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2 wire 4-20mA connection power connections

2 wire 4-20mA connection power connections

(OP)
Hi,

We have transmitters. These will be connected with "2 wire" connection according to project. The problem is about power connections. Do we need an external power connection for "2 wire" connections? If we connect the transmitter's 2 wire to the analog input of PLC is it OK?



I need your tips all.

RE: 2 wire 4-20mA connection power connections

You actually need to look at the specific instruments data sheet. Some need external power to be supplied on 'more' screws.

Otherwise, you should only need the two wires. Of course somewhere in the LOOP you have to have a DC power supply that will provide enough compliance voltage to allow all the instruments in the loop to work.

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

RE: 2 wire 4-20mA connection power connections

(OP)
Is there a possibility of not using any DC source just connecting 2 wires to the analog input of PLC?

RE: 2 wire 4-20mA connection power connections

There is very little chance that will work unless the PLC has special provisions since they don't provide excitation OUT of their analog inputs. You may be able to use the FIELD side DC supply of that block of the PLC to provide the needed excitation for your 2-wire instrument. You really should look at the manual for your particular PLC and see what they recommend.

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

RE: 2 wire 4-20mA connection power connections

Tips:
Those transmitters absolutely need an external DC power supply, like Keith says.

The diagram shows the needed DC power supply as a battery symbol (short vertical line next to longer vertical line).

The typical DC power supply is 24Vdc. The transmitter will operate at the stated minimum 11.5Vdc only when there is no 'load' in the circuit, which is impractical, because the analog input is a load in the circuit. Use a 24Vdc power supply.

Get one with enough capacity (amps) to drive all the transmitters; figure 25mA per transmitter and add 25% for a margin. It pays to get a premium power supply because the transmitters do not work without one and the incremental cost is peanuts in any project.

Make sure you mount the power supply so that it can be convection cooled to get rid of the heat it generates.

You need to wire it as shown. The box labelled 4...20mA represents the PLC analog input whose (+) is at the top of the box and (-) is at the bottom of the box.

I can't recall that I've ever seen a switch installed in a 2 wire, loop powered loop circuit like the one shown in that diagram. Usually the field instrument signal is an input to a control loop and who wants to risk having the control loop's input turned off? Leave out the switch.

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