FAQ / Frequently Asked Questions
Präzisions- und Leistungswiderstände
Why are the ISA-WELD®- and some of the ISA-PLAN®-SMD current-measuring resistors of the component families LMx, BVE, BVS, BVT and BRS only offered as two-wire?
Through a suitable layout of the voltage taps on the circuit board a four-wire tap can be realized in the application with a two-wire resistor. A rise of the TC due to the copper protective resistor is only noticed at values < 5 mOhm. The four-wire resistors are then to be preferred, when the requirements on the TC are correspondingly high.
How does one understand the pulse-load diagram?
The pulse-load diagram specifies the maximum pulse power or energy as a function of the pulse length for a resistance value of the product series, which the component withstands without appreciable resistance drift even after more than 1 million cycles. Since the pulse loadability is dependent on the layout and the foil thickness and therefore on the resistance value, in particularly critical cases this loading limit in the test is to be fixed separately.
Is a continuous loading above the value specified as continuous-/nominal load on the data sheet permitted, if the max. contact point temperature is significantly below the power-derating curve?
Because of the particularly low thermal resistance, e. g. at room temperature, the components can be loaded significantly higher than specified in the data sheet. For this case, after an experimental fuse protection a special release can take place.
Which requirements/specifications are necessary and helpful for choosing the ideal measuring shunt?
Besides the preferred assembling technique and the resistance value, the continuous or pulse loading of the component and the required tolerance are the decisive factors.
For many SMD resistors, in the data sheet under technical data under the point Stability you specify < 1 % after 2000 h. In the diagram of the data sheet this value is significantly lower, namely at approx. < 0.25 %, even up to 5000 h, why?
The typical drift of the components is about < 0.25 %. The specified tolerance < 1 % is the absolute upper limit for the drift, respectively the maximum load according to the load reduction curve.
How can the estimated loading limit be determined for a short-circuit (max. 1 to 5 times, no continuous cyclic load) on the ISA-PLAN® precision resistors?
Only by means of detailed laboratory tests, which we are happy to carry out for our customers.
What happens with an ISA-PLAN® high precision resistor at extreme overloading?
With extreme overloading the resistance path can melt, the component becomes highly resistive or burns through.
Which resistance values are normally available for ISA-PLAN®- and/or ISA-WELD® high precision resistors?
As standard the values are based on the E-12 series. Beyond this, normally the values with 2 and 5 mΩ are also available (e. g. 2 mΩ, 20 mΩ, 200 mΩ, 5 mΩ, 50 mΩ, 500 mΩ). For deviating values, please contact us directly.
Are customer-specific low ohm resistance values also available for ISA-PLAN®- and ISA-WELD® resistors, e. g. 14.7 mOhm? If yes, is there a minimum delivery quantity?
In principle all imaginable values between the technically feasible upper and lower limit can be manufactured. These are linked with a minimum delivery quantity. For ISA-PLAN® this depends on the component size, typically e. g. approx. 37,000 units for the design 1206 or 5,000 units for the design 2512.
For a typical ISA-WELD®-SMD component, e. g. BVS (design 3920) = approx. 200,000, since a separate band must be manufactured as ingoing material (e. g. copper-Manganin®-copper).
You normally use Manganin® as resistance alloy in the resistors. The temperature coefficient of Manganin® is specified with < ± 10 ppm/K. Why do you mostly specify your SMD resistors < ± 50 ppm/K on the data sheets?
For a two-wire, the TC of a component is comprised of the TC of the resistance material (e. g. Manganin®) and the not completely avoidable influence of the supply line or bonding. For that reason we usually specify the TC of a two-wire with < 50 ppm/K.
Between which measuring points on the resistor is the R(thi) determined?
Normaly inbetween the contacts/soldering points and the hot spot (in the middle of the resistor). For ISA-Weld® products between the copper (1mm from the von der welded seam) and the middle of the resistor.
What can happen in the worst case when an ISA-WELD® resistor (electron-beam-welded copper-Manganin®-copper) is overloaded?
The precision resistors of the ISA-WELD® series are very robust and in practice, especially as SMD resistors, almost indestructible. On SMD resistors the soldering point is destroyed, for otherwise bonded ISA-WELD® resistors, the resistance material can melt away at temperatures > 1000 °C.
Especially with the high-impedance SMx series, under certain circumstances a concave sagging of the component can occur after soldering. What influence does this change have on the resistor?
The characteristics of the component are not influenced by this. The cause for this is the separating resin web with its high expansion coefficients.