--- title: Multimeter lang: en type: tuto tags: - eee - tuto prerequis: - instruments-de-mesure-en - niveaux-de-tension-en aa: [] phases: - preuve-de-concept - integration-et-tests draft: false source_fr: embarque/mesure/multimetre.md source_sha256: 3769ac155d3ac1478a948f226260c9ecc357bd59ca9acdeedd5b450ce2327836 --- **The multimeter** is every electronics person's basic measuring instrument: it measures a **voltage**, a **current**, a **resistance**, and tests the **continuity** of a connection. It gives a **single value** — a number, not a waveform: to watch a fast-changing signal, the [[oscilloscope-en|oscilloscope]] is the tool. Sturdy and present on every bench, it answers the vast majority of diagnostic questions: "is there 5 V here?", "is this track broken?", "what is the value of this resistor?". This page is a tool tutorial of the [[instruments-de-mesure-en|measuring instruments]] hub. ![Front panel of a multimeter in functional blocks: the screen at the top, the rotary selector in the middle with its V⎓, V∼, Ω, continuity and A positions, and the three terminals at the bottom — COM for the black lead, V/Ω for voltage, resistance and continuity, 10 A for current only.|600](/ressources/img/multimetre/face-avant.svg) ## What is it for? The common functions of a multimeter: - **DC voltage (V⎓ / DC)** — checking a power supply (5 V, 3.3 V, battery voltage), reading a logic level; - **AC voltage (V∼ / AC)** — measuring an alternating signal (**RMS** value) — rare in a project that lives on DC (mains 230 V stays out of bounds, cf. [[instruments-de-mesure-en|the hub]], *Voltage limits*); - **resistance (Ω)** — reading the value of a resistor, checking a resistive sensor (thermistor, photoresistor); - **continuity** — a beep when the resistance is near zero: testing in seconds whether a track, a solder joint or a cable is properly connected; - **current (A)** — measuring the current drawn by a build (in series, see below); - often a **diode test** too, and sometimes **capacitance** or **frequency**. For signals that **change over time** ([[pwm-en|PWM]], serial frame, contact bounce), the multimeter is not enough. It displays an average or an unstable value: switch to the [[oscilloscope-en|oscilloscope]]. ## Measuring step by step 1. **Choose the function and the range** with the selector: type of quantity (V⎓, V∼, Ω, A) and, on a manual meter, the appropriate range (start wide). *Auto-range* meters pick the range on their own. 2. **Plug the leads into the right terminals.** The black lead always goes into **COM**. The red one goes into the **V/Ω** terminal to measure voltage, resistance and continuity — or into the **current** terminal (often "10 A" or "mA") only to measure a current. *This is the most error-prone point (see Pitfalls).* 3. **Connect to the circuit according to the quantity:** - **voltage** → probes **in parallel** across the two points whose potential difference you want (the circuit stays powered); - **resistance / continuity** → component **with power off**, ideally isolated from the rest of the circuit; - **current** → **in series**, opening the circuit to insert the multimeter. 4. **Read the value** and its unit, taking the range into account — and without over-reading the last digit: [[precision-de-mesure-en|resolution is not accuracy]]. ![Two setups compared: on the left a voltmeter connected in parallel across a resistor (circuit closed), on the right an ammeter inserted in series into the circuit (opened to place it).](/ressources/img/multimetre/serie-parallele.svg) > [!tip] Measuring a current without breaking the circuit > The field reflex: measure the **voltage across a known resistor** of the circuit, then divide — I = V / R. No circuit to open, no fuse at stake. It is the principle of the crossed *Sense* pins in [[lire-une-datasheet-en|the L298N datasheet]]. > [!tip] > **Continuity mode is troubleshooting's best friend.** Before suspecting a component, check with the beep that every link is properly made: a broken cable, a cold solder joint or a cracked track is found in seconds, with the build switched off. ## Example — Checking a board's power supply A microcontroller board does not start. Before anything else, check "is the power getting there, and at the right voltage?". 1. **Supply** — selector on **V⎓**, COM on the board's ground (GND), red probe on the **5 V** pin: expect ≈ 5 V. A very low value (3 V, 0 V) points to an insufficient supply, a *charge-only* USB cable, or a short circuit dragging the voltage down. 2. **Drop across an LED** — probes across a lit LED: you read its forward voltage (≈ 2 V for a red one, ≈ 3 V for a blue one). This confirms it is properly polarised and powered. 3. **Continuity of a track** — build **switched off**, continuity mode, one probe at each end of a suspect link: a beep confirms it passes, silence reveals the break. In three simple measurements, you have narrowed the problem down to the supply, a component or a connection — without desoldering anything blindly. ## Pitfalls **Ammeter left in current position, reconnected in parallel.** The classic mistake: after a current measurement, you forget to put the red lead back into the V/Ω terminal and the selector back on voltage. Next time, you put the probes in parallel "to measure a voltage", but the meter is still an ammeter: short circuit, **blown fuse**. The internal fuse is precisely a [[protection-electronique-en|protection]] device: it sacrificed itself for the meter. Reflex: return to voltage measurement as soon as a current measurement is done. ![In three steps, the chain that blows the fuse: a correct current measurement, red lead in the 10 A terminal; then the selector back on V⎓ while the lead stays, forgotten, in 10 A; then the probes placed in parallel on a 5 V source, where the meter, still an ammeter, behaves as a plain wire — short circuit, melted fuse.|640](/ressources/img/multimetre/fusible-grille.svg) **Measuring a resistance under power.** The ohmmeter function injects its own current: applied to a powered circuit, it reads garbage and can damage the meter. A resistance is measured **with power off**. **Reading the wrong range.** On a manual meter, a range too small saturates ("1" or "OL" on the display), a range too large loses accuracy. Set the range, or take an *auto-range* meter. **Mixing up DC and AC.** Measuring DC on the AC range (or the reverse) gives a wrong value. Check the ⎓ / ∼ position of the selector. **Forgetting the loading effect.** On a very high-impedance node, the small current drawn by the voltmeter can pull the read voltage down. Rare in a project, but worth knowing (see the hub, *Measuring is not neutral*). ## See also - [[instruments-de-mesure-en|Measuring instruments]] — the hub: method and choice of instrument - [[oscilloscope-en|Oscilloscope]] — when the value is not enough and you need to see the *shape* of the signal - [[niveaux-de-tension-en|Logic levels]] — the thresholds you check with the multimeter (prerequisite) - [[lire-une-datasheet-en|Reading a datasheet]] — the expected value to compare the measurement with - [[gpio-en|GPIO]] — checking with the multimeter the real state of a pin (HIGH / LOW)