Triple-duty current loop calibrator
It’s always gratifying when a simple and successful design idea luckily turns out to have additional applications that you didn’t originally envision. Here’s an example.
Wow the engineering world with your unique design: Design Ideas Submission Guide
A while back, the design shown in Figure 1 was accepted for publication:
Figure 1 U1 plus R1 through R5 current steering networks convert a 0/20mA input into a 4/20mA output.
Later, the same circuit, when wired up differently as shown in Figure 2, turned out to be an equally good fit in a different job:
Figure 2 This 4/20mA current loop converter integrates an OFF/ON field contact.
A recent Design Idea by another frequent contributor, Jayapal Ramalingam, addressed the problem of convenient calibration of precision current loop receivers in industrial applications. His design comprises a linear control input that expedites calibration and testing. He explains that it helps to:
…”calibrate the analog input modules of distributed control systems (DCSs) and programmable logic controllers (PLCs) by simulating process signals.”…
This inspired me to wonder if a different approach to the same calibration problem might also be useful. I imagined a design in which the three standard analog test current loop levels: 0, 4mA, and 20mA, were accurately preset and quickly accessed by flipping a switch. I then proceeded to ponder whether that same friendly little converter circuit might work in such an application.
Figure 3 shows the result:
Figure 3 The three-position, center-off, DPDT switch S1 converts this current converter (verbiage redundancy pun-intentional) into a convenient current calibrator.
Not only did it fit, but the calibration procedure for the new role is just as quick, simple, and easy to accomplish in a single pass as it was before.
- Set S1 to the 4mA position.
- Tweak 4mA adj for 4mA output (as measured, for example, with a precision DMM).
- Set S1 to the 20mA position.
- Adjust 20mA adj for 20mA output (ditto).
So, it turns out that the same circuit thriftily fits three related, yet different, applications – a triple-duty design trifecta.
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
- Silly simple precision 0/20mA to 4/20mA converter
- Double-duty current loop transmitter
- Linearly variable two-wire loop current generator
- Full circle current loops: 4mA-20mA to 0mA-20mA
- 4/20mA to 0/20mA loop current converter for grounded loads
The post Triple-duty current loop calibrator appeared first on EDN.
