Skills books › Introduction to Electronics

How to use a multimeter

A basic digital multimeter costs less than a good pair of shoes and lasts for years. One rule covers most of what goes wrong with it: voltage is measured across, and current is measured through.

This guide is for low-voltage, battery-powered circuits. Measuring mains electricity safely needs training and the right equipment, and is not covered here.

Choosing a meter

Look for:

• DC voltage, DC current (milliamps and a 10 A range), resistance, continuity with a beeper, and a diode test;
• auto-ranging, which is easiest for beginners;
• fused current sockets;
• probes with finger guards and flexible leads;
• a clear display with a backlight.

The dial and the sockets

• COM: the black probe, always.
• VΩ: the red probe, for voltage, resistance and continuity.
• mA: the red probe, for small currents.
• 10 A: the red probe, for large currents, for a few seconds only.

Check the dial and the red probe's socket every time you pick the meter up. A red probe left in a current socket from the last job is how fuses get blown.

Measuring voltage

The probes go across the part or battery, while the circuit is working.

1. Black probe in COM, red in VΩ.
2. Turn the dial to DC volts. On a manual meter, choose a range above the voltage you expect.
3. Touch the black probe to the negative side and the red probe to the point you want to measure.
4. Read the display. A minus sign only means that the probes are the wrong way round.

Voltage is the first thing to measure when a circuit misbehaves, because the meter draws almost no current and does not upset the circuit.

Is this battery flat?

• AA or AAA alkaline: about 1.5 to 1.6 V when new, flat below about 1.2 V.
• 9 V alkaline: about 9 to 9.6 V when new, flat below about 7 V.
• 12 V sealed lead-acid at rest: about 12.7 to 12.9 V when full, and in need of charging well before 12.0 V.

A battery can read well with nothing connected and then sag badly under load. If the reading looks good but the device still fails, measure while the device is running.

Measuring current

The meter must become part of the loop.

1. Switch off the power.
2. Black probe in COM, red in mA, or in 10 A if you do not know what to expect.
3. Turn the dial to DC current.
4. Break the circuit at one point and connect the probes into the gap.
5. Switch on, read, switch off, and put the red probe back in VΩ straight away.

Never put the probes across a battery on a current setting. The meter has almost no resistance in that mode, so you would make a short circuit.

Resistance

The meter passes its own small current through the part, so the power must be off. Lift one leg of the part out of the circuit for an accurate reading, and keep your fingers off the probe tips. "OL" means that the resistance is above the range or the path is broken.

Continuity

With the power off, the meter beeps when there is a path between the probes. It is the quickest way to find a broken wire, test a switch, or check a fuse. A good fuse beeps. A blown one does not.

Good habits

Write down what you expect before you measure, and record each reading with its unit and where you took it. A reading within about 10% of your estimate is usually fine. One that is double or half is a clue.

This guide is based on Chapter 4 of Introduction to Electronics.