×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Dual 4-20mA signal but need a single 4-20mA signal output

Dual 4-20mA signal but need a single 4-20mA signal output

Dual 4-20mA signal but need a single 4-20mA signal output

(OP)
Big picture question on instrumentaion logic. My existing setup is a single level transmitter (4-20mA) feeding circuit via a controller. To avoid a failure of the circuit, we would like to add a second level transmitter (so that one LT fails, the other will continue to work and feed the controller and its circuit). However, the controller circuit is too complicated, we would like to do the following:

Take 2 LT inputs (2 x 4-20mA), run it through a black box. Out from the black box, we need a single 4-20mA signal for the controller and the downstream circuit. The black box should be also able to know a failed LT (fail low or fail high or power supply failure) and switch to the working transmitter automatically.

Is such black box device possible? Is anything commercially (readily?) available? If so, please provide the detail.

I appreciate all inputs and responses.

(PS: Controller failure is also similar to the LT failure; but at this point we want to make LT fail proof and worry about controller later)

RE: Dual 4-20mA signal but need a single 4-20mA signal output

Seems to me that your concept reduces reliability, overall, since it's likely your "black box" is at least as unreliable as your LTs, and you've got a single point of failure now, coupled with a single point of failure in the single cable.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Dual 4-20mA signal but need a single 4-20mA signal output

Moore Industries STA safety module takes a single 4-20mA input and alarms on its status, so if the xmtr faults to 21.5mA, it would detect the failure and alarm. The illustrated spec sheet shows 1oo2 voting using an STA for each of the source transmitters, but includes the undefined "logic solver" to pick which of the two is used.

Link to STA spec sheet: Link

app note on web page: Link

RE: Dual 4-20mA signal but need a single 4-20mA signal output

(OP)
Thanks danw2. I looked at the STA. Isn't that STA works with single sensor and provides 2 process alarm points + system fault? I'm in need of ONE black box (or multiple black boxes) that takes 2 level transmitters and provide me with ONE 4-20mA signal output.

RE: Dual 4-20mA signal but need a single 4-20mA signal output

You need to do the fault/reliability analysis to determine how much risk you can tolerate, where that risk is, and whether your solution does anything to the risk at all. Your solution contains two single failure paths, and if they are more likely than a failure in the transmitters, then you've not gained an reliability.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Dual 4-20mA signal but need a single 4-20mA signal output

(OP)
Per vendor, cables meet the highest standard (certified to IEEE 383 - tested and proven to environmental effects) for any non-nuclear use. Also, they are exposed to mild temperatures only. So cable failure is out of the question and hence, two single failure path is not an issue, except for that level transmitter. I clearly understand there are other single point of failures with controller and its circuit but that is something we are also looking for. But the priority is the level transmitter for additional reliability.

RE: Dual 4-20mA signal but need a single 4-20mA signal output

I have never seen a commercial, solid state, stand-along, 4-20mA signal selector module.

Probably because the alternative of PLC availability.

There is the path of using a DPDT relay with gold plated contacts. But you need some discrete signal to drive the coil with.

RE: Dual 4-20mA signal but need a single 4-20mA signal output

(OP)
Okay. Thanks for your inputs

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close