Multiple 2-Wire transmitter inputs, which power supply to use for loop power?

[jfast]

Hello,
I've a question regarding wiring analog inputs (4-20mA) to a compact-logix 1769-IF8 module.

The PLC and module exist already and are wired for 2-wire transmitter analog input, with loop power being derived from a 24VDC power supply in this panel. All 8 Analog input common terminals are jumpered together and bonded to DC- on this power supply (ANLG Com = V/l in 0- = ... = V/l in 7-) to avoid noise.

As a retrofit, I need to wire in two 4-20mA outputs from a device in the field, to spare channels on this slot. The outputs are coming from a Trane Programmable air handler control panel, model MP580. See pg 11 of manual:
http://www.wmitechnologies.com/pdfs/MP581-INFO.pdf

The MP580 has two universal analog outputs configured as 4-20mA. I am assuming they are not loop powered analog outputs.

Can I safely wire this analog output to my IF8 analog input, using my local 24VDC power supply to provide loop power? Or, do I need a loop isolator?

Thanks in advance! ... This stuff throws me through a loop for some reason.

 

[danw]

Your field device must have an analog output to connect to your PLC's analog input.

I'll show you mine if you'll show me yours.

Here's a screen shot from the manual at the link you provide.

The terminal strip labeled analog outputs connects to a load labeled 0-10Vdc, not 4-20mA:

The bottom sensor labeled 4-20mA sensor connects to an analog INPUT and is clearly connected to power supply, as one would expect for a loop powered sensor.

So unless you can show me where the 4-20mA output is, I'd say go buy a 0-10Vdc to 4-20mA signal conditioner.

For some small premium, you can get an isolated converter, to minimize the very high probability of a ground loop between the field instrument ground and the PLC analog input ground.

I would STRONGLY recommend an isolated converter.

There are loop powered converters, whose output is loop powered, like your other 4-20's on the AI.

 

[Mickey]

So unless you can show me where the 4-20mA output is, I'd say go buy a 0-10Vdc to 4-20mA signal conditioner.

From page 4 of the same manual,see picture below.

But it appears to be an active output, so an isolator is still strongly recommended. As mentioned above by danw.

0-20ma to 4-20ma signal conditioner.

 

[danw]

I concede, Mickey. You're incredible, reading through all the documentation!

Duh, I missed the significance of the "Load > 500 ohms".

They're driving the 0-20mA through a 500 ohm load to get the 0-10Vdc.

It bothered me that they mentioned 500 ohms, because I was thinking, "when was there ever a voltage AI with an input impedance as low as 500 ohms? Nowadays a voltage AI is typically 1Meg ohm." 500 ohms?

Too late after a long day at the grindstone.

Yes, still should probably have an isolator.