4-20 Loop 2 Wire vs 3 Wire

Join Date
Dec 2008
Location
AZ
Posts
23
Guys,
I am really confused about 4-20 ma current loop 2 wire vs 3 wire...what is the difference?

Does the difference have anything to do with the sensor being powered separately or in loop?

In My case the sensor is a Gas detector and sends output to PLC Analog Input.So is the sensor being powered from the PLC Analog Input Card?

Thanks..i would really appreciate it if you could clear that up...
 
A picture is worth a thousand words. (or something like that )

Give us the exact part number of your gas sensor or a link to it's manual.

analogWiring.jpg
 
Last edited:
Guys,
Does the difference have anything to do with the sensor being powered separately or in loop?

You are correct! Two wire loop powered sensors are usually simple devices like pressure or temperature transmitters that don't need much power. They use a diode or resistor to create a voltage drop inside the device to power the circuits.

Three and four wire devices are more complex transmiters that need more power than can be easily taken from the loop. Your gas detector is probably three or four wire, but the manufacturer's manual should identify the wiring.

Mickey's post shows the wiring difference perfectly.
 
A picture is worth a thousand words. (or something like that )

Give us the exact part number of your gas sensor or a link to it's manual.

On your picture, is the power supply the analog input card?...or can the analog input card never be a power supply also?
 
On your picture, is the power supply the analog input card?...or can the analog input card never be a power supply also?


That would depend on the card your are using. But most that I am familiar with use a separate power supply.


What analog input card do you have ( complete catalog number)?
 
On the platform i work on all the line of sight and point gas detectors supply 4-20mA to the fire and gas cards ie. they have their own power supply. The catalytic gas detectors are powered from the fire and gas cards and work on resistance.
 
i have some gas monitiors that are two wire 4-20. Google OI-675. Various gas set ups, are Class 1,Div 1 and work from on 12 & 24 VDC systems. Easy as pie but are in the 1.7k range.....
 
A picture is worth a thousand words. (or something like that )

Give us the exact part number of your gas sensor or a link to it's manual.

Looking at the picture the only difference between the 2 wire and 3 wire is the extra ground wire...but what is the significance of that?...
 
Looking at the picture the only difference between the 2 wire and 3 wire is the extra ground wire...but what is the significance of that?...

Read Tom's second paragraph post #5.

Look again at the drawings. The 2 wire device is loop powered. The 3 wire device is first powered by the 24vdc power supply and has a separate output which goes to the PLC's input, it's not loop powered. The power supplies common and the PLC's minus input are tied together to complete the 3 device's output circuit.
 
Read Tom's second paragraph post #5.

Look again at the drawings. The 2 wire device is loop powered. The 3 wire device is first powered by the 24vdc power supply and has a separate output which goes to the PLC's input, it's not loop powered. The power supplies common and the PLC's minus input are tied together to complete the 3 device's output circuit.

I am sorry to keep asking..if like you say that is the case what is the difference between 3 wire and 4 wire they are both separetely powered.....when would you use one over the other
 

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