how is a RTD connected to a PLC

 

>how is a RTD connected to a PLC<

Having struggled with RTDs many years ago, I can understand how confusing this can seem to be. An RTD is a very simple device, the resistance of which changes in proportion to the temperature in the area where the RTD is mounted.

RTDs are usually remotely located--remote in this sense meaning they are not located directly next to the PLC or monitor they are connected to. So, wires have to be used to connect them to the monitors or PLCs.

Wire has resistance, and long wires have more resistance than short wires. So, since the resistance of the RTD circuit is very important the resistance of the wires used to connect the RTD to the PLC or monitor needs to be accounted for (subtracted) in the conversion of the RTD resistance to temperature.

So, there is a third, and sometimes a fourth, wire that is used to calculate the resistance of the wire in the RTD circuit and that resistance is then subtracted from the total resistance of the circuit to arrive at the RTD resistance which is then converted to temperature by the PLC or monitor.

A simple "drawing" of an RTD is shown below. Typically, the two wires coming from one end of the RTD are the same color (quite often white), and the single wire coming from the other end is a different color (red or blue, for example).<pre>

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RTD | |
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| \
| \
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----------- </pre>

You can use an ohm meter to determine which wires are which wires are which if there is any doubt with a particular RTD. The two wires coming from one end of the RTD are sometimes called the "compensation" leads.

So, when you are connecting an RTD to a PLC or monitor you need to pay particular attention to how it is connected, because the single RTD lead must be connected to a particular terminal, and the compensation leads must also be connected to specific terminals.

For more information about RTDs, go to www.omega.com and search for RTDs; there is some really good information there. You can also try www.wikipedia.org for information.

One more thing to note: Many digital controllers (PLCs and monitors) use a function called "multiplexing" to read RTD inputs. This means that its very difficult to use some RTD simulators to simulate RTDs. One needs a decade box, or a potentiometer, to simulate temperature to these kinds of devices.

If you have a specific problem, please provide more details and we can probably provide more assistance.

Hope this helps!

 

>how is a RTD connected to a PLC<

http://lmgtfy.com/?q=wikipedia+resistive+temperature+device

 

can u tell me where the wheatstone bridge will be located and how is the RTD connected to it

 

> can u tell me where the wheatstone bridge will be located

ans: analog input (AI) card in the PLC/DCS/RTU/controller/recorder/indicator

> and how is the RTD connected to it

Depends on model of the AI card, number of leads from the RTD. Read the documentation.

 

My opinion is: do not think too much technically regarding wheatstone's bridge etc.

You know very well, universal temperature transmitters are available in the market. Configure them for RTD 3 /4 wire as the case may be, and convert the value to 4 - 20mA inside the transmitter. So, now you have 4 - 20mA linear signal for the temperature. Give a range to the transmitter & configure the same range inside the PLC (AI card).

Then you can display the value for SCADA or whatever.

Finish!