The multimeter, and what a reading actually tells you

Why this matters

A multimeter is the only instrument in this exam that turns an opinion into a number. Everything else in domain 5 is substitution and inference; this is measurement, and a measurement can be recorded, compared against a specification, and handed to somebody else.

It is also the cheapest useful tool a technician can own. The basic functions — DC voltage, continuity and resistance — cover everything Core 1 asks for, and a meter capable of all three costs very little.

The exam asks which setting answers which question, and it asks about safety. Both matter: the wrong setting produces a meaningless reading or a damaged meter, and the wrong measurement on a mains circuit is the one genuinely dangerous act available in this certification.

The lesson

Voltage, current, resistance and continuity, and which one answers which question

Four functions, four questions.

• Voltage. The difference in potential between two points. Measured in parallel, with the probes across the thing you are interested in, while the circuit is powered. This answers "is the right voltage arriving here?" and it is the function used most.
• DC for anything inside a computer: power supply rails, batteries, fans.
• AC for mains, and mains measurement is covered by the safety section below.
• Continuity. Is there an unbroken path between these two points? The meter emits a tone when resistance is near zero. Measured with the circuit unpowered. This answers "is this cable, fuse or switch intact?" and it is the second most used function.
• Resistance, in ohms. The same idea with a number instead of a tone, measured unpowered. Useful for checking a component against its rating and for distinguishing a slightly degraded path from a broken one.
• Current, in amps. Measured in series, which means breaking the circuit and putting the meter in the path. This is the one rarely needed in computer work, and the one most often done wrongly.

The single most common beginner error is measuring current in parallel. A meter set to current is close to a short circuit by design, so placing it across a voltage source creates one — blowing the meter's fuse at best.

A useful discipline: set the function and range before touching the probes to anything, and return the meter to a voltage setting when you put it down, so the next measurement cannot be made with it still on current.

Safety before anything else: what you do not measure, and why

From lesson 2, restated where it is needed:

• You do not open a power supply. The capacitors inside hold a lethal charge after disconnection. There is nothing serviceable behind the cover, and measuring a supply is done at its output connectors from outside.
• The same applies to cathode ray displays and laser printer high-voltage supplies, both of which are within this exam's scope.
• Measuring mains voltage is not a beginner activity. A meter and probes must be rated for the circuit category, the probes must be in good condition, and the technique — one hand, no contact with anything earthed — matters. If the question is whether a socket is live, a socket tester is the right tool and it is designed to be used safely by anyone.
• Do not wear a grounded anti-static strap while working near mains. The strap's purpose is to hold you at the chassis potential for static reasons, and it is the wrong thing entirely around live circuits.
• Power down and unplug before continuity and resistance measurements. Both functions apply a small voltage of their own, and both give wrong readings — or damage the meter — on a live circuit.

The general rule that covers the cases not listed: if the measurement requires you to be inside an enclosure that carries mains, it is not a Core 1 measurement. The work in scope is on the low-voltage side, at connectors, on cables, and on components that are disconnected.

Measuring in circuit against out of circuit, and the errors each introduces

A measurement taken with the component still connected to everything else is an in-circuit measurement, and it is convenient and often misleading.

• Resistance in circuit reads low, because other components in parallel offer additional paths. A resistor reading well under its rating may be perfectly fine, with the rest of the board measured alongside it.
• Continuity in circuit can produce false continuity for the same reason — a path through neighbouring components looks like a connection.
• Voltage in circuit is the correct way to measure voltage, because voltage is meaningful only in the working system.

So: voltage in circuit and powered; continuity and resistance out of circuit and unpowered. That pairing avoids most of the errors available.

Two more sources of wrong readings worth knowing:

• Bad probe contact. Corroded connectors, an oxidised surface, or a probe resting rather than pressing. A continuity test that fails should be repeated with deliberate contact before it is believed.
• The meter's own leads. Touch the probes together first: the reading should be near zero ohms and the tone should sound. A meter with a broken lead reports everything as open, which looks exactly like a cable fault.

That last check takes two seconds and proves the instrument before the instrument is used to prove anything else — the same discipline as every check in this repository being shown to fail before it is believed.

Continuity testing a cable, a fuse and a switch, and reading the result correctly

Three archetypal continuity tests, all unpowered.

A cable. Probe the same conductor at both ends. Tone means intact; silence means broken. For a multi-conductor cable, test each conductor in turn, and also test between conductors — continuity between two conductors that should be separate is a short, which a per-conductor test would never reveal.

This is what a cable tester automates for network cable, from lesson 17. The meter does the same job one conductor at a time, and it is what you use for a power cable or an adapter lead that no tester covers.

A fuse. A good fuse is a piece of wire and reads continuity. A blown fuse reads open. This is a definitive test, and it is worth doing before replacing anything else — though a fuse that blows again immediately is telling you about a fault downstream rather than about itself.

A switch. With the switch open there should be no continuity; with it closed there should be. A power button that never makes contact is a real and findable fault, and it explains a machine that does nothing when pressed while its board is fine — the lesson 27 scenario, now measurable.

Reading the result correctly means knowing what each state should be before measuring. Continuity across a switch is a fault if the switch is open and correct if it is closed, and a technician who has not decided which state they are testing cannot interpret the tone.

Recording a measurement so it is evidence rather than a recollection

The reason to use a meter at all is that it produces something transferable. Wasting that by not writing it down is common and avoidable.

What a recorded measurement needs:

• What was measured, precisely: "12 V rail at the 24-pin connector, pin 10".
• The value, with units.
• The conditions: powered, unpowered, under load, at idle, machine warm or cold.
• The expected value and tolerance, so the reading has meaning to a reader who does not know the specification.
• The time, which matters for anything intermittent.

Compare two handovers. "The power supply seems weak" cannot be acted on. "12 V rail measures 11.2 V under load at the 24-pin connector, against a specification of 11.4 V minimum; 11.9 V at idle" names the fault, supports the replacement decision, and lets the next person verify it after the change.

This is lesson 34's documentation step applied to the one part of the job that produces numbers. It is also what makes a repair defensible: a part replaced because it measured out of specification is a decision with evidence behind it, and a part replaced because it was suspected is a guess that happened to be recorded.