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2-wire vs. loop-powered device

2-wire vs. loop-powered device

2-wire vs. loop-powered device

According to “precisioin digital”, the term 'loop-powered' is synonymous with the term '2-wire' (meaning that only 2 connections/wires are involved in any of the connections between the transmitter, the power source and the output device).
However, PRtrans and M-System (attached file) distinguish between 2-wire and loop-powered elements. for example PRetrans 6185 is
Loop-powered signal isolation device and PRtrans 5131 is 2-wire device.

Is there really a difference between 2-wire and loop-powered devices? In other words, can a 2-wire device NOT be loop-powered or a loop-powered device not be 2-wire?
Thank you in advance

RE: 2-wire vs. loop-powered device

According to the drawings, one is loop powered from the input, and the other is loop powered from the output. The second would normally only be used with an isolator.

RE: 2-wire vs. loop-powered device

Hi djs
you mean the second should be used with an isolator or it is indeed an isolator itself?

RE: 2-wire vs. loop-powered device

It's all about where the power comes from.

Isolators are cases where there are two 4-20mA signals - one on the input side, one on the output side. Either can be active or passive, because there are different model isolators to cover the different conditions encountered with the field signals.

I take exception to their #2 example, which appears to me to be an active, powered 4-20mA output from a 4 wire device (MD2/MD2Y) running to the BVS signal transmitter which retransmits to an analog input.

It isn't clear to me why an active 4-20mA output would need a signal transmitter, usually a signal conditioner, unless the BVS is an isolator. If the BVS is an isolator, it is loop powered on the input side, using that loop power to drive an active output on its output side.

The "2 wire" must refer to the input side of the BVS, which is loop powered (whatever it is, functionally).

I agree that Precision Digital's definition is the accepted convention for the use of the terms "2 wire" and "loop powered".

RE: 2-wire vs. loop-powered device

Based on the document you shared with us, the 2 wires you have the power supply come from an external power unit, the mentioned the model M2D or M2DY.

These external power supply unit will be connected to your signal transmitter to provide the power to the input signal.

Now, talking about the loop powered, you won't need these external power unit because here the power should come from the input signal, but there is a difference here. it's necessary a load to the 4-20mA signal.

What you need to check is if your input has already a power supply or if you will need an external power supply. If you already have a power supply than you need to follow the loop powered diagram if you need a power supply than you need to follow the 2-wire diagram.

If you still have a question what's the difference between, 4-wire, 2wire and loop powered, I found some articles on the web to help you to understand that.


RE: 2-wire vs. loop-powered device

Loop powered should only refer to the electronics that derives it's energy to operate from the initial 4 milliamps of current, simple as that.

RE: 2-wire vs. loop-powered device

I fully agree with you Roy.

The very definition of "loop powered" means some value of voltage drop AND 4mA at that voltage drop.

My very first job out of college was designing a self modifying loop powered transducer. I date myself, but it used an Intel 8048 µController.

Keith Cress
kcress - http://www.flaminsystems.com

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