MEASURING INSTRUMENTS
Multimeter: voltage, current and resistance
Choose the correct measurement mode and socket and understand the displayed value.
The principle
A digital multimeter converts an electrical measurement into a number. In voltage mode it has high input resistance and measures potential difference between two points. DC indicates magnitude and polarity; AC usually reports an RMS value. A true-RMS meter handles non-sinusoidal AC within its specified limits. To measure current, connect the meter in series using the protected current socket. Resistance and continuity modes pass a small test current through a circuit that must be de-energized. A beep only indicates resistance below the meter’s threshold.
An example
Check a 24 V DC supply using the DC voltage mode between +24 V and 0 V. Measure a 1 kΩ resistor only after disconnecting power; parallel paths can change the reading.
Important
Never measure resistance or continuity on a live circuit. Never place a current-mode meter across a voltage source. Check probe leads, fuses, input ratings and CAT category for the installation.