Using a Multimeter to Check a Circuit

What a multimeter is for

A multimeter combines several instruments in one handheld device. It lets you see what a circuit is actually doing instead of guessing. For most hobby work, three functions cover nearly everything: voltage, continuity or resistance, and, with more care, current.

Safety before you start

Keep to low voltage, such as batteries, USB power and small regulated supplies, while learning. Never probe mains-powered equipment unless you are qualified, and leave that work to a qualified professional. A meter used on the wrong setting or the wrong input can be damaged and can create a hazard.

Good habits:

• Check the meter’s own markings and ratings. Use a meter rated for the kind of work you do, and inspect the leads for cracks and damaged insulation.
• Hold probes by their insulated handles and keep fingers away from the metal tips.
• Do not measure resistance or continuity on a powered circuit.
• Remove the probes from the circuit before changing the dial setting.
• Do not try to measure a battery or supply by setting the meter to current, because that places a very low resistance across it.

The dial and the jacks

Most meters have a rotary dial, a display and two or three sockets. The black probe goes into the socket marked COM, the common. The red probe goes into the socket for voltage and resistance, usually marked with a V and an ohm symbol. A separate socket is used for current, often with its own fuse. Putting the red probe in the current socket while measuring voltage is a classic mistake, because it effectively shorts the circuit and can blow a fuse.

Dial areas to recognize:

• DC voltage, marked V with a straight line or V DC. Used for batteries and most hobby circuits.
• AC voltage, marked with a wave. Used for mains and some signals, which a beginner should not probe.
• Resistance, marked with the ohm symbol.
• Continuity, often shown with a diode or sound symbol, which beeps when two points are connected.
• Diode test, which shows the forward voltage of a diode.
• Current, A or mA, with the meter in series.

If your meter is not auto-ranging, start at a high range and step down so the display shows a useful number.

Measuring voltage

Voltage is measured across two points, so the meter goes in parallel with the part. Touch the black probe to ground and the red probe to the point you want to check, with the circuit powered.

Useful checks:

• Supply voltage. Measure the battery or supply before connecting it to the circuit. A nominal 5 volt supply might read a little above or below that. Large departures are worth investigating.
• Voltage at the part. Measure across a chip’s power pins to confirm it receives the voltage it expects. A reading much lower than the supply may mean a poor connection or a supply that sags under load.
• Voltage drop. Measuring across a resistor, or across an LED, shows how the supply is shared. In a hypothetical LED circuit on 5 volts, you might see roughly 2 volts across the LED and the rest across the resistor, in line with Ohm’s law as explained in the discussion of voltage and current.

Continuity and resistance

With the power off, continuity mode tells you whether two points are connected. Touch the probes to the ends of a wire, a fuse or a switch, and a beep means a continuous path. It is a good way to check:

• whether a wire or jumper is broken
• whether a breadboard rail is split, as often happens on a solderless breadboard
• whether two points that should be separate are shorted together
• whether a switch closes when pressed

Resistance mode gives a value in ohms. For a resistor, remove it from the circuit or lift one leg, because other parts in parallel can distort the reading. Compare the result with the marked value, allowing for tolerance, which for common resistors is often a few percent.

Measuring current

Current is measured by breaking the circuit and inserting the meter in series, so that the current flows through it. This is the riskiest measurement because the meter has very low resistance in this mode. Use it only on low-current circuits, put the leads in the current socket, begin on the higher current range, and never connect the meter across a supply. If the circuit might draw more than the meter’s range, do not measure it this way. Many people simply calculate current from a voltage drop across a known resistor instead.

Reading what you get

A reading is useful only if you know what to expect. Before measuring, write down what each point should read, from your circuit diagram. Then compare:

• Reading matches expectation: that part of the circuit is probably fine.
• A supply point reads zero: look upstream for a break, an off switch or a dead battery.
• A supply point reads low: suspect a weak battery, a poor connection or too much load.
• A point that should be steady wanders: it could be a floating input, as explained in the article on unconnected pins.

A short routine for a dead circuit

1. Power off, check continuity of power wiring and grounds.
2. Power on, measure the supply at its source, then at the board.
3. Measure the supply at each chip’s power pins.
4. Check that inputs and outputs sit at plausible levels.
5. Change one thing at a time, and write down results.

This fits within the wider approach to circuit troubleshooting, where the aim is to narrow down the fault rather than replace parts at random.