rtd issues compactlogix

[elmatador]

Hi Guys,

I have field RTD's which are 4-wire. It has two red wires and two white wires. The input to my RTD compactlogix module (1769-IR6) is a three wire connection.

Currently, all my RTD values are coming in as full scale (32767). The sensor is a Pt385 100 Ohm sensor. When i measured the resistance across the two legs i got 112 Ohms.

For this type of sensor does it mean that at zero degrees it will read 100ohms? Also, could it be the wiring is messed up? Do i jumper the two whites together?

I am missing something quite fundamental here. If the ohms going into the PLC card is only 112ohms why would it interpret it as full scale?

 

[Bob O]

Here is a link to the manual. Page 3-14 explains the wiring.
Chapter 4 talks about channel config.

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1769-um005_-en-p.pdf

Hope this helps.

 

[theDave2]

To use a 4 wire RTD on a 3 wire input, just don't connect one of the wires.

100 ohms is 32 deg F.

I'm guessing that the 32767 is because of the input wring and having the scaling set to RAW.

You will have to set the rtd type and scaling in the input card setup and a few other parameters, like eng. units/raw. Just select the card in the I/O tree and double click it.


All of this is spelled out in the IOM (Installation & Operating Manual) for the card.

 

[Roy Matson]

Yes, your RTD will be 100 Ohms at zero celcius
138.5 Ohms at 100 celcius
As Dave says just ignore one of the wires, The only time 4 wires are used is for super accuracy.
Roy

 

[shooter]

If value is max cables are not correct
use two white on one side of the resistor symbol and one red on the other side .
yes 112 ohms is very nice about 30 degrees. check the resistance of a loop it will be about 3 ohms or so making real temp 24 celsius.