Baby it’s cold outside… Cold Junction Compensation sensors, that is. In this installment of the Tech Files, we’ll look at the various CJC’s for Rockwell Automation thermocouple I/O modules. This includes the I/O modules from Encompass Partner, Spectrum Controls. Typically the CJC sensor comes with the thermocouple input module, but in some cases they do not. This reference is also useful if you need to replace a lost or damaged CJC sensor.
What is Cold Junction Compensation, or CJC?
When using the module with a thermocouple input type, the channel must account for the thermoelectric effect of a junction of the thermocouple field wires and the screw terminals of an RTB or IFM.
The junction at which temperature is measured is the hot junction. The junction where the thermocouple wire interfaces with copper is the cold junction. The module always measures and reports the cold junction temperature in °C. The transition from thermocouple wire to copper typically happens either on the module screw terminal itself or at an IFM.
The thermoelectric effect alters the input signal and must be compensated for to measure temperatures accurately. To accurately compensate the input signal from your module, you must use a cold junction compensation (CJC) sensor to account for the increased voltage.
ControlLogix® Platform (1756)
The CJC sensors come with the 1756-IT6I (qty 2) and 1756sc-IF8U (Qty 2). If using the newer 1756-IRT8I module, the CJC sensors will need to be purchased separately. Note that the CJC sensors are built in to the IFM module, if you are using the remote wiring solution. If you need to purchase or replace the CJC sensor, use the catalog numbers below for each module.
- 1756-IT6I – use CJC PN-C29685 (included when purchasing the module)
- 1756-IRT8I – use CJC 1756-CJC ( not included when purchasing the module)
- 1756sc-IF8U – use CJC 5300004-02 (included when purchasing the module)
Note that the 1756-IT6I, 6-channel thermocouple input module is a discontinued item, and no longer available for purchase as new.
CompactLogix™ Platform (1769)
There are three 1769 modules that can read thermocouples. The 1769-IT6 and 1769sc-IT6I come with the CJC sensors (qty 2) when purchasing the module. The 1769sc-IF8U does not include the CJC (qty 1 needed). All three temperature modules use the same CJC sensor manufactured and sold by Spectrum Controls.
- 1769-IT6 – use CJC 5300004-04 (included when purchasing the module)
- 1769sc-IT6I – use CJC 5300004-04 (included when purchasing the module)
- 1769sc-IF8U – use CJC 5300004-04 (not included when purchasing the module)
Rockwell has a Knowledge Base article that is open to everyone, regardless of TechConnect status, providing additional information on wiring the CJC sensors for the various 1769 temperature modules.
SLC-500 Platform (1746)
The CJC sensors for the 1746 SLC-500 thermocouple input modules are supplied when purchasing the modules. They are either built into the RTB on the input module, or for Spectrum Controls, provided on the module.
- 1746-NT4 – CJC is built into the RTB 1746-RT32
- 1746-NT8 – CJC in built into the RTB 1746-RT34
- 1746sc-NI8U – CJC sensors (qty 2) come with the module
Note that the 1746-NT4 4-channel thermocouple input module is a discontinued item, and no longer available for purchase as new.
MicroLogix Platform (1762)
There are two thermocouple input modules available for the 1762 MicroLogix platform. Both modules ship with the CJC sensors included.
- 1762-IT4 – no replacement catalog number available for CJC sensor
- 1762sc-IF8U – CJC sensor is included when purchasing the module
Micro800 Platform (2080 and 2085)
When using the Micro800 Platform (2080 plug-ins, or 2085 expansion I/O), the CJC sensors are included with the I/O modules when you purchase them. In some cases, the CJC sensors can be purchased separately in case they are lost, or damaged.
- 2080-TC2 – CJC sensor built-in to the plug-in module
- 2085-IRT4 – CJC sensor (qty 1) included with the module
- 2080sc-IF4U – CJC sensor built-in to the plug-in module
- 2085sc-IF8U – use CJC 5300004-02 (included when purchasing the module)
Flex I/O Platform (1794)
The 1794 Flex I/O platform offers four input modules capable of reading thermocouple inputs, two from Allen-Bradley and two from Spectrum Controls. All four modules use the same CJC sensor, 1794-CJC2.
- 1794-IT8 – use CJC 1794-CJC2 (included when purchasing the module)
- 1794-IRT8 – use CJC 1794-CJC2 (included when purchasing the module)
- 1794sc-IF8IU – use CJC 1794-CJC2
- 1794sc-IRT8I – use CJC 1794-CJC2
POINT I/O Platform (1734)
There are two thermocouple input modules available for the 1734 POINT I/O platform, one from Rockwell Automation, and the other from Spectrum Controls. The CJCs are built into the RTB when using the 1734-IT2I, and are supplied by Spectrum Controls with the 1734sc-IF4U. But if you need to replace the CJC sensors, here is what you need to order.
- 1734-IT2I – there are two CJC thermistors on the 1734-RTBCJC RTB
- 1734sc-IF4U – use CJC 5300004-03 (included when purchasing the module)
Rockwell has a Knowledge Base article that is open to everyone, regardless of TechConnect status, providing additional information on the 1734-IT2I Thermocouple input module and use of CJC.
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