Difference between 2 and 4 wire transmitter

A

Thread Starter

Anonymous

Can someone brief on the difference between 2 Wire and 4 Wire Transmitter. Nowadays many use 4 wire transmitters even though it has some advantages and disadvantages?
 
2 wire transmitter mean output sinyal 4-20mA and power supply for transducer in 2 wire. 4 wire mean 2 wire for output and the rest for power supply
 
B

Bill Clemons

2-wire transmitter relies on loop power from the receiving device, usually 24 VDC, to operate the transmitter interface, tranduce, characterize, and send the signal back to the receiving device.

4-wire transmitter, or self-powered, can be 24 VDC or line powered (requires external power supply).
 
A so-called "2 wire" transmitter only has one pair of wires -- a 4-20ma signal circuit connected to the transmitter. The signal is the amount of current that the transmitter passes through the loop. The power for the loop is in the instrument and control system (typically 24 v). The electronics in the transmitter are powered by this one loop.

A "4 wire" transmitter has two pairs: a 4-20 ma. signal, with the power supplied by the transmitter, and a separate pair to carry the power (typically either 24v. dc or 120 v. ac).

The advantage of the 2 wire transmitter is simply the cost of wiring, particularly the cost of running two separate conduits, one for power and one for signal.

The reason for the 4 wire is that some types of instruments require more power than the small amount that can be supplied by the 4-20 ma. signal.

John Shaw
http://www.learnpid.com
http://www.jashaw.com/pid
 
J

Johan Bengtsson

Are you talking about a Pt-100 transmitter?

If that is the case you can have 2, 3 or 4 wires between the Pt-100 and the transmitter The goal for the Pt-100 transmitter is to measure the resistance of the sensor, this can be done using two wires, however the resistance in those wires are added to the sensor when measured in this way. By adding a third wire you can measure the wire resistance (this is a somewhat simplified explanation) and subtract that from the measured value. This gives higher accuracy and the only requirement is that all three wires have the same resistance. By adding a fourth wire you can measure the resistance of the sensor without bothering about wire resistance at all (regardless of if it differs between those four wires or not)

/Johan Bengtsson
 
Two wired transmitters are or used to be limited by the allowable transmission distance due to line resistance generally a 4 wire system is used where long distance transmission signals are required I stand corrected but I think 2 wire systems are limited to approx 100m or that could be 1 km.
 
please tell me when we use 2 wire and when we use 4 wire. do smart transmitters also have 4 wire?
 
The difference between a 2 wire and a 4 wire is the power supply.A 4 wire has 2 wires for power supply and 2 for the control signal.

A 2 wire is a 4/20ma transmitter that uses the leakage current thru the load to power the transmitter.IE a 2 wire prox, versus a 3 wire prox.
 
B
If you are not asking about RTDs...

Two (2) wire transmitters are not self powered and only come with two wires to connect to the measurement/control device. Therefore, power must be supplied to power both the device and the loop.

Four (4) wire transmitters contain an integral power supply. Two (2) wires for the power supply and two (2) wires for the loop.
 
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Wayne Shimanis

The major difference between 2 and 4 wire systems is that for 2 wire systems power and signal are carried on the same set of wires. Also known as loop powered devices.

For 4 wire systems a separate set of wires supplies device power and a separate set is used for signal.

The advantage of 2 wire systems is half the wire runs are required. The disadvantage is relatively low impedance capability that may limit the type and number of devices you can drive with the signal as well as distance that the
signal can be transmitted before circuit loading occurs.

The advantage of 4 wire systems is higher signal side impedance, so better loading and signal transmission distance characteristics. The disadvantage, more wire. Hope this helps.

Wayne Shimanis
Vortex Technologies, Inc.
815-254-9400
 
I must respond to BAD Advice. This reply is wrong. Analog instruments and HART use 4-20 mA signals to transmit the measured value or process variable. Since the transmission uses current flow, it is not limited by the resistance of the wire to any particular length. Once the wire resistance plus the dropping resistor across the receiver, typically 250 ohms, total about 1200 ohms, then a 24 volt power supply will be unable to deliver 20 mA. That is a lot of wire, many miles of copper at the typical 18 AWG. All of this operates on a two-wire transmitter. There are no standards for a 4-wire transmitter for 4-20 mA signals.

Both 3 and 4-wire extension wires are used for RTDs mounted in the field. The RTD is a resistor that must become part of a balanced bridge for measurement using two wires. The extra wire or pair becomes part of the bridge as well to compensate for wire length of the extension. Today, it is more common to locate an RTD transmitter at the thermowell and transmit a two-wire 4-20 mA or fieldbus signal.

Dick Caro
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CMC Associates
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Ricardo Rietveld

The merits and demerits of two and four wires are mentioned in the previous replies. What I would like to add is the fact that in some areas (specified by ATEX, CSA, FM, etc) the power consumption is limited. These areas specified as Intrinsic safe or Non incendive areas are restricted to 2-wire instruments as these have limited power consumption. For HART, the mA-signal represents the process value and is therefore controlled by the transmitter. Acyclic communications are superimposed on the mA signal.

For Foundation Fieldbus and Profibus PA the communication is completely digital. The principle is somewha different, but for all the three above the power consumption is limited.
 
The distance limitation is the same for both 2 wire and 4 wire transmitters and it is ussually 4000 feet assuming you use the proper cable and use the shields properly. The 2 wire transmitter is ussualy powered by the control system. 4 wire transmitters are mostlt transmitters that require more power than the control system can provide like powering external coils or a coriolis device.
 
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Johan Bengtsson

The distance limit is longer for 4-wire transmitters because the loop power the transmitter for 2-wire transmitters and that result in a higher minimum voltage drop over the transmitter. The minimum voltage drop is much lower for a 4-wire transmitter and that means they can handle a longer cable. It does of course depend on wire thickness, shielding and some other things, but the direct limit is total loop resistance and thereby total voltage drop.

/Johan Bengtsson

Do you need education in the area of automation?
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D
This isn't quite right. The 2 signal wires from either transmitter will have the same distance limitation using the same wire and loop voltage. The limit for a DC powered 4-wire transmitter may be shorter if the electronics draw
more than 20mA on their pair.

Danny Scott, P. Eng.
Fredericton NB
Canada
[email protected]
 
J

Johan Bengtsson

For a long distance there is no need to feed the 24VDC power to the transmitter all the way from the same place where you want the signal to go. It is perfectly possible (and in those cases preferable) to put a voltage source closer to the transmitter.

If you intend to strictly feed the 24VDC all that way the limit might be much shorter depending on exactly how low voltage the transmitter is willing to work with.

Let's do an excercise:
A typical 2-wire transmitter have a minimum 10V voltage drop When you have 4mA that means 40mW, that means the electronic need less than that to work.

Now let's say you have a 250ohm resistor at the recieving end. At 20mA you will then get: 24V-10V-5V=9V
that means a maximum voltage drop per wire of 4.5V

Now let's say we use AWG 22, that means 51.7ohm/km and results in a maximum of 4.35km

Now, let's say you have a 4-wire transmitter with the following spec: mimimum 10V, 4mA own power consumption.

First we connect this the way you suggest using the same wire as above: If the transmitter need 4mA by itself the feedign wires will have 24mA when the signal wires have 20mA. Aay we have a voltage drop in the transmitter as high as 1V.

24V-5V-1V=18V to distribute in wire voltage drop This is solved by 4.09V voltage drop in each signal wire and 4.91 in the power wires resulting in 3.96km limit - that's lower alright

Now, by putting a 24VDC voltage source close to the transmitter we can feed it with 24V instead of 14.18V as in the above example. 24V-5V-1V=18V to distribute in wire voltage drop This will be solved by haveing a 9V voltage drop in each wire and 8.7km maximum distance with the same wire.

/Johan Bengtsson

Do you need education in the area of automation?
----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 Hässleholm SWEDEN
Tel: +46 451 74 44 00, Fax: +46 451 898 33
E-mail: [email protected]
Internet: http://www.pol.se/
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D
I don't want to waste bandwidth arguing this but if a transmitter requires 4mA to power it, then in a 2 wire configuration the signal current will vary between 4 and 20mA, not 8 and 24. In a 4 wire configuration, regardless of where you source power, the signal current will still vary between 4 and 20mA. So the voltage drop in the signal wire is the same in either
configuration, thus 2 wire versus 4 wire has no advantage or disadvantage
with respect to distance from transmitter.

Danny Scott, P. Eng.
Fredericton NB
Canada
[email protected]
 
The location of the power source in a current loop is arbitrary. The laws of physics would be violated were this not so. The extra pair of wires in '4-wire' transmitter exist to power the electrical and mechanical components in some instruments, like chemical analyzers, that cannot be sufficiently powered by an instrumentation loop or, for some other reason, should be independently powered or fused.

jk
 
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Johan Bengtsson

I don't think you read or understood the example well enough, but I don't think it really matters enough either.

The signal will of course always vary between 4 and 20mA, but not the power wires. The big difference is that a 2-wire transmitter have a higher minimum voltage drop (typically 10V) than the 4-wire transmitter. That means there are more voltage drop "availiable" for the wire.

/Johan Bengtsson

Do you need education in the area of automation?
----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 Hässleholm SWEDEN
Tel: +46 451 74 44 00, Fax: +46 451 898 33
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------
 
Electrons actually flow from the - pole to the + pole. According to your invalid theory of voltage, placing the transmitter right next to the + pole, after the other loads, puts it in the worst possible location.

jk
 
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