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/

Discontinued Action-Paks? Call us.

As most of you know, many years ago Action Instruments introduced plug-in "Action-Pak" transmitters and alarms.(Note: Action-Pak is an Action Instruments trademark.) Action grew, became a highly regarded company, added other product lines and were eventually acquired by Eurotherm. Their line still includes Action-Paks but some models are no longer available.

One of our favorite stories - a company called us desperately looking for a special version of an old Action module. They told us, not only could Action no longer make it, they insisted they never had! The customer forwarded us a copy of Action's user's manual.

OK - a special - a one-piece order - we had to charge for some engineering, but we did it. They've ordered a few more since.

Over ten years ago another company came to us because Action no longer made some of the (standard) products they used regularly. Our product line included them (still does) and that company has become one of our better customers.

We're not trying to knock Action. They are a well-respected, quality company. But if you need plug-in modules they no longer offer, e-mail or call us. We probably can help. (And, our customers rave about our service.)

Signal Conditioners - our web site: http://www.jhtechnology.com/
e-mail: jhtek@jhtechnology.com
Phone: (941) 927-0300 or US toll-free (800) 808-0300
Fax: (941) 925-8774

Custom Designs

A Very Special Special

A well-known company needed to duplicate a thermocouple alarm they had bought from Action Instruments decades ago. Action was no longer able to provide it so they turned to us.

Action's original "Action Pak" line (Action's trademark) once included alarm modules with extra options such as retransmitting outputs and remote setpoint inputs. 20-pin headers (pin bases) were required to provide the extra connections. As far as we are aware these headers are no longer available.

Our customer's requirement was a nonstandard version of one of those alarms. We replied that we could provide the same function but with a different connection arrangement. This was not acceptable - it was for a nuclear power application and they could not vary the system design. Only one piece, but they were willing to pay for it.

Once again we got lucky. In 1995 we had purchased some 20 pin headers for another customer's need and had a handful left. Our customer bought one (couldn't convince them to buy a spare) - even sent an inspector to Florida to do a source inspection. That was 2008. In 2009 they again bought one and just now ordered two more! Possible future problem - after this order we have only five headers left.

(Note: We do not sell nuclear-approved products. This alarm was sold as "standard commercial quality" for a noncritical application.)

Technical detail: the custom alarm module has two retransmitting 0-1V analog outputs, one proportional to temperature and the other proportional to the setpoint. Our customer was able to supply us with Action's specifications and we designed it to be functionally identical. Here's the functional diagram.

If you have special requirements let us know. Phone (941) 927-0300 or toll-free (800) 808-0300.

E-mail jhtek@jhtechnology.com.

Link to our page on Custom & Special Signal Conditioners.

Signal Conditioners, General - our web site: http://www.jhtechnology.com/

Application Note

Detect Thermocouple Failure Without Upsetting the Process

One from the archives. Our customer needed to alarm on thermocouple failure without shutting down the process.

Thermocouple instruments commonly include upscale burnout indication. If the thermocouple breaks or fails open-circuit the instrument's output goes high, just as if the temperature had gone out of control. This often is used to insure failsafe failure, shutting the process down and sounding an alarm.

Our customer, a utility, needed an overtemperature shutdown on a monitored water temperature. They also wanted to trip an alarm upon thermocouple failure but without interrupting the process. We were able to suggest a simple dual-trip alarm setup to do this.

Our thermocouple instruments, like most others, offer a downscale burnout option. (Thermcouple failure causes the output to go low instead of high.) Using a dual-trip alarm, we set trip #1 for HI trip and trip #2 for LO. The HI trip, of course, was used to shut the process down on excessive temperature. The LO trip was set below freezing. Its relay would trip when the thermocouple failed but never under normal operation.

For the full application note, click here.

Our web site - Signal Conditioners, Process Transmitters: http://www.jhtechnology.com/

E-mail: jhtek@jhtechnology.com

Simple Specials

Standard Product, Nonstandard Connections

Our OEM customer needed a fully compatible replacement for a thermocouple alarm module they once bought from England. Our Model JH1200 plug-in thermcouple alarm filled the bill, except the pin connections were different.

The answer was simple - take our standard product, unsolder the internal wires from the pins and rewire them to be the same as the British product. And, since the the connections to the thermocouple were changed, we also needed to relocate the temperature sensor used for cold junction compensation.

One more step - we made a simple adapter to allow the converted alarm module to plug into our standard production test and burn-in fixtures.

This customer buys only one or a few at a time, but they've become a regular for several years.

Links: Process Alarms
JH1200 Thermocouple Alarm
Custom & Special Signal Conditioners