4-20 mA control loop !

B

BrainEater

[H]ard|Gawd
Joined
Jul 21, 2004
Messages
1,210
I have an issue with a current control loop , I know there's a couple people here with insight. ;D

--

I have a PLC that is running as a 4-20mA 'source' . In an Ideal situation , the pH meter I am trying to interface should be a 4-20mA 'sink' .
It's not, the pH meter runs as a 'source' as well.
---
I know they make converters/"isolation converters" that will work , but they are $$
I can Fabricate PCB's in house , but , my google fu is failing me .

What's the circuit I need ?
Advice ?
 
I'm kind of confused here, and might be missing some of the story. PLC == programmable logic controller?

Sounds like you're trying to make a digitally controlled current source but need to isolate the output. Why not just build a simple analog circuit that provides the 20mA you need? Does it need to vary? Is the pH meter a true current sink (like an LED) or does it run off of a voltage?
 
Yes , programmable logic controller.

Nope , it's a variable current control loop.
Current loop - Wikipedia

Both unit's are trying to source the current , it doesn't work that way.
The unit I linked does it , and 90$ is no problem , I'm just curious about the 'inner workings' at this point.
 
I see. Wouldn't the PLC be a 'sink' in this case because it's just measuring off a constant resistance? The pH meter voltage is changing causing the change in current, correct? When does the pH meter act as a 'sink' for the signal?

do you have the data sheet for the pH meter?

does it look anything like this:
Fig%203_loop%20powered%20device%20applicaiton2346dcb715704a328195262abdf42b17.png
 
That diagram is how the loop should work , yes.
The PLC is actually a source because the 24volt supply is physically built into the PLC module.
This is the problem , the pH meter ALSO has it's own 24vdc supply built in .
 
Why not link any of the parts?

Strange to me the sensor wouldn't be compatible with a normal 4-20mA PLC module. Sometimes in the module you can select different configs.
 
Apologies ,
The PLC module in question is from automation direct :

F2-08AD-1

The pH sensor is ; alpha-pH200 1/8 DIN pH/ORP Controller From Eutech Instruments

-----

Dayaks , you are bang on . The pH meter will interface no problem with a 'sinking' I/O module.

...and I'll just nip this in the bud : I know it say's "external 24 volt source required" on the PLC page.The 24 volts is part of the PLC 'backplane' .....That doesn't mean they will sink......Automation direct does make PLC I/O modules with source/sink capabilities for $$, but I'll just buy their converter/isolator for 90$.I have 3 other channels on that module that need 'sourcing'

At this point , I just wanna know how the converter I linked above works , just because I'm curious.

;)
 
Sorry I missed this one...

Funny this kind of question came up--I used to work as an I&C engineer, so when I saw "4-20mA," my ears perked up. I don't know how the isolation is set up inside the pH meter, but you might be able to connect the "+4-20mA" terminal to the I/O terminal on the PLC, and connect the "-4-20mA" terminal to the PLC 0V somewhere. And just leave the PLC's positive voltage terminal floating.
 
pH sensor document states it can drive a control loop up to 200 Ohm impedance. Looking at the PLC current loop inputs, it appears that they are 250 Ohm inputs.
 
Mohonri said:
Sorry I missed this one...

Funny this kind of question came up--I used to work as an I&C engineer, so when I saw "4-20mA," my ears perked up. I don't know how the isolation is set up inside the pH meter, but you might be able to connect the "+4-20mA" terminal to the I/O terminal on the PLC, and connect the "-4-20mA" terminal to the PLC 0V somewhere. And just leave the PLC's positive voltage terminal floating.
Click to expand...

No worries at all.Thanks for reading !

This is basically how we had it running for the last couple years ....but we had an intermediate programmable transmitter to match the impedances as Powerage mentioned.
It still eventually burnt out the I/O module.

I'm in contact with the Automation Direct people , they have shipped the Isolators and provided hookup schematics etc...looks like it's an easy fix.

:D
 
You must log in or register to reply here.
Back
Top