Temperature coefficient of resistance in current measurement.
Resistance is the combined effect of factors that cause the movement of electrons to deviate from the ideal path in the crystal lattice of a metal or metal alloy.
Cause and Effect
Resistance is the combined effect of factors that cause the movement of electrons to deviate from the ideal path in the crystal lattice of a metal or metal alloy. When an electron encounters defects or imperfections in the lattice, diffusion can occur. This lengthens the path traveled, resulting in increased resistance. The aforementioned defects and imperfections may result from:
- movement in the network under the influence of thermal energy
- the presence of different atoms in the lattice, such as impurities
- partial or complete lack of network (amorphous structure)
- unordered zones at the grain boundaries
- crystal and interstitial defects in the lattice
The temperature coefficient of resistance (TCR), sometimes referred to as the resistance temperature coefficient (RTC), is a feature of the thermal energy component of the above imperfections. The effect of this change in resistance is reversible as the temperature level returns to the reference temperature, assuming that the grain structure is not affected by high temperatures due to extreme impulse or overload. For Power Metal Strip® and Power Metal Plate ™ products, this temperature would be a resistance alloy temperature in excess of 350 ° C.
Said change in resistance due to temperature is measured in parts per million per degree Celsius (ppm / ° C) and varies greatly from material to material. For example, the manganese-copper alloy has a temperature coefficient of resistance (TCR) of <20ppm / ° C (at 20 ° C to 60 ° C), while the copper used in the terminations is around 3900ppm / ° C. The ppm / ° C values can also be represented in another way, perhaps easier to analyze, namely by noting that the value of 3900ppm / ° C corresponds to the value of 0.39% / ° C. The numbers shown may appear small until the change in resistance due to a 100 ° C rise in temperature is taken into account. In the case of copper, this would result in a 39% change in resistance.
An alternative method of visualizing the temperature coefficient of resistance (TCR) effect is to consider it in terms of the material expansion rate with temperature (Figure 1). Let's take a closer look at two different bars A and B, each 100m long. The change of resistance for the rod A is + 500ppm / ° C, and for the rod B - + 20ppm / ° C. Changing the temperature by 145 ° C will extend the rod A by 7.25 m, while the rod B will lengthen only by 0.29 m. Below is a 1/20 scale illustration showing the difference visually. The change in length of bar A is very noticeable, while bar B is not.
This also applies to the resistor as a lower temperature coefficient of resistance (TCR) will result in a more stable measurement with a temperature change which may be due to the applied power (causing the temperature of the resistance element to rise) or to the external environment.
Method of measuring the temperature coefficient of resistance (TCR)
The Temperature Coefficient of Resistance (TCR) as measured by MIL-STD-202 Method 304 is the change in resistance based on a reference temperature of 25 ° C. The temperature is changed and the device under test reaches equilibrium before each resistance value measurement. The difference is used to determine the temperature coefficient of resistance (TCR). For the Power Metal Strip WSL model, the Temperature Coefficient of Resistance (TCR) is measured at a low temperature of -65 ° C, then + 170 ° C. The equation is shown below. Typically, an increase in resistance correlated with an increase in temperature results in a positive Temperature Coefficient of Resistance (TCR). Also note that, due to the temperature coefficient of resistance (TCR), self-heating changes the resistance.
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