Measuring voltage

Checking circuit supply voltage is usually one of the first steps in troubleshooting. This would be performed in a vehicle DC circuit by selecting the V-setting and checking for voltage present or high-/ low-voltage values. Most electronic equipment is powered by DC. For example, home electronic apparatus such as computers, televisions, and stereos use rectifiers to convert household AC voltage to DC voltage.

The waveforms produced by AC voltages can be either sinusoidal (sine waves) or nonsinusoidal (sawtooth, square, ripple). A DMM will display the root mean square (rms) value of these voltage waveforms. The rms value is the effective or equivalent DC value of the AC voltage. Meters described as average responding give accurate rms readings only if the AC voltage signal is a pure sine wave. They will not accurately measure nonsinusoidal signals. DMMs described as true-rms will measure the correct rms value regardless of waveform and should be used for nonsinusoidal signals.

A DMM’s capability to measure voltage can be limited by the signal frequency. The DMM specifications for AC voltage and current will identify the frequency range that the instrument can accurately measure. Voltage measurements determine:

Source voltage

Voltage drop

Voltage imbalance

Ripple voltage

Sensor voltages

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