Comparison of three resistor technologies in terms of resistor power coefficient

The issue of resistor accuracy seems more complicated than imagined. There are three basic types of resistors: Bulk Metal Foil, Thin Film, and Thick Film, which may look similar on the surface and may have similar purchasing specifications. But in reality, all three types of resistors are manufactured in different ways. Intrinsic design and processing will greatly affect electrical performance, so all three resistors will behave differently after installation. These differences become very noticeable and critical when external and internal temperatures change. Additionally, the effects of long-term stability, humidity, and other environmental conditions can have additional effects over time. This should be considered, especially as circuit requirements become more stringent with respect to signal-to-noise ratio (SNR) and impulse response. As a result, some so-called high-precision resistors, when used in circuits, turn out not to be as accurate as one might expect. To produce resistors with high precision and stability, it is important to be able to control the effects of temperature and environmental conditions on resistive devices.

In high-precision resistors with low TCR values, self-heating (Joule effect) can prevent the resistors from strictly meeting their TCR specifications. This inaccuracy can cause errors in the resistor value under the applied power. Before selecting the corresponding high-precision resistor, you should carefully understand how these three different high-precision resistors are manufactured and test them to understand how they perform in use.

　　Resistor Power Coefficient Test Example:

In this test we used the same size (1206)

Three surface mount chip resistors with the same resistance value (1 K). We tested the TCR value of each of the following resistors (MIL range: -55°C ~ +125°C, +25°C reference): foil resistors, thin film chip resistors, and thick film chip resistors. We intentionally choose thin film and thick film resistors to achieve ultra-low TCR values.

During the PCR test, we applied a power of 100 mW to 500 mW to these resistors and tested the change in resistance value throughout the test process.

We used a basic Wheatstone bridge circuit. The resistance value of this bridge leg is chosen to apply high power to the test resistor (Rx) while ensuring that the remaining three legs remain ultra-low power. As power is increased, any changes in Rx are caused by self-heating of Rx.

　　Formula:
　　　　　　Rb×Rx = Rstd×Ra? (1 K×1 K = 10Ω×100 K)??

Ra: 100 K, rated power: 0.3W at +125℃, Rb = 1K, rated power: +125℃ 0.3W at Rstd: 10Ω, rated power: 0.3W at +125℃? Rx = 1 K, rated power: 0.3W at +70℃

　　Conclusion:

High-precision foil resistors achieve the most available stability during resistor temperature changes caused by two factors (ambient temperature changes - TCR and internal temperature rise PCR caused by the Joule effect of load changes).