Multimeter
This is the basic diagnostic instrument that can measure DC/AC voltage, resistance, continuity and, to a limited extent, current. Most measurements made during field installation are done with the DC voltage and continuity ranges. For a correct result, the range, the probe position and the load state of the circuit must be considered together.
A multimeter measures voltage in parallel and current in series. This distinction is the main reason the instrument's fuse blows when it is misused: connecting a meter set to the current range in parallel across the supply terminals creates a short circuit.
- Ranges: DC voltage is the basis of most field measurements. The resistance and continuity ranges are used only on a de-energised circuit.
- Current limit: the direct current input of handheld instruments is typically limited to 10 A and is meant for short measurements; larger currents are measured with a clamp meter.
- Internal resistance: in the voltage range the internal resistance is very high; because of this, voltage can be read even on a broken line. Taking the measurement under load removes this illusion.
- Probe care: a worn probe tip and a broken lead are a silent source of wrong readings; before measuring, touch the probes together and confirm the instrument reads zero.
When starting diagnosis in the field, first measuring how far the supply voltage sags under load quickly narrows down most installation errors.
Context in which this term is used: DC motor and driver fault diagnosis matrix