I really don't think there is an advantage either way – both are correct in their own element. Is one cheaper than the other? It depends on what it's doing. A simple local indication may require only a battery and a scale to read three wires. If you get more complicated – maybe the simple fact that so many devices are available using 4- 20 would make is more cost effective.
Simply – with a three wire you can have not only 1 – 10 but 1 – 5 for example that can be powered up by computer voltage. Usually close to a maximum X to 30 VDC will work with three wires. You simply put a voltage to the device, similar to wiring a rheostat, with a positive, negative and stack or main core. The output can then be run through a meter resulting in Voltage in = Voltage out. So if the device is at half voltage – the meter would read in the middle. Many applications work with 3 wire - you can see a diagram of how one example would work by following the link. See the bottom right picture.
4-20 is an old industry standard with a huge amount of devices built specific including flow, pressure, level etc. Besides having an incredible amount of products that can read 4 –20 it also has the advantage of not losing signal strength over longer leads that cannot be achieved with 3 wires. Books are written on the subject and this is only a small overview. Following this link will show you a basic diagram of how at 4-20 is connected. Click on the picture in the bottom left corner at: "http://www.almegcontrols.com/4-20_converter.htm":http://www.almegcontrols.com/4-20_converter.htm
Thanks Bob Hogg
I have never seen a 2 wire 4-20mA sensor, be sure it is not a digital switch type. Look forward to hearing the answer if your information is correct.
There is none. 4 to 20 ma is a current signal requiring a closed loop to operate. The only impact is of total circuit loading, generally around 600 ohms. As the resistance increases the voltage required to provide 20 mAmps increases (Ohns Law).
The three wire type allows the device to operate using more current than the 4mA low limit would allow. This can allow the manufacturer to have extra features like backlights for displays, or more power for sensing etc.
The 4-20mA signal only requires 2 wires. The only role a third wire would play is that of a dubious shield. You may actually be refering to 3-wire vs. 2-wire at the RTD sensor. Here the third wire provides lead compensation (a good thing).
My experience is actually a little different. Normally a 2 wire transmitter provides power on the same wires as the signal. However, the 3 and 4 wire hookups with which I'm familiar provide power on the extra wires. The purpose of this design is to provide isolation which is not normally characteristic of a 2 wire transmitter. However, the original writer may be refering to the input side of the transmitter in which case your comment is in agreement with my experience.
2 wire sensor is basically a loop powered device without requiring a separate supply voltage (the source voltage is supplied to the destination device) where as the 3 wire sensor is a self powered device meaning, you supply source voltage to the sensor and it can drive a 4-20 ma input device directly without the destination device requiring any supply voltage.
Generally 3 wire devices can supply to larger impedance load compared to 2 wire devices in the same voltage levels.
2 wire devices are easy to connect in circuits with a positive or negative common ie you can insert any where in a loop whereas 3 wire devices connections are usually fixed.
And usually 2 wire sensors can be used in wider voltage ranges compared to 3 wire sensors.
Hope this helps.
2-wire, 3-wire sensors not to be confused with a conditioned, and sometimes isolated control signal (4-20 mA). The third leg acts as a common ground on a multi-leg RTD sensor. Attach the sensor side to your transmitter (typically, there should be 4 terminals...for you guessed it... a 4-leg RTD).
On the conditioned signal side you can have a 2-wire or a 4-wire transmitter. The 2-wire transmitter relys on power from the host device. You feed the power wire into the high side of the transmitter and take the low side back to your host device as the analog signal. In some cases, the overall wiring requires a daisy-chain with the negative side of the loop power output (from the host device) jumper connected to the negative side of the analog (4-20 mA) signal receiver of the host device.
A 4-wire transmitter requires an independent power supply (or self-powered). So look for power supply leads 24VDC or 117AC with a transformer. The analog signal is simply connected high side to high side and low to low between the transmitter and the host device.
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i am an E.C.Eng
i must help u i this topic.
3-wire system is better then 2-wire system in RTD sensor.
4-20 mA is a out put of temparature transmitter
which connected with the sensor wire.
its a analog output.
Three wire rtd sensor is give a better accuracy.
compare to 2-wire.
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Three wire 4-20mA is used as an abreviated version of 4-wire.
This is used when the transmitter cannot sustain itself in the loop 4mA condition.
It may have relays on board which draw 80mA for example.
Do not confuse this with a voltage signal output.
Both are 4-20mA.
3-wire: power supply, signal, and return
2-wire: power supply with signal, and return
Three wire transmitters can be 0-20 mA. A 3-wire transmitter can usually drive the current into higher resistance because the current output can swing to near the power supply. If you power a 3-wire device with 24 VDC they can often drive the current into loads near 24 / 0.02 = 1,200 ohm. That means you can have several input shunts in series as well as very long wires. 3-wires is basically only used by power guzzling devices that can not operate on less than 4 mA.
Two wire devices operate on the current that run through them and therefore must always be 4-20 mA and they always have a voltage drop near 12 V. If you power on 24 VDC the maximum load the current can run through is (24-12) / 0.02 = 600 ohm. Now 600 ohm is a lot and sufficient in most applications.
Having said all that, 2-wire transmitters are my preference.
I must add some to my own message since it became clear we are talking about two things. Half of the answers in this thread address the word "sensor" wire whereas the other half addresses "4-20 mA" transmitter wire.
RTD temperature sensors like Pt100 can be connected using 2-wire, 3-wire or 4-wire schemes. 2-wire means that the voltage drop is measured over the same two wires that are used to source a sensing current through the sensor. This means that with 2-wire you are also measuring the lead wire resistance and therefore get an error. 3-wire means that you create a second circuit,
essentially of just wire going from the transmitter to the sensor and looping back. By measuring the resistance of this circuit and subtracting that amount from the sensor circuit you obtain the true resistance of the sensor - provided the wires are equally long and have an equally good connection etc. 4-wire means that one pair is used to source the current through the sensor, and a second high impedance input pair is used to measure the voltage drop. Here 4-wire is clearly superior.
Transmitters can also be connected using 2-wire, 3-wire or 4-wire arrangements. 2-wire is often called loop-powered because the transmitter is
powered by the same two wires as is its output signal. The output signal is generated simply by controlling its power consumption between 4 and 20 mA. I.e. it always has at least 4 mA to run on. If 4 mA is not sufficient you need to add a 3 rd wire for separate power supply. 4-wire means a
transmitter with one pair for power (typically AC line power) and a separate pair for the 4-20 mA output. Even loop-powered devices often are isolated, don't use non-isolated transmitters. Here 2-wire is best.
I.e. your ideal temperature transmitter has a 2-wire output and a 4-wire input.
Use a 2wire if the loop supplies power. If the loop is passive you'll either have to supply loop power externally, or use 3wire and supply the transmitter locally. (OK, so locally might be from a long ways away if you run the power wires from a cabinet somewhere.) In the 3wire case the transmitter is suppling the loop current.