There is more to resistor precision than meets the eye. The three basic types of resistors -- bulk metal foil, thin film, and thick film -- are alike on the surface and may often have similar purchase specifications. However, beneath the surface, all three are made differently. Inherent design and processing will strongly influence electrical performances, so that all three behave differently after mounting. These differences will become apparent and vital as the external and internal temperatures change In addition, the effects of long-term stability, moisture, and other environmental conditions take their additional toll with time. This should be taken into account, particularly when the circuit requirements become stricter for signal-to-noise ratio (SNR) and pulse response. Thus, some so-called precision resistors turn out to be not quite as precise as one might expect after being used in the circuit. To produce a resistor with high-precision and high-stability characteristics, it is important to be able to control the influence of temperature and environmental conditions on the device’s resistance.
In precision resistors with low temperature coefficient of resistance (TCR), the self-heating (Joule effect) causes the resistors to not perform strictly to their TCR specifications. This inaccuracy will result in an error in the resistance value under applied power. Before selecting an appropriate precision resistor, one should take a hard look at how the three different types of precision resistors are made, and test them to see how they will perform in service.
Example of power coefficient of resistance test
In the test, we used three surface-mount chip resistors of the same size (1206) and same resistance value (1 K). The TCR was measured in each of the resistors (MIL range: -55°C to +125°C, +25°C Ref): foil resistor chip, thin film chip, and thick film chip. The thin film and thick film resistors were intentionally selected to have very low TCR.
During the PCR test, we applied power to the resistors from 100 mW to 500 mW and measured the resistance change during the entire test run.
Figure 1. Wheatstone Bridge Circuit