How to use a multimeter: practice on a virtual one first
- 192parts on the bench
- 25boards running now
- 1.00xreal time, on every board
A multimeter measures three things most of the time: voltage, current and resistance. It is simple to use once you know one rule for each, and easy to damage, or to get a wrong reading from, if you do not. The meter in the circuit above behaves like a real handheld: the dial changes what is between the leads, the current jack has a fuse, and the leads you connect decide what you are measuring.
The practice bench is a 9 V supply, a 1 k resistor and a red LED. The meter starts on DC volts across the resistor and reads 7.40 V. Work through the sections below in order. Each one says where to put the leads and what you should see.
The jacks and the dial
There are three jacks. COM is the black lead, every time. V is the red lead for volts, ohms, continuity, diode, capacitance and frequency. A is the red lead for current. Many real meters have two current jacks, a milliamp one and a 10 A one; this meter has one.
The dial picks what to measure: V with a straight line for DC volts, V with a wave for AC volts, the same two for amps, the omega symbol for ohms, and the sound-wave symbol for continuity. On this meter, turning the dial restarts the simulation. On a real meter, the equivalent habit is to take the leads off before you turn the dial.
Measure voltage: across the part
Voltage is a difference between two points, so the leads go on either side of the part, with the circuit running. On the bench, the meter reads 7.40 V across the 1 k resistor. Move the red lead to the LED’s anode and the black lead to ground and it reads about 1.60 V, the LED’s share. Add them and you get the 9 V the supply gives.
Swap the red and black leads and the reading goes negative. Nothing is wrong: the meter is telling you the red lead is on the lower side.
Measure current: in series, never across
Current is a flow, so the meter has to be in its path. That means breaking the circuit and putting the meter in the gap: disconnect the wire from the supply to the resistor, move the red lead to the A jack, connect the supply to A and COM to the resistor. Set the dial to DC amps. It reads 7.4 mA, which is the 7.40 V across 1 k from the last step. Ohm’s law agrees with the meter.
On amps, the meter is nearly a short circuit: about 1 ohm between A and COM. That is what makes it accurate in series, and what makes it dangerous anywhere else.
The blown-fuse mistake
Everyone does this once. The meter is still on amps from the last job, and you put the leads across a supply to check its voltage. The meter is now a 1 ohm short across the supply. On a real meter the fuse blows, if you are lucky; on a meter with an unfused 10 A jack, the leads or the supply can get very hot.
Open the blown-fuse circuit and press Run. The supply is set to allow 3 A and the meter’s fuse is rated 2 A, so the fuse goes almost at once and the face says so. After that, the current range reads zero through everything, which is the confusing symptom of a blown fuse on a real meter. Press the button on the face to fit a new fuse, move the red lead to V and set the dial to DC volts, and it reads the supply’s 5 V.
Measure resistance and continuity: power off
The ohms range pushes its own small test current through the part and measures the voltage it makes. Any other current in the circuit spoils that, so turn the supply off first. Ideally lift one leg of the part out of the circuit too, so you measure the resistor and not everything wired around it. Across the 1 k resistor alone it reads 1.000 k.
Continuity is the same measurement with a beep: the meter sounds below about 30 ohms. Use it to check that a wire, a jumper or a switch actually connects. Across a resistor it stays silent. The rest of the dial, including diode test and capacitance, is on the unpowered board in the second practice circuit linked below.
On a real meter: safety first
Start with batteries and low-voltage circuits, 12 V or less, like everything on this page. Do not measure a wall outlet or anything connected to one until someone qualified has shown you how, and only with a meter rated for it. Real meters carry a CAT rating on the front for that reason.
Check the leads before every use, and look for cracked insulation or exposed metal. Keep your fingers behind the finger guards on the probes. Replace a blown fuse only with the exact type the manual lists. And put the dial back on volts when you finish, so the next person does not start on amps.
Questions
Why does my multimeter read zero current?
Most often the meter is not in series with the current, or the fuse in the current jack has blown. Check the red lead is in the A jack, the circuit is broken where the meter goes, and then check the fuse.
Why does my resistance reading look wrong?
The circuit is powered, or other parts are in parallel with the one you are measuring. Turn the power off and lift one leg of the part out of the circuit.
Is the virtual meter accurate?
It reads the simulator’s own answer with no noise or drift, and its 10 megohm input and 1 ohm current shunt are modeled, so it loads a circuit the way a real meter does. A real meter adds an accuracy tolerance on top, printed in its manual.
Build this for real
Open the editor, change a value and watch the number move with it. Nothing to install, and no account needed.