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View Full Version : 4-20 Loop 2 Wire vs 3 Wire


iahi_2005@yahoo.com
May 4th, 2011, 09:26 PM
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...

Mickey
May 4th, 2011, 10:28 PM
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.

labeledas
May 4th, 2011, 11:59 PM
if it is resistance based it is probably used for compensation, simialr to an rtd

Vic
May 5th, 2011, 12:45 AM
if it is resistance based it is probably used for compensation, simialr to an rtd

huh?

Tom Jenkins
May 5th, 2011, 08:13 AM
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.

iahi_2005@yahoo.com
May 5th, 2011, 08:38 AM
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?

Mickey
May 5th, 2011, 09:12 AM
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)?

iahi_2005@yahoo.com
May 5th, 2011, 10:44 AM
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)?

I am using a 1756-IF16 Card (Allen Bradley)

Mickey
May 5th, 2011, 11:07 AM
I am using a 1756-IF16 Card (Allen Bradley)

The card does not supply the power. You will need an external power supply.

inst_tech
May 5th, 2011, 02:40 PM
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 is a noob
May 5th, 2011, 02:51 PM
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.....

albertabound
May 5th, 2011, 02:55 PM
http://www.bapihvac.com/content/uploads/2010/07/4-20-mA-Current-Loop-Configurations.pdf

iahi_2005@yahoo.com
May 5th, 2011, 10:03 PM
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?...

Mickey
May 5th, 2011, 10:29 PM
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.

iahi_2005@yahoo.com
May 5th, 2011, 11:17 PM
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

Mickey
May 6th, 2011, 12:08 AM
Yes but the four wire device does not connect the power supply's common to the PLC.

Some four wire devices require more power then a loop can provide. 4 wire devices can also be powered by different voltages (e.g. 120vac)

Less wiring for loop powered/3 wire then 4 wire.



I'm sure others with more knowledge of how and why vendors build their devices will give a more knowledgeable answer then mine.

cbuysse
May 6th, 2011, 10:04 AM
Since a 2-wire device cannot directly reference the power supply's common, this configuration limits the device's power requirements. It cannot use the full source voltage since it must leave enough voltage on the signal wire to drive the current through the input impedence of the PLC input card. It also must be able to operate at 4mA.
Since it directly references the power supply's common, a 3-wire device can use the power supply's full voltage and is not limited on current.
A 4-wire device provides the same advantages as a 3-wire, but it also allows for the use of a completely different supply voltage (like 120AC) and isolated commons.

shooter
May 6th, 2011, 02:58 PM
inside gas sensor is a heater and that takes too much power in a loop (4 mA on 10 volts is only 40 mW not enough for the heater so extra wire to get enough power.

dialc
July 8th, 2011, 02:50 PM
Can anyone help me how to configure a 1756-IF16 in a controllogix using ladder ??
Where can I find information about it ??

Thanks,
Diego

Tom Jenkins
July 8th, 2011, 04:43 PM
Diego, you will probably get more and better responses if you start a new thread with your question.

az1
September 11th, 2012, 09:26 PM
is there any way (circuit interfacing, etc) to convert the 3 wire system to 2 wire system? my detector uses 3 wire but my DCS only support 2 wire system..

danw
September 11th, 2012, 11:07 PM
Whose DCS requires loop power to a field device? There is no alternative to not have sourcing power voltage on the analog input terminal? Are you sure?

If you really must, the 3 wire output can be fed by a power supply and feed into a loop isolator input. There are loop isolators whose output is then a 2 wire current output.

az1
September 11th, 2012, 11:43 PM
yup.. that's the problem actually.. the DCS card only has 2 wires input (24V & GND) and the detector have 3 wires (24V, GND & 4-20mA)..

danw
September 12th, 2012, 07:39 AM
The Phoenix Contact model MCR-1CLP-I/I-00 (2814016) isolator's output is 2 wire loop powered.

You'd also have to provide a separate power supply to the 3 wire device.

cbuysse
September 12th, 2012, 08:34 AM
yup.. that's the problem actually.. the DCS card only has 2 wires input (24V & GND) and the detector have 3 wires (24V, GND & 4-20mA)..

Are you sure this is an analog input and not a digital? I've never seen an analog input with the terminal labeled 24V. What is the model part number for your DCS card?

All inputs are only going to have 2 wires, but they are usually labeled (4-20mA) or (0-5V) or (I+) or (V+) along with (GND) or (-). "3-wire" or "4-wire" only refer to the sensors.

Lancie1
September 12th, 2012, 08:41 AM
they are usually labeled (4-20mA) or (0-5V) or (I+) or (V+) along with (GND)
I hope that they are not usually labeled with "GND", but maybe "COM" or "-".

cbuysse
September 12th, 2012, 08:50 AM
Lancie - you're right.. kinda goes with your post on the one of the other threads

Lancie1
September 12th, 2012, 09:05 AM
Cbuysse,

It is not surprising that electricians and others get the wrong ideas, when even otherwise-knowledgable vendors such as Allen Bradley put the wrong labels on signal terminals in their user manuals. Here is an example from the Allen Bradley SLC 500 Analog Input Module User Manual that has been wrong for years:

kirk
September 12th, 2012, 09:24 AM
This is how I think of it.

2 wire-- I power the sensor with 24 Volt + and read the "Signal" on the -

3 wire-- The sensor need 24 Volt + AND - and the signal is on it's own independant 3rd wire.

Lancie1
September 12th, 2012, 09:35 AM
Here is another way that the Analog Module wiring diagram could be fixed:

az1
September 18th, 2012, 01:12 AM
thanks for the reply.. i'm using Yokogawa Singal Condition Card (GS 33M50H85-40E) Model EA1.. it uses 2 wire system as in attachment.. my gas detector uses 3 wire system which is 24V,GND and 4-20mA signal.. if can't view the attached images, the images can be found at http://www.imagebam.com/image/801eef211082433 and http://www.imagebam.com/image/289494211082630.. thanks..

http://www.imagebam.com/image/801eef211082433
http://www.imagebam.com/image/289494211082630

cbuysse
September 18th, 2012, 07:38 AM
I'm just guessing here. I'm not familiar with Yokogawa and that documentation is just about as ambiguous as you can get. It appears to me that the card can supply 24 VDC from A to B and then there's a 250ohm resistsor connected from B to C so that all it's really doing is converting the signal to 1-5VDC. I can't tell for sure. My first attempt would be to connect it this way:
DCS Transmitter
A --- 24V
B --- GND
C --- 4-20mA

If that doesn't work, I'd go with a separate power supply. Connect the (+) and (-) from the power supply to 24V and GND on the transmitter. Also connect the (-) from the power supply to B on the DCS and then connect 4-20mA from the transmitter to C on the DCS. It'll end up looking exactly like the middle diagram on one of the earlier posts.