Thermocouples – How to Configure Them Using PCBs
Thermocouples, also known as resistors, are electronic devices to detect temperature; they consist of two different conductive materials forming an electronic junction. A thermocouple produces a temperature-based voltage that is read to determine temperature. Industrial thermocouples have many uses and are used in a variety of applications. They are economical, reusable, are available in several different types, and are supplied with standard electrical connectors.
Thermocouples can be either conductive or non-conductive; the most common variety is made from a non-conductive material. Thermocouples use two different temperature sensors, called resistance thermometers. These are constructed to detect changes in a layer of a metal by the change in its electromagnetic field. In other words, if a layer of metal has a low electromagnetic field, the thermocouple will sense that change, translating this into a change in temperature.
Thermocouples have been used for industrial and scientific purposes for nearly a century. Today, they continue to be widely used, ranging from temperature measurement in scientific experiments to industrial and commercial applications. They are widely used in the scientific and medical communities because of their accuracy and wide range of temperature measurement capabilities. In addition, Thermocouples have become widely used in applications requiring precise temperature measurements, such as in welding, automotive, marine, aerospace, communications, power generation, petroleum refining, and other industries.
Industrial Thermocouples comes in several different varieties, depending on their use. You can use a single-mode or dual-mode meter for more accurate measurements, or you can use a multi-step mode, which measures temperature gradation over a wide range. They are also available with variable pulse width, which allows the user to determine the time it takes for the measurement to complete the measurement.
A reference junction sensor is an integrated circuit that houses the thermocouple. A bridge rectifier is necessary to convert the measured temperature into a useful voltage. The measurement circuitry then converts this voltage back into an output signal, which can be amplified to make the signal strong enough to use as a voltage at the reference junction. Alternatively, the reference junction could use a diode or a combination of diodes. Either way, the accuracy of the measurement is still dependent on the accuracy of the reference junction and the current through it.
With a multilayer PCB, thermocouple wiring can be incorporated within the PCB itself. This makes them easier to install, which frees up some valuable room in the production area. However, thermocouples can also be installed directly onto a substrate. There are two approaches to configuring thermocouples: one is to configure the entire device as a reference junction and the other is to edit the device using a PCB editing software package such as Eagleworks or Xilento.
The first approach is preferable for low-cost products where each wire can be separately configured. For this configuration, simply connect the terminals to their corresponding pins on the board. Then connect the thermocouple to one terminal and the reference junction to its corresponding pin on the board. You can then modify the settings to your liking. However, if you want to configure all thermocouples to the same level of sensitivity (high sensitivity for low temperatures), this is not recommended, as using low-temperature sensors will give false readings if there is a slight change in the environment.
The second approach is less preferred, as it requires the PCB to have higher layer thicknesses. To test this point, disconnect one wire from each junction and place them into a small box with a fan at low speed. Once the thermocouples have cooled, you can reconnect the terminals to their terminals and see if the readings change. In this case, thermal expansion will have altered the temperature of the wires, causing the readings to become incorrect. This is a simple test that can confirm the presence of low-temperature cross-connections.
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