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

Convert a 0-200 Microamp Signal for use in PLC?

Convert a 0-200 Microamp Signal for use in PLC?

Convert a 0-200 Microamp Signal for use in PLC?

I have an old turbine control system that uses black boxes for temperature and speed control. I'm trying to get the speed (passive Mag pick-up) and temperature signals (Type K Thermocouple) in to a PLC for monitoring. I think piggy backing off the Mag pick and taking the signal into a high speed counter card at the PLC should be straightforward.

The temperature signal is more complicated. There's only one Type K signal (wire) going straight to the black box. I'd rather not molest/piggyback the thermocouple signal (splitter has delays, drift, etc) so am looking at a 0-200 MICROamp output signal that is available off of the temp control black box (proportional to the input t/c signal) that I could use for monitoring. How in the world can I convert or get a 0-200 microamp signal in to a PLC?

RE: Convert a 0-200 Microamp Signal for use in PLC?

Accuracy isn't too critical. 0.5% - 1.0% or better preferred. The 0-200 microamp signal is just for monitoring, not precise control.

RE: Convert a 0-200 Microamp Signal for use in PLC?

Parallel the Type K thermocouple with a length Type K wire from the input on the black box into a commercial temperature transmitter which will give you a 4-20mA output that any PLC can handle. Transmitters are typically "2-wire, loop powered", meaning you also have to provide a 24Vdc power supply to power the transmitter, but that power runs over the 2 wires going to the transmitter.

I suggest isolated inputs on temperature transmitters, because many commercial thermocouple junctions are grounded and grounding can produce ground loop problems; an isolated transmitter input avoids that issue.

There are two categories of temperature transmitters - dumb and smart. The dumb transmitter is factory configured for some input range, say, 0-500 Deg F. with a corresponding proportional 4-20mA output.

The smart transmitter's input can be configured zero/span for the 4-20mA output. Problem is, unless you're in an industry where the software/modem or handheld programming communicator is readily available, then you're stuck buying and learning whatever gear is needed to set the range.

A dumb transmitter should be pre-ranged; my recommendation for a smart transmitter one is to find a vendor who will configure the range for you so you don't have deal with the configuration end.

It pays to physically label a field transmitter with its configured input range so the next guy knows what the 4-20mA output signal represents.

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


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