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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??

Hi,

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.

Regards,
Vinodh

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.

Steve
MTL Instruments
www.mtl-inst.com

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.

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. Donīt 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!