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5.1 The multimeter

Outcome 1 of 6 in domain 5 · Instrumentation and measurement

Voltage, current, resistance, continuity, diode; its limits and its traps.

My targets

I declare:

Measure DC and AC voltage, resistance, continuity and diode drop, choosing the jack and the range without hesitation.
Measure a current in series, and explain why the multimeter’s fuse blows when you get it wrong.
Explain the multimeter’s input impedance and show a case where it falsifies the reading (high-impedance divider).
Say what "true RMS" changes and when an ordinary multimeter lies on a non-sinusoidal wave.
Measure an AC current with the clamp meter and check for voltage with the non-contact tester, knowing what each doesn’t see.
Diagnosis: a "ghost" voltage on a disconnected wire, explain parasitic capacitance and prove it with a load.

Read

Kuphaldt, Lessons in Electric Circuits, volume I (DC), chapter 8: DC metering circuits. "What is a meter?" and "Voltmeter design" for the moving coil and the series resistor; "Voltmeter impact on measured circuit" for ohms per volt, with a loaded divider; "Ammeter design" and "Ammeter impact on measured circuit" for the shunt and for the clamp; "Ohmmeter design" for the battery inside and the rule that follows, never on a live circuit; "High voltage ohmmeters" for the megohmmeter; "Multimeters" for the box that holds it all. He writes for the needle; all of it holds for the digital one, whose input is a fixed resistance.