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How to Wire and Calibrate a Thermocouple?
Wiring a thermocouple
| Temperature is provide by a thermocouple sensor connected to a ACE PLC
ACE 1450, 3090, 7096,.. have thermocouples inputs. Our Type K thermocouple is wired as follows:
Reverse the wire if the temperature is not correct. Sometimes, with other thermocouples, the + and – can have different colours. See the thermocouple manual.
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Precautions for wiring a thermocouple probe to a ACE PLC input
- Use compensation wires of the same type as the thermocouple to avoid measurement errors
- Cable extensions, if unavoidable, should be as short as possible.
- Observe the polarity of the thermocouple when connecting to the PLC input.
- Avoid unnecessary junctions and loose connections that create parasitic voltages and unstable temperatures.
- Separate the thermocouple wiring from the power cables to limit electromagnetic interference.
- Ground the shielding on one side only if the cable is shielded. Ensure that the GND quality is adequate.
- or connect the cable braid to 0 VDC
- Check that the controller input is configured for the correct type of thermocouple and measurement range.
Calibrate the thermocouples
After selecting the correct thermocouple (Type K) in the configuration wizard…
Choose “Calibrate Thermocouple” from the vBuilder menu.
| Check therocouple you’d like to calibrate
Inputs F1, F2, F3 or F4 forACE-3090 as example Set the current temperature, then, press calibrate You can select a second point to obtain a more accurate measurement. For example, 21°C for the first point and 0°C for the second point.
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If you copy the thermocouple input (InFloatF4 in our case), into any other tag, just to see the value in debug mode, you will see the temperature appear.
Abnormal temperature spikes/drops on thermocouple measurements
The most common problem is poor connections in ACE connectors. You should use crimp ferrules, especially if the wire is very thin.
Symptom: brief, simultaneous pics/drops on several thermocouples, synchronized with machine state changes (compressor, contactor or SSR switching).
| Probable cause | Solution |
|---|---|
| Outdated controller firmware (version other than 9.1) | Check the firmware version and update to 9.1 |
| EMI noise from motors, VFDs, contactor coils and relays during switching | Fit RC snubbers or varistors on AC coils and flyback diodes on DC relays. Use output filters and shielded motor cables on VFDs |
| Ground loop: thermocouple junction touching a metal ground | Use insulated-junction thermocouples or a galvanically isolated input module |
| Unshielded or untwisted thermocouple cables | Use shielded, twisted-pair thermocouple extension cable |
| Shield wrongly connected (grounded at both ends, or not at all) | Ground the shield at one end only, on the controller side |
| Measurement cables routed with power cables | Separate the cable routes (≥ 20 cm) and cross at 90° if needed |
| Intermittent wiring fault: loose terminals, or vibration-induced micro-cuts when the compressor starts | Retighten all terminals, use ferrules, secure the cables against vibration, and inspect the extension wire for damage |
| 24 V supply dip when loads start, disturbing cold-junction compensation and the ADC | Check the 24 V with an oscilloscope and separate the controller supply from the actuator supply |
| Cold junction compensation (CJC) issue: faulty CJC sensor, or a sudden temperature change in the cabinet (fan or heater switching) | Check the CJC reading. Keep the input module away from heat sources and airflow, and replace the module if the CJC is faulty |
| No software filtering on analog inputs | Enable filtering or a moving average in the input configuration |
We can also add a capacitor with a capacitance of between 100 – 470 µF and a voltage rating of 35 or 50 V to the ACE’s 2-pin power supply input.