The multimeter (or multitester) is an instrument that measures electrical quantities such as voltage, current, and resistance.
If you could only have one electronic tool, this would be it.
Although there are models ranging from 10€ to over 500€ (Fluke), they all have three basic parts: the display, the function selector, and the ports for the probes.
Multimeter Connection Table
| What you measure | Symbol | Connection | Circuit powered? | Red Port |
|---|---|---|---|---|
| Continuity | 🔊 | Between two points | NO | VΩ |
| DC Voltage | V⎓ | Parallel | YES | VΩ |
| Resistance | Ω | Between terminals | NO | VΩ |
| Current | A | Series | YES | mA or A, depending on the device |
The Cables: Red and Black
Before measuring anything, you need to plug in the probes.
Black Cable: Always goes to the port marked as COM (Common / Ground). It never moves from there.
Red Cable: Here you have to choose:
- Port
:: For measuring voltage, resistance, continuity, and small currents (milliamps). The red cable will live here 99% of the time. - Port 10A ⚠️: Only used for measuring large currents (motors).
Port 10A: This port typically does not go through any fuse or protection. If you misuse it, you can fry the multimeter, or worse, cause a short circuit in whatever you are measuring.
Measuring Voltage (Tension)
We want to know the potential difference between two points. Voltage is measured in parallel.
- You don’t have to disconnect anything.
- The circuit must be powered on.
DC Voltage (V⎓ or DCV)
This is for batteries, Arduino, digital electronics.
Turn the dial to V= (or to the 20V number if it’s not autoranging).
Black probe to the negative (GND).
Red probe to the point you want to measure.
If it shows negative (e.g., -5V), it means you put the probes in reverse. No problem, just swap them.
AC Voltage (V~ or ACV)
This is what we find, for example, in the mains electricity. Measuring in a socket is not a suitable practice for beginners.
Turn the dial to V~ (Usually 600V or 750V).
Do not touch the metal parts of the probes.
Insert the probes into the socket. There is no polarity here (red or black doesn’t matter).
Voltage measurement is the “least dangerous.” The multimeter acts like an enormous resistor, so it’s almost as if you aren’t connecting anything to the machine (unless you touch the metal of the probe where you shouldn’t, etc.).
Even so, do not measure the mains if you lack the necessary training. The multimeter and probes must have an appropriate CAT rating and nominal voltage for the measurement point; the selector showing 600 V is not enough to guarantee safety.
Measuring Resistance (Ω)
We want to know the value of a resistor or a potentiometer.
Turn the dial to the Omega symbol (Ω).
Place one probe on each side of the resistor.
If it shows “1” or “OL” (Over Limit), increase the scale on the dial.
The component must be disconnected from the power supply.
If you measure a resistor while the circuit is powered, the reading will be false, and you could damage the multimeter. Also discharge capacitors before measuring.
How to Measure Current ⚠️
To measure current, it must flow through the multimeter. Therefore, current is measured in series.
Turn off the circuit.
Open the circuit at the point where the current you want to measure flows.
Connect the red probe to the appropriate port. If you don’t know the current, start with the highest input and range the device allows.
Set the selector to the correct current function (AC or DC).
Insert the multimeter in series.
Turn on the circuit.
In current mode, the multimeter has a very low internal resistance.
If you connect it in parallel with a source, you can cause a short circuit, blow the fuse, or damage the instrument and the circuit.
Continuity Mode (The Beep) 🔊
It is represented by the symbol of a sound wave or a diode. The multimeter sends a small current. If the current flows from one probe to the other (near zero resistance), it beeps.
- What is it for?
- To know if a wire is broken internally.
- To check connections and solder joints.
- To look for short circuits with the circuit off and discharged.
As with measuring resistances (it is essentially the same thing), the component must be disconnected from the power supply.
