4-20 mA power loop with single source (pressure probe) and three receivers (controllers)

Question

First of all, I am mechanical engineer, not electrical, so please forgive me for possibly silly question. I need a help connecting a single 4-20 mA source to three receivers. The source is pressure probe with supply voltage 7 … 30 V DC.

I would like to connect it to the pressure controller and to an electronic expansion valve controller (two branches for the control of two valves). The dirty solution would be to use three separate probes, but that doesn't seem “right”, since all three pressure probes would be connected to the same suction pipe and will be measuring the same pressure.

So I want to ask if it would be possible to use just a single pressure probe and somehow organize a current loop?

All three receivers have just two connection points, without the requirement of external power supply. If I understand this correctly, the power supplies are integrated into every controller and one end of the receiver is connected to the power supply inside of the controller. According to the pressure controller documentation, the (internal) power supply is 12 V DC. In the documentation of EEV controller the power supply is not specified, but I measure 12V DC as well.

The question is how to wire the current loop (if possible). Let me designate the +12 V DC output of the first controller OUT1, and other connection of the controller will be IN1 (the same principle for the controllers 2 and 3). Then a connection of current loop seems straightforward:

• OUT1 -> Probe_IN;
• Probe_OUT ->IN3;
• OUT3 -> IN2;
• OUT2 -> IN1.

The loop seems closed, but I have no idea if such loop is valid.

Also, since every controller has its power supply, we will have three power supplies connected in series in the loop. Could somebody help me with advice here? Do I need some additional equipment installed in the loop to make it work, or is it not possible at all? I can find some discussions considering current loops with multiple receivers, but in all the diagrams the external power source is used and the receivers have no power sources.

P.S. The controllers do not have external grounding (they may be connected to the same 24 V AC (transformer), but I can use separate transformers if it will help. The EEV controller has two branches in a single device, so I do not know if there is a common ground inside. But when I measure with a tester, I do not find any connection between OUT1 and OUT2 or between IN1 and IN2 (when the power supply is not connected to the controller).

Edited When the EEV (Electronic Expansion Valve) controller is not connected to 24 V AC, I cannot measure resistance between OUT1 and IN1 as well as between OUT2 and IN2 (measure infinity). I guess this is because the internal power source is connected in series with the resistance. I do not have access point to the resistance alone.

When the controller is connected to 24 V AC, I measure 12 V DC between OUT1 and IN1. I measure 0 V between OUT2 and IN2 (maybe because the second circuit has to be enabled in the parameters of the controller). Also, I measure no voltage between OUT1 and OUT2, OUT1 and IN2 or OUT2 and IN1.

The pressure probe is emerson PT5-07M. Supply voltage 7 ... 30 V DC, class 2; Protection class IP65. All the other info in the datasheet is considering the refrigeration part.

The EEV controller is Emerson EXD-HP2. In the datasheet just the before mentioned pressure probe is specified, without any additional info (just where to connect brown wire from the probe and where to connect white wire). Once again, the remaining info in the datasheet is considering programming, error codes, refrigeration.

As a pressure controller I intend to use Carel ir33 universal controller. In the manual I find 12 V DC voltage to the probe, 4-20 mA (2 wires). Nothing about connecting three branches to one probe.

once more edited

It looks like in my case the DAQ are active, because the external power supply is not required.

The Datasheet for Emerson contoller can be found by this link http://www.emersonclimate.com/europe/ProductDocuments/AlcoLiterature/EN_EXD-HP12_35076.pdf

The manual for the Carel ir33 universal controller (DN33W9MR20 in my case) is here http://www.temperature-house.com/cms-files/Carel_IR33_Manual_for_Warming_Oven.pdf

I can not find manual for the pressure probe from Emerson, but any standard industrial probe would do, like danfoss AKS 3000 series (my low reputation does not allow to post more links)

Once again, the DAQ is active.

Answer 1

An acquisition equipment (DAQ) comes in two main differences: active and passive. The active type is sourcing the power supply and measures the current, while the passive type sinks and measures the current.

^{simulate this circuit – Schematic created using CircuitLab}

Theoretically you can connect many passive DAQs in chain, or one active and two passives. As long the total resistance is not too much: 20mA * 750ohm = 18.7V.

But as said it's theoretically possible, usually the low contact is tied to minus of PSU or ground. Only good and expensive DAQ have differential way measuring the current with a limited common mode voltage and possibly galvanic isolation.

first of all you need to make clear what type of DAQ you have and what is the input resistance.

Post some more data about the probe and equipment, links to datasheet, manuals.

Answer 2

Current loop isolator modules are available from many suppliers for just this sort of application. In effect, referring to Marko Buršič's answer, these can convert an active receiver into a passive one.

You'll need to check that your source has enough supply voltage for the voltage burden of all of the loads.