Price, cost, risk, and selecting the best resistor technology for the application
The choice 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. But 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 – 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.
But 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.
Another aspect that should be reviewed is post manufacturing operations (PMOs). The PMO was first established a few decades ago when the demand from the military and space was for production methods that would bring the ΔR (“the shift of resistance value”) of the resistors to a minimum after launch into outer space. The PMO today combines two elements: short time overload and accelerated load life. The PMO should only be considered when the level of stability required is beyond the published limits for standard products.
Specific types of circuits requiring ultra-precision resistors, particularly when implemented in end systems operating in extreme environments, are too numerous to list. But for an example, consider the current mirror, which could be found anywhere electronics are found.
The function of the current mirror circuit is to duplicate, attenuate, or amplify a specific current source or current signal in such a way that the output current is identical to the input but just scaled by a constant gain ratio: A. In the case of a ratio, A = 1, the circuit behaves like a buffer. When the gain is less than 1, the circuit performs as an attenuator, and when the gain is greater than 1, the circuit performs as an amplifier. In the circuit shown, a current signal (I-ref, which is passed through R1) is input to the current mirror. The output signal, in an ideal situation, will be the exact same signal except scaled by the gain ratio A = R1/R2.
The ratio of R1/R2 must remain constant throughout the operation of the product for this circuit to give the most accurate reproduction of I-ref. When the gain ratio “A” changes because of resistance changes due to the effects of temperature, operating time, operating power, or other environmental conditions such as humidity, the output current, I-out, will change as well even if I-ref remains constant. The unwanted distortions of the original signal can be called noise, drift, or various other terms for error.
In the case of the example above with gain ratio of 100, in order to balance the input voltages of the op amp, a relatively large current (500mA, 2.5W) is supplied to R2 when the input current is 5mA. In this case
R2 may become very hot due to self-heating, which means that various changes of the sensed voltage are
experienced due to temperature coefficient of resistance, power coefficient of resistance, thermal EMF
generation, and current noise; thus the output signal is distorted and no longer represents the exact shape of the input signal.
An example of a non-standard solution for this circuit is given below. Two resistors of VCS332Z are chosen to assure the stability of R1/R2. The output terminals have been configured to behave like a voltage divider. This takes advantage of the self-heating of one resistor to bring the other resistor to the same temperature resulting in an identical response to maintain the ratio. The VCS332Z was selected for its very low TCR to negate changes due to ambient temperature and for its low power coefficient of resistance (PCR) to reduce ratio changes when power is suddenly applied.
Figure 2: Bulk Metal Foil VCS332Z Current sensing power Resistor with Z-Foil technology
Figure 3: Current Mirror Circuit
Different applications require different resistor technologies; whatever the application, some price/performance tradeoff is always involved. An effective price-cost-benefit-risk analysis must be conducted for each application to ensure selection of the appropriate resistor for the application. The basis for such an analysis is a thorough understanding of the performance characteristics and reliability implications of each technology in each application.
About the author
Yuval Hernik holds a B.Sc in electrical engineering from the Technion (Israel Institute of Technology). He has been a director of application engineering at Vishay Precision Group – Bulk Metal Foil resistors – since 2008.