AC Measurement Application Guide
a precision op-amp rectifier circuit to provide a general-purpose
voltmeter which is calibrated to display the RMS value of sinusoidal
signals. The signal is AC-coupled to eliminate the effect of DC
bias. True-RMS readings are not obtained for complex waveforms with
harmonics, such as square waves or AC signals which have been half-wave
rectified or chopped by an SCR or triac controller.
True-RMS voltmeters use an integrated circuit which computes
the true root-mean-square value of complex signals. AC coupling
allows the measurement of the AC component (or ripple) from a signal
with both AC and DC components. DC coupling allows the measurement
of total RMS, or "heating value", including DC and AC components
to over 30 kHz. (Maximum crest factor (Vp/Vrms) is 3:1). Screw terminals
allow the selection of AC or DC coupling. True-RMS meters are especially
useful in monitoring voltage and current supplied to motors or heaters
controlled by an SCR or triac circuit.
AC power line monitors use a simple half-wave rectifier to provide
an economical AC voltmeter, which is calibrated to be accurate for
sinusoidal signals around 115 or 230 Vac. Because of nonlinearity
introduced by the rectified diode, the accuracy of power line monitors
decreases away from the calibration point. Power line monitors are
not suitable at the low end of the voltage scale, for instance with
AC current shunts or current transformers.
meters are used to measure the RMS value of sine waves, such as
AC power-line voltage.
True-RMS meters are required to measure the RMS value of complex
waveforms, such as sine waves chopped by a triac or square waves.
True-RMS meters can be connected to read AC plus DC components
or AC components (AC ripple) only.
shows the relationships in a pure sine wave between the amplitude
Vm, the mathematical half-wave rectified average Vav, and the true-RMS
value VRMS. The gain of Newport's AC-average meters is calibrated
so that meters display RMS value for sinusoidal signals.
Current Transformer For AC currents over 5 A, a donut-shaped current
transformer (CT) is normally used to step down the maximum current
to 5 A and isolate the meter at the same time, thus avoiding common
mode problems. Current transformers are available from most Newport
distributors. They are specified by a turns ratio such as 100:5,
where the first number is the rated input current in amps and the
second number is the 5 A output. When specifying an AC or true-RMS
meter for use with a 5 A CT, specify the reading desired at 5 A
input to the meter.
All models provide a special 5 A range, which allows direct connection
to a 5 A CT, with no need for an intermediate stepdown transformer.
A gain resistor inside the meter may need to be changed to provide
the desired display reading. This is normally accomplished by an
authorized NEWPORT distributor.
INFCAC 204B, 2003B, 2004 and QUANTA®
include a special range, which allows direct connection to a 5 A
current transformer, with no need for a stepdown transformer. The
current transformer adds to safety and avoids common mode noise
by isolating the signal.