# How loop wiring works?



## Ctsparky93 (Sep 17, 2016)

Was it a 2 wire sensor? One is a +24vdc and the other is a - that gets hooked directly up to the plc card for 4-20 MA 


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## gpop (May 14, 2018)

24vdc+ transformer to sensor +

Sensor - to plc card +

plc card to - to transformer - 


A lot of people struggle with the idea that the sensor negative can go to the plc plus terminal. Ive heard people refer to it as a less positive wire.

In loop its simple if you think of the circuit as water. (amps equals flow)

A pump (24v power supply) pushes water through the sensor. The sensor acts as a valve restricts the water flow (based on what ever its measuring), The water then goes to a flow meter (the plc card). The plc reads the flow. The water returns to the pump.


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## gpop (May 14, 2018)

Kevhuff26 said:


> Hello guys, I have been troubleshooting this system. The transmitter installed was having issues showing readings back through the PLC to the Display for the program. Turns out the issue was a bad probe that attaches to the transmitter and all readings are good now. My question is this, looking at the electrical wiring schematic, the voltage is supplied 24vdc to the same terminals (for power). As well as the 4-20amp ma loop wiring(for PLC communication) Can anyone explain how this can create feedback to the input of a PLC? I’m just not understanding fully how these two terminals for the feed can serve both purposes. Any information is appreciated. Please see pictures attached!


A picture would make this easier but just guessing.

you have a plc power supply thats 24vdc and they are using the same power supply for some anolog sensors. This is simply 2 items using the same power supply. They are not interlinked.


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## Kevhuff26 (Dec 17, 2018)

*Thank you everyone!*

I believed you guys answered my question that makes sense. Those were some good explanations. I also referenced the prints for the PLC and can see that the negative goes into an input for the 4-20ma which proves what you guys answered. I guess this one just stumped me. I couldn’t attach the pictures because I’m new to the website and it says I need at least 20 posts. Sorry about that.


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## LARMGUY (Aug 22, 2010)

Just go to the controversial part of the forum. You'll fill up those 20 posts fast!


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## gpop (May 14, 2018)

Kevhuff26 said:


> I believed you guys answered my question that makes sense. Those were some good explanations. I also referenced the prints for the PLC and can see that the negative goes into an input for the 4-20ma which proves what you guys answered. I guess this one just stumped me. I couldn’t attach the pictures because I’m new to the website and it says I need at least 20 posts. Sorry about that.


Glad we can help.

Just remember that your meter is your friend.

measure across a transmitter terminals and 4 to 20ma will roughly equal 1-5vdc on the meter.

Set your meter to ma and put the leads in the right holes (on the meter) then you can put your meter into the loop any place and see what the plc is seeing.

A 4 to 20 transmitter that is not reading its sensor will read either 4 or 20+ ma. (fail high or low)

A reading of 0 on a 4-20 either means you hooked something up backwards, wiring problem or the transmitter has a problem. 

If you unhook the transmitter the wires should read 24vdc (or close)

If the transmitter is feed from a different power supply (24 or 110ac) check the manual as some of these have a trick where you wire them backwards depending if you want to use loop or internal power.


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## bill39 (Sep 4, 2009)

Think of a device that takes several AAA batteries. They need in a flashlight to be installed + to - to + to -, etc. Instrumentation loops are wired the same way.

There are 2-wire instruments and 4-wire instruments. 4-wire instruments have two terminals for power only and 2 terminals for the signal outpost.

2-wire instruments are loop powered and only have + and - terminals.

I made a drawing that clearly shows the different hookups for instrumentation and I’ll try to post it soon.


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## Rora (Jan 31, 2017)

Voltage drops along a circuit, but current is the constant throughout. A 2 wire transistor produces an effective resistance between nominal 1,200 to 6,000 ohms to regulate the current 4 to 20 mA assuming a perfect 24V input, and since the current remains constant it can be read anywhere.

In reality, the transistor has internal circuitry to regulate current regardless of the input voltage, down to a certain point, as it's common to get more than 24V from the supply or less if there are other components in the loop (for instance, 250 ohm HART resistor).

The difference with a 4 wire transistor is that it is "producing" its own voltage on the loop circuit and and likewise regulates the current based on the sensor value.


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## splatz (May 23, 2015)

> Electrical looping is loop/loops created between two light with 1 single wire connected to multiple fittings. Usually looping is done for neutral wire. Neutral wire is a return wire for the current in an electrical circuit. That is it carries electricity from the output device back to the service panel/board.
> 
> As neutral wire carries charge while returning, it doesn’t get heated up much. Also I t becomes necessary to loop neutral wire is it is difficult to pull neutral wire for longer distances and have separate neutral wire for each and every light/fan connection.
> 
> *For more details refer contractordouchebhai.com..Here you will get whole information related to this..*


There it was, I was waiting for it! 

This is some fairly labor intensive spamming but that's what it is, spamming.


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## Kevhuff26 (Dec 17, 2018)

*Thanks again*

Thanks again everyone, I feel I have a much better understanding.


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## bill39 (Sep 4, 2009)

Kevhuff26 said:


> Thanks again everyone, I feel I have a much better understanding.



Found it. The attached drawing is what I put together as a reference. It covers both 2 & 4-wire 4-20mA instruments to PLC input and output modules. It's always worked for me but if anyone sees any errors let me know.


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## mpetro (Jan 6, 2016)

bill39 said:


> Found it. The attached drawing is what I put together as a reference. It covers both 2 & 4-wire 4-20mA instruments to PLC input and output modules. It's always worked for me but if anyone sees any errors let me know.



As someone who does instrumentation and controls every day, it's kind of embarrassing to say that I didn't know until now that 2 of the 4 wires on a 4-wire transmitter were just for power. :vs_blush: I've never actually used them in a project.


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## gpop (May 14, 2018)

Some 4 wires are 3 wires just for fun as they use 24v for power.


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## cmdr_suds (Jul 29, 2016)

I sure hope you don't power a 4-20mA current loop with a transformer. Life might get interesting. Use a DC power supply instead.

On 2 wire devices, its the fact that there is always at least 4mA flowing through the sensor. this keeps the electronics in the sensor functioning. More then 4, no problem.


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## emtnut (Mar 1, 2015)

priyankagupta said:


> 24vdc+ transformer to sensor +
> Hi, I'm a di-ck



Why the **** isn't this guy "registered" yet ???


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