Can someone give me brief reply on this..

the 4 wire transmitter is power from the i/o module or have the separate power supply in the power marshaling, and some transmitter have 120v. so from where it will get the supply?

 

2 wire means the transmitter is loop powered. There are just two wires going to the device.

3 or 4 wire means the device is not loop powered. it could still be powered by the same loop power supply though.

 

To rephrase your question:

If you have a transmitter that must be powered by 120v AC, can that power be supplied at the I/O termination rack and sent on a pair of wires that is part of a 4-wire connection to the field instrument?

Yes, but not all manufacturers of I/O marshalling racks allow 120v AC power to be applied through rack terminations. The most common power distributed to I/O marshaling racks is 24v DC. Most field instruments that require 120/240v AC power do not expect to obtain that power from the instrumentation wiring, but expect the site contractor to supply an AC power receptacle or junction box close to the field instrument so it can be directly wired. Then ordinary 2-wire 4-20ma is used for the data connection.

In most locations, running any AC power on a 4-wire instrument cable is considered a safety hazard, and is not recommended, or may be banned.

Dick Caro

 

2-wire transmitter relies on loop power
4-wire is for external Power supply.
Then what is the use of 3 wire transmitter (RTD)?

 

RTDs can be either 2, 3, or 4 wire. It is a completely different thing than how the transmitter itself is powered.

Typically a fixed small current is forced through the RTD and the voltage drop is read back. A 2 wire RTD will include the wiring out to the RTD as part of the voltage drop. A 3 or 4 wire RTD setup allows the voltage drop in the wiring to be ingorned so it is more accurate.

 

RTD is different thing in 2 and 4 wire concepts..
in RTD it has a many configuration like 2,3,4 wire..
simply RTD worked on positive temp. coefficient..
so as temp change resister change..
in temp transmission LEAD wire resistance also added in measure temp. resistance.
so our temp. data is wrong or can say that not actually measured value.
so 3rd wire added for this lead wire resistance compensation.

for ex.
suppose measure temp. resistance 130 ohm
and lead wire resistance 5 ohm
so at transmission end there will be a 135 ohm..so our measured temp. data change.
so third wire subtract that 5 ohm resistance.