4-20ma and loop powered


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Can someone explain to me the different between 4-20ma loop powered and 4-20ma?

To my knowledge, 4-20 ma loop powered means that 24Vdc power supply is coming from the DCS Analogue input card for any transmitter connected to the DCS. But, how do you explain 4-20 ma input from a transmitter that is not loop power from the DCS and yet connected to the DCS Analogue input card??

There are 2 types of tramsmitters that can be connected to DCS/PLC or any controller - 2 WIRE & 4 WIRE. By 2-wire it means the transmitter is powered form the system to which it is connected... normally 24VDC and 4-20mA signal is connected to the control system using only one single pair of wires, by 4-wire it means the transmitter is powered from a external power supply and signal 4-20mA which is a separate pair of wires is connected to the system, e.g. a mag flow meter is powered 24VDC or 110VAC or 230VAC and output 4-20mA is connected to DCS/PLC AI board channel.

Hope your confusion is cleared.

Any 4/20mA field device using 2 wires is probably going to be "loop powered", ie it derives its power from the first 4mA of the signal. There are many other devices that are separately powered but still emit a 4/20mA signal that can be picked up by a DCS input card. In this case the input card would be "passive" and supply no power to the loop.

MTL Instruments
The norm is to refer to Active or Passive loops. Active loops are powered from the A/I cards in the DCS and Passive loops are powered from the Instrument, you must also be aware that normally DCS A/I cards have different terminations for Active and Passive circuits.

Robert Scott

A 4-20ma loop-powered device extracts the power it needs to run its circuits from the loop itself. There is no separate power feed from the Analog input card or from anywhere else. The current that the device draws becomes part of the 4 to 20 ma. that flows in the loop, therefore the device must not require more than 4 ma. to operate.

A non-loop powered device gets its power from somewhere else. It could be a separate power supply, or it could be power from the analog input card. But it is still considered to be non-loop powered if the power does not come from those two wires in the loop.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting
You can connect a device (for example a pt100-4-20mA transducer) to the PLC using the PLC analogue input card power supply or you can use an other power supply to "power" the loop.
The input card (or plc) should have the ability to get the signal from a device that has its own power supply.

That is, like a simple mili-amperometer, meaning you only read the current value, no matter where the power comes from.

Most PLC, or like you say, DCS, can do that, you must look at the manual and connect it the right way. Don´t worry, Most of these devices have their protections, so it can be, like in my own experience have always been, a try, and try, and try, and then get to the right connection!
Transmitters are called 4 wire or 2 wire. A 2 wire transmitter only has 2 wires connected to it. The power to run it comes from the device it is connected to (in your case, an analog input card) which internally measures the current the instrument is drawing from the card. That is why it is important that there be enough voltage reaching the transmitter to allow the transmitter to operate. You will see a specification for the minimum voltage the transmitter needs. This is measured at the transmitter, not the source.

A 4 wire transmitter, has 4 wires, 2 for the 4-20MA output, and 2 for the power to supply the transmitter. This is usually an AC Voltage. In this case, the transmitter provides the power for the device it is connected to, to measure the 4-20 MA coming from the transmitter. Sometimes it is a device which simply measures the voltage drop of the 4-20 MA across a resistor. For example 4-20 MA across a 250 Ohm resistor gives a 1-5 VDC signal into the analog input card. Most input devices have separate terminals to use either a 2 wire, (loop powered) or 4 wire (field powered) input. But in either case, there will still be only 2 wires connected to the input card. If you can get a schematic for the internals of the input card you will be able to see how the two different inputs are treated.
I am having a loop powered tx with DCS. Now if my tx got earthed from field side then what could be the affect at DCS AI card. Suppose in normal conditions if I am getting 24 Vdc output at card then after connecting this tx having earth fault what would be the voltage (approx) at DCS terminals.
If you read Robert Scott's response, he explained it well. The 4 mA powers the electronics in the measuring circuit

Issac Issachar

There are 0-20mA analog field signals. But they don't come from 2 wire loop powered instruments.

Loop powered field devices get the electrical power needed to operate the field instrument from the current below the minimum output of 4mA (generally about 3.5mA or below, because the lower fail-safe output can be various values, like 3.6 or 3.8m8.

When the signal range starts at zero, 0-10mA or 0-20mA, there would be no current available for the field device if the output were 0mA.

So analog outputs that start at zero cannot be, by definition, 2 wire loop powered devices and must be 4 wire devices, with electrical power need for operation supplied from a power supply that is not part of the output loop.

Arthur Mayclin

Another benefit of a 4-20ma signal over a 0-20mA signal is the ability to detect a failed loop.
why we have to use 4 wire connection we can measure with 2 wire itself.........

i need to know difference between two wire and four wire............
I am trying to explain 2 wire & 4 wire system differently.

a] 2 wire xtr[ loop power ] -- means 24vdc is
required to maintain the current loop of 4-20
madc.ie 24vdc PS & 4-20 madc
signal is both through the same pair of cable.
In this event xtr is passive device.[sink]

b] 4-wire xtr[transducer ] -- in this device ps
line [ +ve & -ve ] & signal line [ +ve , -ve ]
through seperate cables. In this event xtr[transducer ] is active device[ source ].

Important issue here is, whenever active component is involved in the field, the life of transducer is much less compared to passive device[2 wire ]. to increase the reliability of process control application 2 wire system have been introduced,today, except loadcells & analyse all majority of instruments, commonly used have
2 wire concept.

we would like to know the name of this genius. He should be awarded Noble prize of Instrumentation Industry.

thanks in advance

>Why 4-20ma and not 0-20ma or 0-16ma.
>GapeTM [AT] eskom.co.za

By having an offset of 4 ma, you can tell if the transmitter is at a zero process value reading (i.e. when you are getting 4 ma), or the transmitter has failed (you are getting 0 ma).
Can an Active Device be used as a Passive Device? Or all 4 wire devices need without a doubt the external power supply?

I have an instrument which is passive, but the isolator/converter at the PLC is for Active instruments...would the instrument work just the same? or is ther a specific characteristic on active devices that prevent the from being loop powered?

Thank you!
It isn't clear to me how an isolator/converter can be for only "active" instruments/signals. The isolator might be intended for active outputs because active outputs are prone to ground loop and common mode problems, but the isolator converter 'sees' only the signal, it has no way of knowing or discerning where the power comes from to power the signal/instrument.

If the instrument is passive, it expects an external power supply to power the loop.

So, use a DC power supply to power the instrument to get a signal to the instrument side of the
isolator converter.

The output of the isolator converter goes to whatever analog input you've got.

If it were me, I'd try connecting the 'passive' instrument/power supply directly to the analog input because it'll probably work OK.

> or is there a specific characteristic on active devices that prevent the from being loop powered? <

An active output is already powered. It doesn't need a power supply, it already has one. Put another power supply in the loop and they'll buck each other. Why would you do that?

I have one analog input card where 4-20ma transmitters are connected in two different ways. The first one is as follows: One wire from a 24VDC power supply is connected to one wire of the transmitter, the second wire of the transmitter is connected to the input screw on the analog input card while the other wire from the 24vdc power supply is connected to the "ground screw right next to the input screw on the analog input card. This will close the loop.

The other wiring option I have seen is that the two wires from the transmitters goes straight to the two screws on the analog input cards.

can anyone explain the difference on these two wiring scheme?

any help will be appreciated