Tuesday, April 26, 2011

True RMS Measurement Includes DC

Simple Modifications meet Customer's Needs

When measuring AC voltages, DC components or offsets usually should be ignored. Examples include measuring AC ripple from a DC power supply, and data or audio signals superimposed on DC power lines. As shown in the figure, a blocking capacitor removes any DC component.

Therm-O-Link, a wire manufacturer in El Paso, uses rectified but triac-modulated power in their annealing operation. To monitor and control the annealing process they need to know the total voltage, DC plus AC. They also need true-RMS measurement for accurate power monitoring regardless of the waveform.

A simple modification to our true-RMS process transmitter, Model JH6010IR, removed the blocking capacitor for full DC response. Additional changes speeded the transmitter's response time for proper operation in their control system.

For the complete application note, True RMS: AC or DC Response?, click here.

True RMS Measurement Basics

RMS measurements express the power capability of an AC source. Low-cost meters and transmitters often make a simpler average measurement rather than true RMS. The two are not the same, and the relationship between average and RMS measurements varies for different waveform shapes. The figure shows a pure sine wave, a chopped and rectified sine wave and a square wave pulse.

True RMS measurements require more complex and expensive circuitry, generally available in slightly higher priced instruments.

To learn the fundamentals of RMS measurement click for our application note, What is True RMS - and When do I Need It?

JH Technology manufactures:
Plug-In True RMS AC Process Transmitter
DIN Rail Mount True RMS AC Process Transmitter
Field Mound True RMS AC Transmitter with Display
Plug-In AC Alarm Trips with True RMS Response

Our web site: http://www.jhtechnology.com/

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