LessonUsing a multimeter · 13 min

Measuring resistance and continuity

On Ω the meter becomes a tiny power source: it pushes a small known current through whatever is between its leads and measures the voltage that results. That is why resistance is measured with the circuit switched off - any other voltage would upset the reading (in the simulator, the meter then shows "- - - -").

The in-circuit trap

The meter measures everything connected between its probes. In a circuit, other parts almost always form parallel paths, so the reading comes out lower than the part's real value. To measure a part properly, unsolder (lift) at least one of its legs.

• Power off, and let big capacitors discharge first.
• Lift one leg of the part, or measure it before soldering.
• Do not hold both probe tips: your body (100 kΩ or more) adds a parallel path - it matters when measuring high values.
• Short the probes together first for small values: the leads themselves read 0.1 - 0.3 Ω; subtract that.

Continuity: the beeper

Continuity mode beeps when the resistance between the probes is very low (below roughly 30 - 50 Ω, depending on the meter). You do not need the number - only "joined or not". It is the quickest fault-finding tool there is.