LessonUsing a multimeter · 14 min

Meet the multimeter

A multimeter is the electronics engineer's first and most-used instrument: a voltmeter, an ammeter and an ohmmeter in one box. Used well, it answers nearly every "why does this not work?". Used badly, it blows its fuse - or misleads you. Electronics Studio's multimeter behaves like a real one, so the habits you build here carry straight over.

Its parts

• The display: the reading, with its unit and a minus sign when the leads are the other way round. "OL" means overload - more than the range can show (or an open circuit on Ω).
• The dial: what it measures. V⎓ (DC volts), V~ (AC volts), A⎓ / A~ (current), Ω (resistance, often with the continuity beeper and diode test on the same position).
• The sockets: COM - the black lead, always. VΩ - the red lead for volts, ohms and continuity. 10A (and often a separate mA) - the red lead for current only.

What is inside each mode

• Volts: a very high resistance (about 10 MΩ) between VΩ and COM. It takes almost no current, so it can be connected across anything without disturbing it much.
• Amps: a tiny resistance (a shunt, about 0.02 Ω on the 10 A range) between the A socket and COM, behind a fuse. The meter measures the small voltage across the shunt - Ohm's law again. Because it is almost a short circuit, it must only ever be put IN SERIES.
• Ohms: the meter pushes a small known current through the part and measures the voltage that results (R = V / I). Any other voltage in the circuit spoils this - hence power off.

DC, AC, ranges and accuracy

DC (⎓) is steady: batteries, USB, solar panels. AC (~) reverses direction: the 230 V 50 Hz mains. On AC a meter shows the RMS value - the DC voltage that would give the same heating. "230 V" mains actually swings to ±325 V peak (230 × √2).

Most modern meters are auto-ranging. On a manual meter, start on the highest range and turn down. A meter's accuracy is given like ±(0.5 % + 2 digits): on the 20.00 V range, a 12.00 V reading is good to ±(0.06 + 0.02) = ±0.08 V.