Current Repeater - Active/Passive

<...Is it possible to connect active inputs (sources of 4-20 mA power) on both sides of the repeater?...>

Each repeater runs only one signal, unless it is a multiplexing transmitter/receiver system, which is not stated in the post. Use multiple repeaters or a single multiplexer instead if more than one signal is to be transmitted. If there were sources on both sides then either

• the signal will be corrupted, or
• the signal will be excessive and unrepresentative, or
• equipment failure, or
• a circuit protective device will operate, or
• any combination of the above

will be the outcome

A 2 wire, loop powered transmitter is a passive device, requiring an external power supply in its loop to drive the loop current.

The wiring diagram for that model MTL shows a passive 2 wire loop powered transmitter on the hazardous side, with the MTL powering the passive transmitter.

I would not call the hazardous side an 'active input', because the MTL is powering the loop.

On the 'safe' side, it shows the MTL generating an active 4-20mA output, powered by the DC power source that powers the MTL repeater.

I'm not sure what you mean by an 'active card'. Is it an analog input that can supply DC power to passive field instruments, like a 2 wire loop powered transmitter?

If so, is there some means of not using the analog input's DC power, like wiring to a different terminal, or a DIP switch that disables the DC power?

If the analog signal connections to your analog input must be powered by the receiver, then the I'd have to answer to your question with a "No, the active output of the MTL is not compatible with an input that also is attempting to powered an already powered loop current."

Firstly thank you all for very quick replies .

In the scheme on the hazardous side I can see the current source (terminals 3,1), so the transmitter could also power the loop. Am I right?

'active card' ... yes, it is an AI card that can supply power to passive field instruments, like a 2 wire loop powered transmitter... I cannot use different terminals, or a DIP switch to disable the DC power... I will have to use/buy the different AI card.

...or buy a current mirror (usual disclaimer) and insert it between the above MTL barrier and the active input in the safe area wiring.

However, the above arrangement is not needed if a passsive barrier device is used instead of a retransmit barrier. The MTL 706 and 787 (usual disclaimer) are both worthy of investigation, as they do away for the need for galvanic isolation, powering the transmitter from the active input power already available, and getting rid of the other barrier and the current mirror device.

Either way, job done!

Were the arrangement to be like #2↑, it is not safe to connect the input to that particular barrier repeater to an active input source in the hazardous area wired from the safe area, as that would defeat the concept of intrinsic safety that the barrier is intended to provide. In the case of equipment configured in this way, there is no need to provide galvanic isolation in the circuit.

If galvanic isolation is needed on an active safe area signal, then it would be appropriate to use a different current repeater for that purpose. A separate supply for the equipment will be required to provide the output, which will be an active one.

>on the hazardous side I can see the current source (terminals 3,1), so the transmitter could also power the loop. Am I right?

No. A 2 wire loop powered transmitter can never power the loop. They are always 'passive', requiring an external power supply.

The whole idea of loop powered power supplies is to keep the power supply in the safe area and use the signal wiring to also supply the power to the transmitter, which works quite well in practice.

The MTL is powering the transmitter on the hazardous side.

The terminals functions are defined in the table:

It isn't clear to me why you're using the barrier power repeater.

Do you have ground loop problem (which I would not expect with a passive transmitter) ?
Is the barrier for I/S protection?

If it were me, I'd look for a different isolating barrier; one that powers the loop on the hazardous side (like this one) and offers a passive output on the safe side, one that is compatible with your active analog input. Consider calling your vendor.

Agreed. That particular barrier is the wrong piece of equipment for the job at hand. GA