4-20 Loop 2 Wire vs 3 Wire

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.
 
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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.
 
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.
 
Can anyone help me how to configure a 1756-IF16 in a controllogix using ladder ??
Where can I find information about it ??

Thanks,
Diego
 
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..
 
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.
 
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)..
 
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.
 
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.
 
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:

Errors in Analog Signal Labels.jpg
 
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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.
 

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