Electronics
How to use a multimeter
A multimeter answers two main questions: how hard is the push, in volts, and how much flows, in amps. To measure voltage, put the probes across a part, one each side. To measure current, break the loop and put the meter in the gap, so the current flows through it.
Volts: across
Voltage is always between two points, so a voltmeter compares two points. Set the dial to volts, black lead in COM and red in V, and touch the probes either side of the part with the circuit running.
A good voltmeter has a huge resistance, about 10 MΩ, so across 9 V it draws under 1 µA and the circuit barely notices. To read the voltage "at" a point, put the black probe on ground, usually the battery's − terminal, and the red one on the point. A meter between two points reads the difference: 5 V and 2 V from ground are 3 V apart.
Amps: in the loop
An ammeter counts the charge flowing through it, so it has to be part of the loop. Switch off, move the red lead to the A (or mA) socket, open the circuit at one point and connect the meter into the gap. In a single loop the current is the same everywhere, so any gap gives the same reading.
Live experiment
Plug the meter in
A 9 V battery, a 330 Ω resistor and Pip, a red LED, with about 21 mA flowing. Set the meter to volts or amps, then put it across Pip, into a gap in the loop, or across the battery. One of the six blows the fuse.
“Measuring with meters” is lesson 3 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 9 before it first.
An ammeter's resistance is nearly zero, so it barely slows the current it measures. That's exactly why it's dangerous in the wrong place.
The opposite slip is harmless but confusing: a voltmeter in the loop adds 10 MΩ, almost no current flows, and the LED goes out.
Ohms, and meter loading
To measure resistance, switch the circuit off. The meter pushes its own small current through the part and works out R = V ÷ I, so any other voltage confuses it, and parts in parallel change the reading. Lift one leg of the part to be sure.
No meter is perfect. Across a high-resistance circuit, the voltmeter's 10 MΩ sits in parallel with what you're measuring. Two 1 MΩ resistors on 9 V should give 4.5 V in the middle, but 1 MΩ ∥ 10 MΩ ≈ 0.91 MΩ, so the meter reads about 4.29 V, roughly 5% low. With two 100 kΩ resistors the error shrinks to about 0.5%.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Measuring with meters” is lesson 3 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 9 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 3 a second. In one loop the current is the same everywhere. Charge can't pile up or go missing, so whatever passes A also passes B.
- Why not “None at all”? If B counted nothing, charge would be piling up somewhere between the windows. In one loop it all keeps moving.
- Why not “More than 3”? There's nowhere extra charge could come from in a single loop. Whatever passes A also passes B.
- Why not “Fewer than 3”? Charge can't pile up or vanish anywhere in the loop, so nothing is lost between A and B.
Show the answer
Answer: 0.25 A. Milli means one-thousandth, so 250 mA = 250 ÷ 1000 = 0.25 A.
- Why not “0.025 A”? That divides by 10,000. 0.025 A is 25 mA.
- Why not “2.5 A”? That divides by 100 instead of 1000. 2.5 A would be 2500 mA.
- Why not “25 A”? That divides by 10 instead of 1000. 25 A would be 25,000 mA.
Show the answer
Answer: 4,700 Ω. k (kilo) means × 1000, so 4.7 kΩ = 4.7 × 1000 = 4,700 Ω.
- Why not “47,000 Ω”? That multiplies by 10,000. 47,000 Ω is 47 kΩ.
- Why not “47 Ω”? That multiplies by 10 instead of 1000. 47 Ω is only 0.047 kΩ.
- Why not “470 Ω”? That's 0.47 kΩ. Kilo means a thousand, so 4.7 kΩ is ten times bigger.
“Measuring with meters” is lesson 3 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 9 before it first.
Questions people ask
How do you measure voltage with a multimeter?
Set it to DC volts, black lead in COM, red lead in the V socket. Touch the probes to the two points you want to compare, one each side of the part, with the circuit running. You don't disconnect anything. Red on the more positive point gives a positive reading.
How do you measure current with a multimeter?
Move the red lead to the amps socket and set the dial to amps. Switch off, break the loop at one point, and connect the meter into the gap so the current flows through it. Switch on and read. Then move the red lead back to V, or the next voltage reading becomes a short.
Why did my multimeter's fuse blow?
Almost always because it was on amps and put across something, often straight across a battery. On amps the meter is nearly a plain wire, so it shorts whatever it touches and a huge current flows. The fuse in the amps socket blows to save the meter.
How do you measure resistance with a multimeter?
Switch the circuit off first, and ideally take the part out, because other parts in parallel change the reading. Set the meter to Ω and touch the probes to its two ends. The meter pushes its own small current through it and works out R = V ÷ I.
Related topics
- What is voltage?The push that makes charge flow: what a volt is, and why stacked cells add up.
- What is electric current?How much charge flows past a point each second, in amps, and why it's the same all round a loop.
- Kirchhoff's laws explained: KCL and KVLWhat flows into a junction flows out, and the drops round a loop add up to the rise.
- Voltage divider explainedTwo resistors that split a voltage: the formula, potentiometers, and why a load drags the output down.