Multimeter basics: voltage, continuity, resistance and current, safely

A multimeter is the first tool to own, even before a soldering iron. Most “why doesn’t it work” questions are really “is power getting there?”, “is this wire broken inside?” or “is this the right resistor?”, and a meter answers each in ten seconds. This lesson covers the four modes you actually use on the bench (voltage, continuity, resistance and current) and the one mistake that blows fuses most often.

Every example stays in low-voltage territory: batteries, USB 5 V cables and 3.3 V ESP32 boards.

The dial and the jacks

A typical digital multimeter has three jacks:

• COM: the black lead always goes here.
• V Ω mA (some meters split this into VΩ and mA): the red lead goes here for voltage, resistance, continuity and small currents.
• 10A: move the red lead here to measure large currents.

On the dial, V with a straight line (V⎓) is DC, which is almost everything we measure. V with a wavy line (V~) is AC. The sound-wave symbol is continuity.

On a manual-range meter, turn to a range above the value you expect. The 20 V range reads 0.00 to 19.99 V. If the display shows only a 1, or OL, the value is over the range; step up.

Next to the jacks you will see a CAT rating (CAT II, CAT III). It tells you which part of an electrical system the meter is built to survive transients from. The lowest, CAT I, is protected electronic equipment not wired straight to the mains, which covers everything on this site. CAT II and up are plug-in appliances and building wiring.

Mains wiring is not part of this lesson, whatever voltage the meter claims to handle. Anything that plugs into the wall is an electrician’s job; we stay with batteries and USB.

DC voltage: in parallel, circuit powered

Voltage is the difference between two points, so you measure across those two points with the circuit powered.

1. Black in COM, red in V.
2. Turn to V DC, on a range above the expected value.
3. Black on negative or GND, red on the point you want to know.

A negative reading means the leads are swapped. Nothing is harmed; swap and measure again.

Reference voltages for the 3.7 V lithium cells (LiPo/Li-ion) common in maker projects:

State	Voltage per cell
Fully charged	4.2 V
Nominal (average)	3.7 V
Considered empty	3.4 V
Protection circuit cuts off	3.0 V

Example: a LiPo in a toy reads 3.92 V. That sits between 3.7 and 4.2, so it still has charge. A reading of 3.35 V means it is empty; recharge it only with a charger designed for lithium cells. If a cell is swollen, hot or smells, stop using it at once.

Worked example: the USB cable that resets the board

Symptom: an ESP32 resets on its own when Wi-Fi connects or several LEDs turn on. You suspect the power.

USB 2.0 allows the port to supply 5 V ±5%, i.e. 4.75 to 5.25 V, and allows the cable and connectors to drop it to about 4.35 V at the far end. Measure step by step:

1. Plug the board into the computer with the usual cable. Black on the board’s GND pin, red on the 5V pin (sometimes marked VIN or VBUS).
2. With the board running normally: 4.85 V.
3. Swap in a thin 2 m cable and measure again: 4.30 V, below the 4.35 V the standard allows for.
4. Conclusion: 4.85 − 4.30 = 0.55 V is lost in the cable. Go back to a short, thicker cable and the resets stop.

One fault a meter cannot catch: the voltage is a healthy 5 V but the computer never sees the board. The cable may be charge-only, with no data wires. Try a cable you know transfers files before blaming drivers.

Continuity: is the wire broken inside?

In this mode the meter beeps when the two probes are connected through a few ohms or less. Use it to find jumper wires broken inside their insulation, joints that didn’t take, or breadboard rails that are split halfway.

1. Power off first, always. Unplug the battery and the USB cable.
2. Turn to the sound-wave symbol and touch the probe tips together; you should hear the beep.
3. Touch the two ends of whatever you are checking.

Probing across a power rail with a capacitor on it may give a short beep that then stops. The capacitor is charging from the meter. That is normal.

Resistance: power off, part out

1. Power off. If you can, pull the resistor out and measure it on its own; anything connected in parallel skews the reading.
2. Ω mode, range above the expected value.
3. Touch the two legs. Don’t hold both legs with your fingers; your body becomes a resistor in parallel.

Example: a 220 Ω resistor with ±5% tolerance (gold band).

• 5% of 220 = 11 Ω
• Acceptable range = 220 − 11 to 220 + 11 = 209 to 231 Ω
• A reading of 218 Ω is fine. 150 Ω means you grabbed the wrong value.

On the 20 kΩ range of a manual meter the display shows 0.22, which you read as 0.22 kΩ = 220 Ω. Readings include the resistance of the test leads too: touch the tips together to see your meter’s baseline and subtract it when measuring small values.

Current: break the circuit, let it flow through the meter

Current is how much flows along a path, so you measure it in series: open the circuit at one point and put the meter in the gap. In current mode the inside of the meter is close to a plain wire. Probe across a battery, or across 5V and GND as if measuring voltage, and you short the supply through the meter; the mA fuse blows instantly.

Example: the current through an LED driven from a 3.3 V GPIO pin through 220 Ω (the maths is in Wire an LED to an ESP32).

1. Expect first: if a red LED drops about 2.0 V, current ≈ (3.3 − 2.0) ÷ 220 ≈ 5.9 mA.
2. Power off. Pull the resistor’s LED-side leg out of the breadboard.
3. Move the red lead to the mA jack; turn to the 200 mA DC range.
4. Red on the loose resistor leg, black on the row where the LED leg sits. Current now has to pass through the meter.
5. Power on. It reads about 5–6 mA, matching the calculation.
6. Power off, move the red lead back to V, then restore the circuit.

The reading may wander a little because the meter averages over time. If you are not sure the current will stay inside the mA range (say, a whole board with Wi-Fi, or a motor), start on the 10A jack. That jack only resolves to about 0.01 A, so if the reading is small, move down to mA.

The four modes in one table

Mode	Red lead in	Circuit power	Connected
DC voltage	V	On	Parallel (across two points)
Continuity	V Ω	Off	Across the two ends
Resistance	V Ω	Off	Across the part (best removed)
Current	mA or 10A	On while reading	Series (in the path)

Common mistakes

• Probing across a supply in current mode. The fuse blows, or with a lithium cell that can deliver a lot of current, the leads can get dangerously hot.
• Forgetting to move the red lead back to V. Your next voltage measurement becomes a short circuit.
• Replacing a blown fuse with a bigger one. The fuse is the only thing protecting the meter and your hand. Use only the type and rating printed on the meter.
• Measuring resistance with the power on. The number means nothing and can damage the meter.
• Using leads with cracked insulation. Check them before every use; replace damaged leads.
• Leaving the meter switched on. Its battery runs down and readings drift without you noticing.

Next: Soldering basics

Sources

• How to Use a Multimeter — SparkFun Learn
• Guide to Digital Multimeter Safety — Fluke
• IEC Category Ratings: Use the Right Tools for the Job — Fluke
• Li-Ion & LiPoly Batteries: Voltages — Adafruit Learning System
• TIDUF26 Design guide (USB VBUS range) — Texas Instruments