TC-32 User's Guide
Functional Details
12
You can add another 32 differential TC input connections to mini-jack connectors labeled CH0 through CH31
(refer to Figure 2).
The device supports type J, K, S, R, B, E, T, N thermocouples.
Caution!
MCC strongly recommends that you ground yourself using a wrist strap before handling the
thermocouple sensors.
Connecting TCs to common-mode voltage sources
You can connect a thermocouple to a common voltage source (AC or DC) as long as the magnitude of the
voltage is within the common mode voltage range specification of 48V (DC) or 3.5V p-p (AC).
Each applied common mode voltage must be less than or equal to the
average
common mode voltage ±1.4 V.
For example, in Figure 4, there are three TCs connected to three different common mode voltage sources and
two thermocouples that are not connected to anything (floating).
In this example, the average common mode voltage is calculated as follows: (11.4 V+ 10 V + 8.6 V)/3 = 10 V
Each TC channel becomes biased to the average common mode voltage of 10 V.
Each common mode voltage must then be within the 10 V ±1.4 V limits or 8.6 V to 11.4 V.
Figure 4. TC-32 common mode voltage connections example
Cold junction compensation (CJC)
Each TC input on the TC-32 has a high-resolution CJC sensor. When you connect the thermocouple sensor
leads to the sensor input channel, the dissimilar metals at the TC-32 terminal blocks produce an additional
thermocouple junction. This junction creates a small voltage error term which must be removed from the overall
sensor measurement using a CJC technique.
The measured voltage includes both the thermocouple voltage and the cold junction voltage. To compensate for
the additional cold junction voltage, the TC-32 subtracts the cold junction
voltage from the thermocouple
voltage.
Increasing the thermocouple length
If you need to increase the length of your thermocouple, use the same type of TC wires to minimize the error
introduced by thermal EMFs.
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