2-wire and 4-wire sensor wiring for analog module

There are numerous 2 wire temperature transmitters, particularly low cost, low end transmitters (hockey puck style), that are non-isolated, input to output.

So, the installation uses a grounded thermocouple, and that ground potential is transferred through the transmitter to whatever is connected at the other end, in this case the PLC AI.

Add another grounding point from another non-isolated 2 wire transmitter, with who-knows-where and at-what-ground-potential with respect to the PLC AI and you've got a recipe for creating enormous potential (get the pun?) for ground loops and excessive common mode.

As a rule, industrial grade pressure transmitters are isolated, but temperature transmitters might or might not be, depends on the model.
Dan
 
I do not have any part numbers and the ones I was supplied may or may not be available elsewhere or probably are with different brands on them.
Basically, negative 24VDC connected straight to the analogue input card - positive into the positive connection on the hockey puck and then out of the hockey puck negative to the positive input on the analogue card - classic straight forward loop powered device.
Beleive me, I did have the problem and only when I connected more than 1 of the devices to the analogue card. If I disconnected 2, the one left connected worked fine. I also had this confirmed by another SI friend of mine and he had the same problem once and now only uses isolated devices when connecting more than 1.
I must also say that I have not had the problem before with other devices, they have worked perfectly - I cannot explain why it happened but it did happen
 
BobB,
There are plenty of "Hockey Pucks" that are 2 wire and isolated. Even if they aren't provided your probes isolated from ground you should be OK.
Roy
 
Twido analog module changed - but same issue

I have now changed the twido module to TWDAMI2HT. The Twido documentation says is supports differential inputs.

First analog input i connect is Mettler Toledo-M300 Conductivity Transmitter which has external 230VAC supply and gives output of 4-20mA. This from the documentation states, it is galvanically isolated.

The other analog input is 2-wire Level transmitter from Anderson. Loop powered.

These are both conected to the TWD AMI 2HT module of Twido PLC. One is loop powered another is not loop powered.

I had the same problem, and my analog channel is now not working. If i have to use isolatio, what has to be isolated??? The 2-wire sensor, or the 4-wire sensor.

I am attaching the module specifications and the wiring from catalog.

Also is this a module problem, because i do not face the same problem with any other PLC from Schneider, for eg. TSX Micro, M340, TSX Premium.


Regards,
Parvez

TWD AMI 2HT Module Specifications.jpg TWD AMI 2HT Wiring.jpg
 
2ns9qhz.jpg


I suppose one could argue that subtracting the value on the negative terminal from the value on the positive terminal constitutes a 'differential input', but usually there is some isolation from ground inferred in the statement.

For this AI card, the commons are tied together, hence you've got a ground loop problem.

>The problem i am facing now is that, when i connect the 4-wire transmitter, all my 2-wire transmitters give max reading at 11 to 12 mA instead of 20mA.
It sounds like all of your temp transmitters work OK by themselves, until you connect the conductivity 4 wire signal?

If so, the first step is to put a loop isolator on the condutivity signal and see if that cures the problem, even though the conductivity output claims to be 'galvanically isolated'.

If the 4 wire output is galvanically isolated, then the likelihood is that the 2 wire devices are causing the problem.
 

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