4-20 mA Loop Resistance

Contact MTL (usual disclaimer) and discuss the problem with them.

I use one of these.

The barrier isn't there to drive the transmitter, Captain. The barrier is there to prevent an incendive voltage/current/spark from happening in the hazardous area. By the way, the transmitter is rated EEx i for the gas group, temperature class and zone you've mounted it in, isn't it?

15V into 300ohms is 50mA, so at first glance the system will work.

The cable drop at 20mA is going to be negligible.

I don't think 15V is sufficient to drive 300 ohms in the loop.

OP did not, like usual, state a brand or model, but let's take Siemens DSIII as an example. Most quality transmitters provide a supply voltage vs loop resistance ( or maximum loop resistance) chart in their documentation. As an example, below is a Siemens DSIII pressure transmitter supply voltage vs loop resistance chart.

Where the curve crosses the bottom at zero ohms is the voltage required just to run the transmitter, with no load. Note that the Siemens transmitter takes 10.5 volts to turn on and run with zero ohm loop load, for instance, a bench test with a DC power supply connected right across the (+) and (-) terminals to see if the thing turns on.

The OP says his also takes 10.5V to run the transmitter.

If it takes 10.5V to run the transmitter, that only leaves 4.5 volts (15.0 - 10.5) to drive 300 ohms in the loop.

4.5V/300ohms = 15mA. 15mA = 68.75% of 4-20mA full scale.

15V is too little supply voltage. The Siemens chart shows that ~17V is necessary to drive 300 ohms for its DSIII.

It isn't clear whether the OP has included the IS barrier in the 300 ohms number.

Note Rosemount's explicit statement (3051 HART manual) that loop resistance INCLUDES an I/S barrier:

Your question is not very clear.

I can only say that the loop resistance should be 500Ω to convert 20mA to 10V. (since you are talking about loop...)

Otherwise, the litterature of the transmitter should be consulted .....

If the manufacturer says that it can drive 300 Ohm with a supply voltage of 15 V, that means that he (the manufacturer) has considered the maximum voltage drop on the sensing resistor (300 Ohm)

20mA X 300 Ohm = 6V

If it says that 15 V would be enough to drive your line, that means that the electronics of that transmitter will need 9 V in order to function properly.

The barrier has nothing to do with these voltages (unless it has some resistance by itself in which case you should consider the total resistance -300 Ohm- the sum of the resistance of the barrier and your sensing resistor).

Manimaran

Assuming you have a passive zener barrier then the power supply in your loop must be greater than the total of the voltage drops across all of the elements in the loop, ie the transmitter, the safe area load, the cable and the barrier.

If you have a 24V supply, a 250R load (=5V @20mA) and the transmitter takes 10.5V, then this leaves 8.5V (=24V-5V-10.5V) for the drop across the barrier and the cable.

From memory a typical MTL 2 channel barrier has a total voltage drop of 8.1V so you would be close to the limit. Ideally you should increase the power supply to 26V or use a semi-active barrier that will compensate for the voltage drop across the barrier.

If in doubt, contact your local MTL office or download some of the Application Notes from the MTL website (ww.mtl-inst.com)

Steve