selecting the best resistor technology for the application

7/30/2019 Selecting the Best Resistor Technology for the Application

http://slidepdf.com/reader/full/selecting-the-best-resistor-technology-for-the-application 1/4

Selecting the best resistor technology f or the

application

Yuval Hernik, Director Application Engineering, Vishay Precision Group (VPG) - Decem ber 17, 2012

The choice of com ponents f or every application involves tradeof f s. When it comes to resistors, several device

technologies are available to designers and each of them makes sense f or a certain subset of applications

depending on cost-benef it analyses. But when the application requires stability over time and load, initial

accuracy, minimal change with temperature, resistance to moisture, and a num ber of other characteristics, the

choices are more limited.

The purchase price of each resistor technology generally f alls along the lines of thick f ilms being the least

expensive, thin f ilms being more costly, and f oil being more costly yet. But as we all know, purchase price and

“cost of usage” are two very dif f erent things. The inexpensive device that f ails can wind up costing many timesmore in terms of replacement costs bef ore shipment, f ailures in operating systems af ter shipments, scrubbed

missions, and f uture business.

Thin f ilm resistors are more precise than thick f ilm resistors. They are also more costly. This technology is best

suited f or applications requiring greater precision, as in analog circuits where the stability of specif ic values is

important, rather than just the mere presence or absence of a signal. Here, the designer makes both economical

and perf ormance analyses and determines that the requirements f or precision and stability are satisf ied by the

more-costly thin f ilms with acceptable risk and consequences of f ailure f or the application.

In some applications, however, the consequences of f ailure are so costly that only the use of very high precision,very high reliability resistors, such as f oil devices, can be justif ied. 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 conf idence; there is no replacement

opportunity and the cost of getting the system into operating locations is astronomical.

Automatic test equipment perf orming hundreds of almost instantaneous tests on semiconductors as they come of

the production line must perf orm with precision and reliability or hundreds of thousands of dollars’ worth of

materials could be lost. Medical equipment cannot give f alse or undependable readings and still saf eguard

people’s health and lives.

The choice of resistor technology of ten depends on the designer’s view of the overall error budget (TEB – total

error budget). The designer may choose to use a percentage of f ull deviation error budget if the equipment will

never see f ull-scale stress conditions. For exam ple, a laboratory instrument that is expected to be permanently

installed in an air-conditioned laboratory does not need an end-of -lif e allowance f or excessive heat.

But there are other reasons f or 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 f oil resistor. Also, the drif t of the resistor without any

stress f actor considerations at all will still experience in a base-level shif t over time that must be considered. Foil



resistors have the least amount of time shif t. The equipment manuf acturer’s recommended recalibration cycle is a

f actor in the marketability of his product and the longer the cycle, the more acceptable the product. Foil resistors

contribute signif icantly to a longer calibration cycle.

Since the stress levels of each application are dif f erent, the designer must make an estimation of what the level of

stress might be and assign a stress f actor to each one. In some applications the operating stress level might be

low, but the non-operating stress levels can still be high. For exam ple, if the resistor is installed in a piece of

equipment that is expected to go out into an oil f ield in the back of a pickup truck, then shock, vibration, rain,su barctic cold, or heat f rom the sun are obvious f actors.

Industry standards f or 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 specif ications may include higher levels of shock and vibration sustainability. This applies

to jet aircraf t, truck-, tank-, and ship-mounted military equipment, air-drop emergency equipment, missiles, and

so on.

Figure 1. There are several f actors taken into account in the total error budget of a precision resistor

It may need to be increased due to perf ormance inconsistencies between resistors.

Selecting the best resistor technology (cont.)

Another aspect that should be reviewed is post manuf acturing operations (PMOs). The PMO was f irst

established a f ew decades ago when the demand f rom the military and space was f or production methods that

would bring the ΔR (“the shif t of resistance value”) of the resistors to a minimum af ter launch into outer space.

The PMO today combines two elements: short time overload and accelerated load lif e. The PMO should only

be considered when the level of stability required is beyond the published limits f or standard products.

Specif ic ty pes of circuits requiring ultra-precision resistors, particularly when implemented in end systems

operating in extreme environments, are too numerous to list. But f or an example, consider the current mirror,

which could be f ound anywhere electronics are f ound.

The f unction of the current mirror circuit is to duplicate, attenuate, or amplif y a specif ic current source or current

signal in such a way that the output current is identical to the in put but just scaled by a constant gain ratio: A. In



the case of a ratio, A = 1, the circuit behaves like a buf f er. When the gain is less than 1, the circuit perf orms as

an attenuator, and when the gain is greater than 1, the circuit perf orms as an amplif ier. 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 = R 1/R 2.

The ratio of R 1/R 2 must remain constant throughout the operation of the product f or this circuit to give the most

accurate reproduction of I-ref . When the gain ratio “A” changes because of resistance changes due to the ef f ects

of tem perature, operating time, operating power, or other environmental conditions such as humidity, the outputcurrent, I-out, will change as well even if I-ref remains constant. The unwanted distortions of the original signal

can be called noise, drif t, or various other terms f or error.

In the case of the exam ple above with gain ratio of 100, in order to balance the input voltages of the op am p, a

relatively large current (500mA, 2.5W) is supplied to R 2 when the in put 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 coef f icient of resistance, power coef f icient 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 f or this circuit is given below. Two resistors of VCS332Z are chosen toassure the stability of R 1/R 2. The output terminals have been conf igured to behave like a voltage divider. This

takes advantage of the self -heating of one resistor to bring the other resistor to the same tem perature resulting in

an identical response to maintain the ratio. The VCS332Z was selected f or its very low TCR to negate changes

due to am bient temperature and f or its low power coef f icient 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

Dif f erent applications require dif f erent resistor technologies; whatever the application, some price/perf ormance

tradeof f is always involved. An ef f ective price-cost-benef it-risk analysis must be conducted f or each applicationto ensure selection of the appropriate resistor f or the application. The basis f or such an analysis is a thorough

understanding of the perf ormance characteristics and reliability implications of each technology in each

application.

About the author

Yuval Hernik holds a B.Sc in electrical engineering f rom the Technion (Israel Institute of Technology). He has

been a director of application engineering at Vishay Precision Grou p – Bulk Metal Foil resistors – since 2008.

T his ar ticle or iginall y a p peared on EE T imes Euro pe.

selecting the best resistor technology for the application

Documents