[+] Q. Why use Vishay Foil resistors for accurate circuitry?
A. Laboratory instruments require 10 times more accuracy than the devices they are monitoring. For example, a digital bridge with reading accuracy of 0.1% must have reference resistors in the first, most significant decade; devices as accurate as 0.01% and 0.005% are preferred. Vishay Foil is the only technology that allows resistor adjustment to these tight tolerances and ensures resistor stability inside the tolerance limits thereafter. Hermetic versions of the molded Foil resistor can be produced with a 0.001% tolerance and are documented by test data, collected for more than 20 years, to retain that level of stability.
[+] Q. What is the importance of resistor stability in an electronic circuit?
A. The more stable the resistors, the longer the interval between calibrations. The more stable the resistors, the more reliable the in-circuit reference resistors. The more stable the resistors, the more reliable the equipment for many applications, including military, avionics, deep-space probes, automatic test equipment, sense and measurement equipment, and medical equipment. The more stable the resistors, the less the need for correcting circuits, on-board thermal stabilization circuit ovens, and so forth. Vishay Foil resistors withstand surges in excess of 25 kV for several milliseconds; the same type of event could seriously damage or open thin film resistors.
[+] Q. Why use Vishay Foil resistor in a non-precision application?
A. Circuit noise elimination. If your circuit suffers from noisy resistors, replace them with Vishay Foil resistors instead of trying to suppress the noise. For precision ratio stability, where tight absolute tolerances are not required, replace a pair of thin film resistors with one Vishay Foil divider. A 1% Vishay Foil resistor instead of a 1% thin film resistor will significantly improve the long-term system accuracy. A Vishay Foil trimmer can significantly reduce the adjustment time/manufacturing cost over wirewound or cermet trimmers.
[+] Q. What does TCR mean?
A. Temperature coefficient of resistance (TCR) is the relative change of resistance per degree of temperature change. It is measured in ppm/°C (1 ppm = 0.0001%) and defined by the following formula:
(Temperature change ΔT is typically referenced to +25 °C). For example, a resistor with a TCR of 100 ppm/°C will change 0.1% over a 10°C change in temperature and 1% over a 100°C change in temperature. In the context of a resistor network, the TCR value is called the absolute TCR in that it defines the TCR of a specific resistor element. Vishay Foil resistors have the lowest TCR in the industry.
[+] Q. How TCR is represented in a graph?
A. On ΔR/R = f (T) graph the TCR is expressed as the slopes of two chords, each chord joining two points of the curve corresponding to two temperatures, e.g :
Hot chord: +25°C to +125°C
Cold chord: -55°C to +25°C
[+] Q. What does TCR tracking mean?
A. TCR tracking defines of how closely the resistance of one resistor "follows" the resistance of one or more other resistors over a given temperature range. It shows how well do two or more resistors maintain fixed resistance ratios when the ambient temperature changes.
[+] Q. How can I gain the best tracking of one resistor to the others in my differential amplifier?
A. For the best maintenance of resistance ratios in real-life applications, both absolute and tracking TCR are important. The tracking component of the resistor ratios assumes that all the related resistors are at the same temperature. In fact, the temperatures are rarely at the same temperature due to different rates of power dissipation, different local temperatures from different levels of heat radiation from adjacent components, different internal thermal efficiencies, etc. The ratio error is the sum of the tracking errors plus the differences in absolute TCR times the differences in resistor temperature. The low absolute TCRs of the Foil resistors greatly improve one component of ratio error, while putting the Vishay Foil resistors in a hermetic packages or PRND maintains the initial ratio and is an improvement over loose resistors. Vishay Foil resistors are step-and-repeat printed on a sheet of foil, and any two adjacent resistor chips track each other better than the loose selection of chips. A significant tracking gain is achieved by using chips that have been diced in pairs (ratio 1:1) and put in a hermetic network package. The ultimate gain is the use of "Z" foil which has a + 0.05 ppm/°C TCR on the hot or cold side and with which two adjacent chips will track within 0.1ppm/ °C. Once more the Vishay Foil resistor provides the best tracking. To get the best tracking it is necessary to use resistors with very low absolute TCR. A very low absolute TCR also avoids problems with temperature gradients.
[+] Q. What does absolute TCR mean?
A. Absolute TCR describes the resistor’s individual response to temperature change without reference to the response of any other resistor. This is as opposed to tracking TCR which describes the resistor's response to a specific environmental temperature change relative to other resistors' responses to the same environmental temperature change.
TCR values specified in datasheets of precision thin-film resistors often are applicable to narrow temperature ranges. For example, to achieve a lower TCR in the 25°C to 125°C range , the normal parabolic TCR curve of thin-film resistor might be rotated by heat treatment to be flatter in this range. But the result of such manipulation is to make the TCR curve steeper in the -55°C to +25°C range. In other words, the TCR in the 'cold" temperature range may exceed the claimed low TCR in the "hot" temperature range. This fact is often obscured in the datasheets because full and complete specifications are avoided. The TCR tracking figures of thin film resistors are also often applicable to a limited temperature range. The problem stems from limitations of thin film technology. It is not capable of ensuring low TCR over the entire temperature range because the adjustment is applied to the resistive element material only.
Foil resistor technology has additional "degrees of freedom" for TCR adjustment. The exceptionally low TCR of the Bulk Metal® Foil resistor across wide temperature ranges stems from the adjustable balance of two physical phenomena: (a) foil material resistivity and (b) temperature-induced compressive stress, which results from different coefficients of thermal expansion between the foil and the ceramic substrate.
[+] Q. What influences the resistor temperature?
A. Three factors determine the temperature of the resistive material:
External factors: ambient temperature and heat transfer from adjacent heat producing components.
Internal factor: self heating of the resistor (Joule effect) due to the dissipated electrical power. The relative resistance change per temperature degree caused by self heating is also known as Power Coefficient of Resistance (PCR or power TCR).
Thermal efficiency, or inefficiency, that prevents adequate heat dissipation from the resistive element.
[+] Q. How does the Vishay Foil resistor perform under pulse applications?
A. Vishay Foil resistors are the best available devices for pulse applications. For pulses of 2ms or shorter, foil resistors are the best because every resistive strand is in contact with a heat sink, and the pulse has little effect on the resistor's value.
[+] Q. What is the noise level of the Vishay Foil resistors vs. resistors manufactured using other technologies?
A. The noise level of a Vishay Foil resistor is -40dB. The comparable values for thin film and cermet resistors are 23dB and -10dB respectively.
[+] Q. Why do Vishay Foil resistors contribute the least noise to electronic circuit?
A. An electrical DC voltage applied to a resistor results in the generation of so-called excess noise. The noise level depends on the structure of resistive element material or, in other words, on the “smoothness” of the electrical current paths inside the materials.
Current passing through a resistor experiences path discontinuities depending on the resistive element. These discontinuities (particle-to-particle paths) provide a locus for bunching and releasing of electrons. Carbon film, cermet and to a lesser extent thin film resistors are sources of current noise. Vishay Foil resistors and some wirewounds, on the other hand, are formed of a continuous alloy of metal and the current path is therefore across intergranular boundaries which have many parallel paths , eliminating the limitations of noise generation in a single path. The noise value of Vishay Foil resistors is -40dB, which is even below the measuring range of highly sensitive Quantec equipment.
[+] Q. What does ESD mean?
A. Electrostatic discharge (ESD) can be defined as a rapid transfer of charge between bodies at different electrical potentials – either by direct contact, arcing, or induction – in an attempt to become electrically neutral. The most common cause of ESD damage is direct transfer of an electric charge from either a human body (handling) or a charged material to an ESD sensitive device.
[+] Q. What exactly does ESD or a large transient do to the resistive segment in a Vishay Foil resistor?
A. The foil in Vishay Bulk Metal Foil resistors is a hundred-plus microinches-thick piece of metal which can safely absorb the joules of energy associated with ESD. A uniform metal structure results in the uniform density of discharge current throughout the element, without local overheating. For comparison, the resistive element in a thin-film resistor is 100 to 200 times thinner than the foil and therefore is much more sensitive to ESD.
[+] Q. Can Vishay Foil resistors withstand a high temperature exposure?
A. High temperature exposure due to self heating of the resistors or high ambient temperatures can render resistors and the instruments using them temporarily or permanently out of tolerance. The use of a Vishay Foil resistor with a low TCR and PCR will keep the instrument in tolerance even when exposed to high temperatures. Vishay Foil resistors are used in down hole applications at temperatures up to +275 °C.
[+] Q. How do Vishay Foil resistors behave under continuous load?
A. The load life curve of a thin-film resistor shows a continuous change in resistance over the operating life of the resistor. The longer the service life, the more the film deterioration, and finally the resistance moves out of tolerance. By contrast, a foil resistive element, which is 100 times thicker, shows only a small change in the beginning of the service and levels off to little or no further change. This small initial change can be prevented by Post Manufacturing Operations (PMO) leaving only a few parts per million resistance change over the life of the resistor.
[+] Q. What are the physics of NiCr when exposed to electrical bias and moisture?
A. Condensation of microscopic quantities of water vapor on the surface of NiCr thin-film resistive element results in the dissolution of ionic contaminations available on the surface and formation of electrolyte solution. Since all plastics and all epoxies are hydroscopic, thermal cycling causes the resistor to “breathe in” water vapor that picks up encapsulation contaminates which are then condensed inside the package. Under low-power DC voltage, the ionic etching of thin NiCr films can cause rapid and significant resistance changes in a few minutes, completely destroying the resistor (open circuit) within a few hours. That is why any damage of humidity protection (coating, package) in thin-film resistor inevitably results in its failure. Bulk metal resistive elements in Vishay Foil resistors are hundred times thicker than thin-film resistive elements and therefore are much less vulnerable to the etching process. Hermetic versions of Vishay Foil resistors are available to completely eliminate any moisture influence whatsoever.
[+] Q. What resistor type withstands high temperature exposure best?
A. It depends on: (1) temperature level, (2) duration of exposure, (3) construction of the resistor. Any resistor with internal solder connections is out of the game at the temperature exceeding melting point of the solder. Any resistor with pressed on end caps is likely to see a reduced contact pressure at elevated temperatures (expansion of the metal cap over the ceramic blank) that results in resistance shift or open circuit.
Any thin film resistor is likely to see film degradation at elevated temperatures due to negligible film thickness (hundreds of Angstroms). Any wirewound resistor is likely to see distortion of the bobbin resulting in resistance shift.
A Vishay Foil based resistor has no solder connections, no thin resistive element (the foil is 25,000 Angstroms thick), and no plastic bobbin . It has a ceramic substrate, bulk metal resistive element (foil), and welded terminations. The bonding resin between the foil and the ceramic substrate may deteriorate over time at extremely elevated temperatures, but the foil itself and its connections to terminals will remain intact. Table 1 shows resistance stability of Vishay Foil resistors at elevated temperatures.
[+] Q. Can construction materials and resistive materials used in Vishay Foil resistors withstand mechanical stress?
A. Yes. The silicon coatings, ceramic packages, molded jackets, and RTV coatings used in different constructions of Vishay Foil resistors are capable of managing significant mechanical stresses. In addition, the Vishay Foil resistive element can handle a significant mechanical stress as well.
[+] Q. How do Vishay Foil resistances react after repetitive pulses?
A. Repetitive pulses have essentially no effect on Vishay Foil resistors.
[+] Q. Do Vishay Foil resistors exhibit inductance and capacitance?
A. Vishay Foil resistors are characterized by no more than 0.08 µH of inductance and 0.5 pF of capacitance. By comparison, wirewound resistors are essentially inductive. “Non-inductive” wirewound resistors with bifilar winding retain noticeable inductance because there is no way of get rid of entire inductance using winding techniques. Vishay Foil resistors, on the other hand, have the current paths laid out in such a way that magnetic field generated by one path is opposed by the field generated in an adjacent path. Inter-path capacitances are oriented in series, thereby reducing the total capacitance.
[+] Q. Do Vishay Foil resistors exhibit moisture problems?
A. Moisture is "mortal danger" for NiCr metal film (thin-film) resistive elements. Non-protected film simply begins to dissolves electrochemically within several seconds under influence of moisture, ionic contaminations and DC voltage applied to the resistor. Reliability of metal film resistors in humid ambient environs depend exclusively on the integrity of moisture repelling coatings. The foil resistive element is a hundred times thicker than the thin-film resistive element. It may be affected to some extent by moisture but the respective resistance change is not measurable within the instrument recalibration cycle. A hermetically sealed version of Vishay Foil resistor is recommended for long term exposure to high humidity.
[+] Q. How much do I increase the value of my instrument by using Vishay Foil resistors instead of thin film resistors?
A. Let’s suppose you swapped out 10 thin-film resistors for Vishay Foil resistors supposing that foil resistor’s greater stability would affect the instrument performance. For example you increased the cost of the instrument by $68 and therefore the selling price by $150. But the upgraded more precise 0.01% instrument with a dramatically increased recalibration cycle has more value than its predecessor (1% instrument). $150 is a small price to pay for eliminating one or more recalibrations per year at a cost that includes the line down time, the instrument idled, the technician time to do the recalibration, and any associated shipping cost. All in all, the replacement of several thin-film resistors by foil resistors is justifiable.
[+] Q. What are the most demanding resistor applications and why are Vishay Foil resistors the ultimate solution?
A. There are a number of differences between Vishay Foil resistors and other types of resistors that make Vishay Foil resistors uniquely suited to the demands of specific applications. Here are the highlights:
Precision Measuring Instruments - Resistors in a precision application such as a measurement bridge need to maintain their initial installed value for a protracted period of time (between periodic recalibrations) and under conditions of normal use. A laboratory environment is not likely to produce mishandling or environmental abuse but initial accuracies may require tolerances beyond the range of other resistor types. Foil resistors with tolerances of 0.01% are routinely manufactured, resistors, and tolerances of 0.005% and 0.001% are commonly required.
Precision circuits in service - Some service equipment is called upon to provide accurate readings in spite of temperature excursions, temperature cycling, moisture, extended periods of service, and overload. Vishay Foil resistors are the most stable resistors under all conditions of use. End-of-life stabilities of 350 ppm (tolerance + load life shift+ TCR+ PCR+ mounting) are common and unavailable with other types of resistors.
Field service equipment - Measurement equipment installed on the back of a pickup truck in an oil field is subjected to shock, vibration, high and low temperatures, moisture, and on occasion, induced surge current from a lightning strike. The Vishay Foil resistor is the best solution when measuring equipment is maintained under severe environmental conditions. A surge from a 25kV pulse will destroy a thin-film resistor while leaving the Vishay Foil resistor unharmed.
Audio - Audio applications are characterized by stringent requirements of low intrinsic noise, high linearity of amplification, and minimal dynamic distortions. Vishay Foil resistor is preferable choice for audio applications because its bulk metal resistive element has the lowest (when compared to thin-film and thick-film resistors) excess noise and the lowest voltage coefficient of resistance (best linearity). Extremely low TCR and PCR of foil resistor eliminates temperature-induced nonlinearity of amplification. In addition, the low reactance of foil resistors results in negligible distortion of pulse signals and immunity to circuit self-oscillation in audio amplifiers.
Inductance-free and capacitance-free applications - Pure resistance is called for in many applications. Signals in circuits operating at high frequency are influenced by any inductance and or capacitance introduced by the resistors. The Vishay Foil resistor adds only 0.08 µH of inductance or no more than 0.5 pF of capacitance. This low level of inductance (compared to a wirewound resistors and spiraled thin-film resistors) may be explained as the following. Adjacent etched patterns in the foil resistive element conduct the current in opposing directions. Therefore, the magnetic field generated by one pattern is opposed by the field generated by the adjacent pattern, dramatically reducing total inductance. Inter-pattern capacitances of adjacent etched patterns are oriented in series, thereby reducing the total capacitance.
Down hole applications - In oil field applications electronic equipment has to be inserted down into the well where the temperatures can be as high as +275°C. This is hot enough to melt the solder terminations of a wirewound resistor and high enough to degrade the film (with time) in a thin-film (metal film) resistor. Even Vishay Foil resistors succumb at this temperature eventually, but they have the best survivability due to: (a) the absence of internal solder connections, (b) 100 times higher thickness of the resistive element when compared to thin-film resistor.
[+] Q. What are the best applications for Vishay Foil resistors?
A. Differential amplifiers, reference voltage sources, Wheatstone bridge arms, A/D converters, sources of bias currents and voltages, sources of threshold voltage, and precise and stable current sensors.
[+] Q. What is the required minimum quantity of Vishay Foil resistors to order?