Can i get two 4-20ma output signals from one PTx 5

[INSDE08]

Hi All
I have project where i need i require to take two 4-20ma signals.

1. 4-20ma to contorl valves
2. to connect to the PLC .
The transmitter is yokogawa pressure transmitter , loop -powered .

As i need one 4-20ma signal to control the exisiting control valves.
And if possible second 4-20ma signal DCS via PLC in order to alarm .
Thank You

 

[danw2]

Can you get two 4-20mA signals proportional to temperature from a single pressure transmitter? No.

Can you take the voltage from a dropping resistor across the input of the PLC and use the voltage? Yes.

Can you wire the 4-20mA in series with the PLC and the DCS? In all probability, yes.

Can you buy a signal multiplier, one 4-20mA in, two 4-20mA out and use it for getting a separate 4-20mA signal to the PLC and the DCS? yes.

something like Phoenix Contacts' MCR-FL-C-UI-2UI-DCI-NC
2814867

http://rd.phoenixcon.com/products/interface/db/datasheets/MCR-2UI.pdf

 

[roydm]

You can also use just a signal isolator either loop powered or line powered, use the original signal where you need the best reliability and the isolated signal for the second.
Roy

 

[Standingback]

The lowest cost option is to wire the control valve and the PLC in series with the transmitter 4-20mA loop

There are two issues to watch out for:-
1. That the total loop resistance remains less than 600 Ohms.(which shouldn't be difficult)
2. Wire the PLC in the loop after the valve positioner. (PLC's often ground the negative terminal,this can cause issues if there is more than one mA input on the PLC). Placing the PLC last in the loop should over come this.

I trust this helps,
Mlv

http://www.coulton.com

 

[roydm]

Hey, there's something missing in this loop. Where is the controller?
Roy

 

[Standingback]

Sorry,
I had assumed that we we talking in general terms about splitting a 4-20mA loop.

To implement this technique within a control loop the diagram would be modified as per the attached.

Thanks Roy for preventing further confusion.

I trust this helps,
Mlv

http://www.coulton.com

 

[roydm]

Standingback, So you put the PLC on the wrong side of controller just to see if I was paying attention.
I have often used a 1-5V PLC input across the controller 250 Ohm, that works as well.
BTW, I was refering to the original post, not your's, INSDE08 doesn't mention the controller, I think he has 2 (to) valves as well.
Regards
Roy

 

[sparky970]

You will need some type of controller for the valve and a loop isolater should work fine.

 

[INSDE08]

Thank You All
I have redesigned using your advises and ideas.

Cheers

 

[waross]

Do you want to send the process variable or the valve position to the PLC??


Bill
--------------------
"Why not the best?"
Jimmy Carter