4-wire isolated to 3-wire non-solated 4-20 mA

einnh · Sep 9, 2020

I am stumped. I haven't run into this before.

We have a four wire 4-20mA sensor
+PWR
-PWR
+SIG
-SIG

We have tested it with a process meter and it works as desired however I am trying to hook it into an analogue input block

PIN 1 - 24V PWR
PIN 2 - INPUT 0
PIN 3 - COM
PIN 4 - INPUT 1
PIN 5 - NOT USED

I thought I could just tie my +PWR AND +SIG together and read the input on -SIG on INPUT 0 but that doesn't work. I tried the opposite as well, -PWR AND -SIG tired to COM and reading on +SIG at INPUT 0. No luck.

Is what I am trying to do even possible? I don't need the signal to be isolated, it is just how this particular transmitter came from the manufacturer.

Thoughts? Ideas? Any information would be greatly appreciated.

Mickey · Sep 9, 2020

Can you provide the complete catalog number of your transmitter and the complete catalog number of your analog input module? (Brand names also)

GaryS · Sep 9, 2020

+PWR - 24V PWR Pin 1
- PWR - COM Pin 3
- SIG - COM Pin 3
+SIG - Input Pin 2 or Pin 4
Should work COM is connected internally to the power supply common and the Input Common
There should be an instruction sheet with both

Bit_Bucket_07 · Sep 9, 2020

einnh said:
I am stumped. I haven't run into this before.

We have a four wire 4-20mA sensor
+PWR
-PWR
+SIG
-SIG

We have tested it with a process meter and it works as desired however I am trying to hook it into an analogue input block

PIN 1 - 24V PWR
PIN 2 - INPUT 0
PIN 3 - COM
PIN 4 - INPUT 1
PIN 5 - NOT USED

I thought I could just tie my +PWR AND +SIG together and read the input on -SIG on INPUT 0 but that doesn't work. I tried the opposite as well, -PWR AND -SIG tired to COM and reading on +SIG at INPUT 0. No luck.

Is what I am trying to do even possible? I don't need the signal to be isolated, it is just how this particular transmitter came from the manufacturer.

Thoughts? Ideas? Any information would be greatly appreciated.

I don't understand why you're trying to tie your power and signal wires together. Power up the instrument and connect the + & - signal wires to IN0 and Common, as you would with any 4 wire transmitter.

einnh · Sep 9, 2020

The input block is a BALLUFF BNI00AJ.

The transmitter connected to it is unknown. It is embedded in a custom build sensor.

There are four wires coming out of the sensor:

WH (+PWR)
BU (-PWR)
RD (+SIG)
BN (-SIG)

The BALLUFF block port each has M12-5F/A-Code.
P1 - 24VDC
P2 - IN 0
P3 - COM
P4 - IN 1
P5 - NOT USED

So you are saying connec:

WH - P1
RD - P2
BU - P3
BN - P3

I thought I tried that already but I can check again in the morning.

I do have the block configured for 4-20mA with the correct units.

danw · Sep 9, 2020

einnh said:
So you are saying connect:

WH - P1
RD - P2
BU - P3
BN - P3

Yes, that's what I'd start with

CanSpellPLC · Sep 9, 2020

+PWR - 24V PWR Pin 1
- PWR - COM Pin 3
- SIG - COM Pin 3
+SIG - Input Pin 2 or Pin 4
Should work COM is connected internally to the power supply common and the Input Common
There should be an instruction sheet with both

^ Seconded. The COM terminal should receive the -SIG and -PWR. Tying the +PWR and +SIG won't work as you'll be receiving the current that it takes to power the device as well as the 4-20 mA. If it doesn't ruin the electronics, the signal will almost certainly appear to be saturated on your input.

einnh · Sep 10, 2020

So we tried this morning and it isn't working. Manufacturer of the sensor also said we could try with a resistor between INPUT+ and the COMs but that didn't work yet either..

Running out of things to test *eyeroll*

einnh · Sep 10, 2020

Sorry for the second post. So during our testing of tying the commons together we were seeing the loop saturate to ~20mA

parky · Sep 11, 2020

It sounds to me like this is an isolated output sensor so requires a supply from the source as well, i.e. on normal analogue cards the card is active.
The only suggestion is you connect the 24v & + sig together, use the -sig out to the + in of the block and connect the ov (-pwr) to com

WH (+PWR)-----P1 (24DC)
BU (-PWR)------P3 (COM)
RD (+SIG)------P1 (24DC)
BN (-SIG)-------P2 (IN 0)

danw · Sep 11, 2020

If it were me, I'd be testing each end independent of the other to determine if each was still functional at this point.

Does the AI still function with a different, active 4-20ma source?

Does the sensor still output 4-20mA when powered with a stsand-alone power supply?

parky · Sep 11, 2020

Danw is correct, I would set up a psu for the sensor, see if there is an output, if not and you believe the sensor is ok connect 24v to the + sig a meter to the -sig and ground of the PSU. I suspect the output is passive and requires a supply for example a two wire sensor relies on the +out to have an energy source, the -out is then the current (or voltage) output.

einnh · Sep 11, 2020

As stated previously the sensor works fine in isolated 4-wire mode still.

Bit_Bucket_07 · Sep 11, 2020

einnh said:
As stated previously the sensor works fine in isolated 4-wire mode still.

Sounds like you need an isolation transmitter then.

Does the analog input work with a test signal as well?

cidchase · Sep 11, 2020

FWIW:

From BNI IOL-719-002-Z012 User Guide:
One sensor can be connected to each port. In case of voltage/current
type sensor the input pin of the analog port can be configured. The
analog input signal can be connected either to pin2 or pin4. In this case
the BNI IOL-719-002-Z012-XXX has a +24V sensor supply. In case of
voltage/current type sensors you can only use 3-wire method.

Bit_Bucket: Isolation Transmitter, Yes but I don't see this available.
I'm hoping to learn something here about this conversion...
It appears that the Balluff module is not directly compatible.

Maybe this? https://www.se.com/uk/en/product/RMCN22BD/analog-converter---4..20-ma---for-zelio-analog/

4-wire isolated to 3-wire non-solated 4-20 mA

einnh

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