Double-duty current loop transmitter

Tracking down rodentia (or otherwise)-caused cable cuts, and differentiating them from normal open circuits, is critical. Evolving the circuit design for expanded functionality makes it even more valuable.

It’s just part of the job.  Every design engineer learns early (if not so happily) about the inevitable necessity of detecting, confronting, and swatting “bugs” in circuitry.

Wow the engineering world with your unique design: Design Ideas Submission Guide

In a recent Design Idea, frequent contributor Jayapal Ramalingam extends this art of circuit defect detection and deletion from dealing with mere insects to coping with something much more formidable: rats!

With so many rodents and creatures around the plant, a cable cut can happen at any time

The cables being subjected to those toothy threats transport signals from field contacts monitoring pressure, temperature, valve position, limit switches, manual operator inputs, etc., to process control systems. The possible consequences of mistaking an undetected cable break for an open contact range from the merely inconvenient to the catastrophic. An example of the latter might be a critical valve that’s actually open but erroneously read as closed—viz., Three Mile Island?

Mr. Ramalingam’s clever solution to the problem of undetected cable cuts is a current transmitter design that adds a third current level to the two that are inherent to an ON/OFF contact.  Thusly.

20mA = contact closed, cable intact
4mA = contact open, cable intact
0mA = cable cut, contact state unknown

It therefore explicitly verifies cable continuity, preventing the mistaking of an open circuit for an open contact. See his article for details.

Mr. Ramalingam’s circuit works, is proven, and has nothing significantly wrong with it.  Its utility, however, is limited to that single function.  It might be significantly more convenient and thrifty if its role could be combined with another in a multipurpose design, provided, of course, that said design would be of no greater cost or complexity than the single-purpose transmitter.  Figure 1 and Figure 2 show such a circuit adapted from an earlier article.

Figure 1  0/20mA to 4/20mA current loop converter.

Figure 2 Field contact OFF/ON to 4/20mA current loop converter.

Note that the circuits are identical, so that only one design needs to be fabricated, documented, and stocked.

Calibration in this new role is quick and simple and completed in a single pass:

1. Open contact.
2. Tweak 4mA adj for 4mA output.
3. Close contact.
4. Tweak 20mA adj for 20mA output.

Stephen Woodward‘s relationship with EDN’s DI column goes back quite a long way. Over 200 submissions have been accepted since his first contribution back in 1974.  They have included best Design Idea of the year in 1974 and 2001.

Related Content 

• Is your PLC/DCS reading the field contacts reliably?
• Silly simple precision 0/20mA to 4/20mA converter

The post Double-duty current loop transmitter appeared first on EDN.