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Choosing the best resistor technology for particular apps

Posted: 12 Sep 2012 ?? ?Print Version ?Bookmark and Share

Keywords:resistors? cost-benefit analyses? total error budget?

The selection of components for every application involves tradeoffs. When it comes to resistors, several device technologies are available to designers and each of them makes sense for a certain subset of applications depending on cost-benefit analyses. However, when the application requires stability over time and load, initial accuracy, minimal change with temperature, resistance to moisture, and a number of other characteristics, the choices are more limited.

The purchase price of each resistor technology generally falls along the lines of thick films being the least expensive, thin films being more costly, and foil being more costly yet. But as we all know, purchase price and "cost of usage" are two very different things. The inexpensive device that fails can wind up costing many times more in terms of replacement costs before shipment, failures in operating systems after shipments, scrubbed missions, and future business.

Thin film resistors are more precise than thick film resistors. They are also more costly. This technology is best suited for applications requiring greater precision, as in analog circuits where the stability of specific values is important, rather than just the mere presence or absence of a signal. Here, the designer makes both economical and performance analyses and determines that the requirements for precision and stability are satisfied by the more-costly thin films with acceptable risk and consequences of failure for the application.

In some applications, however, the consequences of failure are so costly that only the use of very high precision, very high reliability resistors, such as foil devices, can be justified. For example, telemetry equipment in remote earth locations may be extremely expensive to access and repair, and lives could be lost if the signal goes down. Systems in space must work as required with the greatest degree of confidence; there is no replacement opportunity and the cost of getting the system into operating locations is astronomical. Automatic test equipment performing hundreds of almost instantaneous tests on semiconductors as they come off the production line must perform with precision and reliability or hundreds of thousands of dollars' worth of materials could be lost. Medical equipment cannot give false or undependable readings and still safeguard people's health and lives.

The choice of resistor technology often depends on the designer's view of the overall error budget (TEB C total error budget). The designer may choose to use a percentage of full deviation error budget if the equipment will never see full-scale stress conditions. For example, a laboratory instrument that is expected to be permanently installed in an air-conditioned laboratory does not need an end-of-life allowance for excessive heat.

However, there are other reasons for making the tolerances of the resistors tighter than the initial calculation. Measurement equipment accuracy is traditionally 10 times better than the expected accuracy of the devices under test, so these tighter tolerance applications require a foil resistor. Also, the drift of the resistor without any stress factor considerations at all will still experience in a base-level shift over time that must be considered. Foil resistors have the least amount of time shift. The equipment manufacturer's recommended recalibration cycle is a factor in the marketability of his product and the longer the cycle, the more acceptable the product. Foil resistors contribute significantly to a longer calibration cycle.

Since the stress levels of each application are different, the designer must make an estimation of what the level of stress might be and assign a stress factor to each one. In some applications the operating stress level might be low, but the non-operating stress levels can still be high. For example, if the resistor is installed in a piece of equipment that is expected to go out into an oil field in the back of a pickup truck, then shock, vibration, rain, subarctic cold, or heat from the sun are obvious factors.

Industry standards for shock and vibration are based on the robustness of end products considered as the sum of their parts, and the threshold is what the most susceptible part can withstand. Above and beyond the industry standard, individual part specifications may include higher levels of shock and vibration sustainability. This applies to jet aircraft, truck-, tank-, and ship-mounted military equipment, air-drop emergency equipment, missiles, and so on.

Figure 1: There are several factors taken into account in the total error budget of a precision resistor. It may need to be increased due to performance inconsistencies between resistors.

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