Reading a multimeter

A multimeter is the one instrument that turns an invisible circuit into numbers you can compare with your calculations. It is also the instrument most likely to be used wrongly by beginners, because the three main measurements (voltage, current, resistance) each need the meter connected in a different way. If you understand why, you will not need to memorise the rules.

What is inside

A basic digital multimeter is a sensitive voltmeter plus some switchable helpers. The dial selects what the helpers do:

• Voltage mode: the meter measures the voltage between its two probes through a very large internal resistance, typically 10 MΩ.
• Resistance mode: the meter pushes a small known current of its own through the part and measures the voltage that appears, then computes R = V / I.
• Current mode: the meter routes the current through a small internal shunt resistor and measures the voltage across it, again using Ohm's law.

It has two or three sockets. COM is the common (black) probe. One socket is for voltage, resistance and continuity (marked V, Ω). A separate socket, usually marked mA or A, is for current and is connected to the shunt and, importantly, to a fuse.

Measuring voltage: in parallel

Voltage is a difference between two points, so you touch the probes to those two points without changing the circuit. The meter is connected in parallel with the part you care about, which is why measuring voltage is the easy, safe one.

1. Plug the black probe into COM and the red into the V socket.
2. Set the dial to DC voltage (a straight line, sometimes a dashed line) for batteries and microcontroller boards, or AC (a wave) for mains-derived signals.
3. Touch black to ground, red to the point you want. A negative number means the red probe is at the lower voltage.

Because the meter's input resistance is 10 MΩ, it draws almost nothing. Measuring across a 5 V supply takes 5 V / 10 MΩ = 0.5 µA. But "almost nothing" is not nothing. Measure the mid-point of a divider made from two 1 MΩ resistors on 5 V. The meter in parallel with the lower resistor gives 1 MΩ || 10 MΩ = 0.909 MΩ, so:

Vout = 5 V * 0.909 / (1 + 0.909) = 2.38 V

The meter reads 2.38 V where an ideal 2.5 V should be. This is loading. With resistors of 10 kΩ or smaller the error is negligible, but in high-resistance circuits remember that your meter is part of the circuit.

Measuring resistance: power off

To measure a resistor, the meter drives its own current through it, so the part must not be connected to anything else that has a voltage or a parallel path.

• Switch the circuit off, and discharge any capacitors.
• Ideally lift one leg of the component out of the board. A resistor still soldered into a circuit is measured in parallel with everything else attached to it, and the answer will be lower than the printed value.
• Touch the probes to both ends. Do not pinch the metal tips between your fingers: your body is a parallel resistance of hundreds of kΩ and will skew a high reading.

Compare the reading to the colour code and tolerance. A 10 kΩ resistor with a 5 percent tolerance should read between 9.5 kΩ and 10.5 kΩ.

Continuity: the beeper

The continuity setting (a speaker or diode-like symbol) beeps when the resistance between the probes is below roughly 50 Ω. It is a yes or no test of whether two points are connected, and it is the fastest way to find a broken wire, a cold solder joint, or an unintended short between neighbouring pins.

Measuring current: in series

Current is a flow through a wire, so to count it the meter must sit in the path of that flow. You have to break the circuit and insert the meter as part of the loop, so the whole current passes through it.

1. Switch the circuit off.
2. Move the red probe to the mA (or A) socket.
3. Turn the dial to the matching current range.
4. Open the circuit at one point, for example disconnect the LED's resistor leg, and connect the meter probes across that gap.
5. Switch on, read, switch off, then restore the circuit.

The meter must go in series because the shunt that does the measuring has a very small resistance. If you now connect this current meter across a voltage source, such as the 5 V and GND pins, you are connecting a near-short-circuit. Suppose the shunt is 0.5 Ω:

I = V / R = 5 V / 0.5 Ω = 10 A

The current-input fuse is typically rated 200 mA to 500 mA, so it blows instantly. The supply may also be damaged. Placing a current meter in series with a 300 Ω LED resistor in the same 5 V circuit is harmless: I = 5 V / (300 + 0.5) Ω = 16.6 mA, and the shunt drops only 16.6 mA * 0.5 Ω = 8 mV. That small drop is the burden voltage, the price of inserting a meter into the loop.

Common mistakes

• Measuring current in parallel (see above). This is the number one fuse killer.
• Measuring resistance on a powered circuit. The external voltage confuses the reading and can damage the meter.
• Wrong range or mode, such as AC when you meant DC. A DC voltage read in AC mode is typically shown as zero or garbage.
• Probe tips slipping and shorting two adjacent pins. Use sharp tips, steady hands and, better, hook clips.
• Trusting a reading from a weak meter battery. Low batteries cause erratic numbers. Replace them when the low-battery symbol appears.
• Forgetting that fast-changing signals such as PWM or a motor's noise are not DC. A cheap meter averages them: a 5 V signal on for 50 percent of the time reads about 2.5 V.

Bench checklist