2-W P transmitter/ PLC


Thread Starter


Hi everyone here,

We have a 2-wire differential pressure ABB transmitter (0-2 bar), and supplied by (29 VDC).

The problem is that: the operators said there was a different readings between the DCS and local screen of the transmitter. I've checked it on the site, and found there was some water leaking inside the transmitter. Then I dried it up after removing the supply wires, and then I get the wires back, but unfortunately it doesn't power up again. While I've tested it with an external 24 v source, it does work ok. I guess the problem lies somewhere either in the PLC's analog input card (S7-300) or the supply cable.

Would anyone lend me a hand, and help me on how to test the Analog Input terminals assigned to this transmitter?

Note: After taking the supply wires back in the transmitter , I measured the voltage. it has been about 3.4 v (rather than 29 v). when I take them off the transmitter, it reads (29 v)again. What is the reason do you think behind such voltage drop?

Thank you so much in advance..
My best regards.
1. Use a 4-20mA source/calibrator to test the analog input.

If the DC power wire got shorted directly to the conductor leading to the analog input, the resistor across the input could have been damaged by the overload. They're typically 1ow wattage resistors and can not handle 24V for any period of time.

2. Water in a transmitter can do all sorts of damage. Residue or moisture can be conductive and cause a ground loop, which can be the cause of a difference between the transmitter's output value and the loop current.

3. Some facilities fuse the DC power line. Does yours? Is the fuse good?

4. Out of curiosity, is the DC power supply voltage actually 29V? At the power supply?

5. The voltage measured across the transmitter terminals is the sum of the analog input resistor drop, wire resistance and the IR drop the transmitter creates to modulate the loop current. For most purposes, the wire resistance is negligible.

In a 24Vdc system, one gets these kinds of voltage drops across the transmitter's (+) and (-) in a loop with a 250 ohm input resistor:

4.0mA drops 1.0V, drop across the transmitter is 23V
8.0mA drops 2.0V, drop across the transmitter is 22V
12.0mA drops 3.0V, drop across the transmitter is 21V
16.0mA drops 4.0V, drop across the transmitter is 20V
20.0mA drops 5.0V, drop across the transmitter is 19V
Thank you so much David for the useful response. I do admit that this reply is too late, and I'm really sorry for the delay, but -anyway- late is better than never. Now, let me share with you guys how we've got to figure the problem out:

*The analog input card in question has temporarily been replaced with another working one in the same panel, and then we checked all the transmitters wired to this card (including the suspect one). They're all OK as the DCS shows. This indicates that the card is down.

** The next step: I've found out that the terminals in the Analog Input Card interconnecting the transmitter's circuit are opened (Typically there's a 25 Ohm resistor).
>** The next step: I've found out that the terminals in the
>Analog Input Card interconnecting the transmitter's circuit
>are opened (Typically there's a 25 Ohm resistor).

The input impedance of most analog inputs is 250 Ohms, if you were to short out the transmitter you would get 24/250 about 96 mA and the input resistor would have to dissipate 2.3 Watts.

If your inputs are indeed 25 Ohms and I'm not doubting it 24/25 = 960 mA and the resistor would be subject to 23 Watts, soon burn out
The good thing is apart from a couple of burnt components your card is probably fixable, just compare the damaged channel to another good one and replace the parts.

Analog input cards measure the Voltage across the Input Resistor, I doubt the A-D has been damaged.