Isolate current signal

[Pandiani]

Hello guys,
I'm not sure if I used correct term in the topic's name but basically I need the following.
I have one pressure transmitter that gives current signal (4-20 mA) and I want to use that signal on two places:
1. as input to analog PLC card
2. as input to instrument
I know I can just put instrument in series and everything is fine. Actually that is exact way how it is done now. I need some way to isolate this signal so I have one "part" that goes to instrument and other "part" that goes to PLC so I can safely replace instrument in case of need without disturbing PLC.
I hear that there is some kind of galvanic isolator circuits but not really sure.
Can you suggest me solution for this problem?

 

[Mickey]

Some good sources:

http://www.miinet.com/products/ca_signal.shtml

http://www.actionio.com/

http://web4.automationdirect.com/adc/Shopping/Catalog/Process_Controls/Signal_Conditioners

 

[Tom Jenkins]

A quick solution, if possible, is to use an instrument with a 0-5 VDC input. Chart recorders and so on are often available that way. Then you just put a 250 Ohm resistor across two field terminals. The recorder can be removed at will without disturbing continuity.

Another trick I've use is to have a reversed Zener diode across the instrument terminal. When the recorder is pulled, the diode conducts. You have to make sure the voltage and current rating of the Diode are consistent with your instrument.

 

[Krcedinac]

Galvanic isolator

Have a look here...

http://www.prelectronics.com/filer/6185v.pdf

 

[Pandiani]

Thank you guys, I'll check these links. Tom Jenkins, I like your suggestion about Zener diode...

 

[Pandiani]

Can this solution (in attachment) solve this issue?
Thanks for help

 

[krk]

I'd say no.

If channel 2 is the same as channel 1 and terminal 43 is internally grounded (as is terminal 41) then you've effectively shorted channel 2 input.


Kevin H

 

[Pandiani]

Thank you for your reply krk.
Yes, internal ground was my dillema too, that's why I post this attachment.
What would you do in this situation? Can I use this type of isolator at all?
I was thinking to use two separate devices whose input would be connected in series but not really sure about this.

 

[krk]

Ideally you need a signal multiplier for this application.

The 28 14 86 7 is a Phoenix Contact device that would do the job. (data sheet attached)

 

[Pandiani]

Yes, it seems it's just what I need.
Besides that, I think that using two isolators should do the job.
In other words, I can't think any reason why solution in new attachmnet wouldn't work.

 

[krk]

As long as the transmitter can handle the combined input load it should work OK

 

[danw]

KRK's Phoenix Contact solution does the job with one single module that provides two isolated outputs, rather than using two, separate isolators.

Dan