In some applications, dc voltage measurements may be taken in circuits that include ac voltage.
To ensure maximum accuracy of a dc voltage measurement, first measure and record the ac voltage. Then measure dc voltage by selecting a dc voltage range (using the RANGE button) that is the same or higher than the ac voltage range.
Some DMMs can simultaneously measure and display the ac and dc components of a signal. The DMM display can show results three ways (see illustration below):
The ac portion of the signal appears in the primary display and the dc portion in the smaller secondary display.
The dc reading can be switched to the primary display while the ac drops to the secondary (on most DMMs).
The combined AC+DC value – the signal’s equivalent rms signal value.