transistor dies at switch-off, first time or hundredth, but it dies.

**Picking a board up by its pins.** The kilovolts of an electrostatic discharge are not even felt, but the component feels them. By the edges, after touching a ground.

**Believing that the input stage protects everything.** It protects the supply, not the pins: a GPIO that is shorted or that receives too high a voltage is protected locally: series resistor, and compatibility of [[niveaux-de-tension-en|logic levels]].

## See also

- [[alimentation-electronique-en|Designing a power supply]] — the architecture note of which this stage is one of the moves
- [[niveaux-de-tension-en|Logic levels]] — overvoltage on the signal side (5 V on a 3.3 V pin), cousin of the one treated here
- [[pcb-en|Designing a board (PCB)]] — fitting the protection stage onto the board
- [[securite-et-qualite-en|Safety and quality]] — the safety of people, a subject distinct from protecting the hardware
- [[emc-en|Electromagnetic compatibility (EMC)]] — ESD and overvoltage protection is the hardware side of EMC immunity