Frequently Asked Questions
Welcome to our FAQ page. No matter whether they are questions regarding the pulse diagram or the requirements/specifications for selecting the ideal measuring shunt, you will find answers here to the most frequently asked questions.
Why are the ISA-WELD®- and some of the ISA-PLAN®-SMD current-measuring resistors 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.
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.
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 more information please contact us directly.
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 < 10 ppm/K.
What is Rthi?
Rthi is the thermal internal resistance of the component. For SMD components, this is usually determined between the contact points / soldering points and the hot spot (center of the resistor material). For ISA-WELD® components intended for the copper rail mounting, this is determined between the copper (1 mm from the weld seam) and the center of the resistor).
What can happen in the worst case during an overload of an ISA-WELD® resistor?
The components of the ISA-WELD® series are very robust and are almost indestructible in practice (especially as SMD components). The solder point can be destroyed first with SMD components. For differently contacted ISA-WELD® components, the resistor material may melt at temperatures > 1,000 °C.
In certain circumstances, a concave deflection of the component may occur after soldering, especially with the high-resistance SMx series. What effect does this change have on the resistor?
The concave bending after soldering affects the following series: SMK, SMP, SMS, SMT. This bending is absolutely safe and there is no risk for the reliability of the components. The component properties are not affected by this. See information on the corresponding data sheets: "Slight deformations during soldering do not affect technical properties of the component."
What is to be observed for BVS and BVE resistors with Aluchrom?
The resistance material Aluchrom is only recommended for DCB or ceramics during assembly due to the low expansion coefficient, whereby the following information about Aluchrom must be observed: There may be a strong distortion of the voltage signal during AC current applications due to the ferromagnetic properties of Aluchrom. With DC current applications too, a quick remagnetization of the material at high currents may create undesired voltage peaks, which can be very disruptive in sensitive applications. Alternatively, components can be used with the resistance material made of MANGANIN®, ISAOHM® or NOVENTIN®, depending on the resistance value.
What are the differences between ISAOHM® and NOVENTIN®?
ISAOHM® is used with various SMD-ISA-WELD® products due to its relatively high specific resistance for high-resistance values. The material has very good temperature coefficient values and an excellent long-term stability and is otherwise very robust in the event of short-term overheating. The disadvantage with this material is the difficult process control and adjustment of the temperature coefficient in the process chain. The new resistance material NOVENTIN® can be used as a very good substitute for ISAOHM®, whereby the specific resistance value is somewhat lower and the technical properties are not as pronounced with respect to stability and Rthi, which however is only noticeable at very high loads on the components.
How are the solderability and processability of ISA-WELD® components guaranteed?
All ISA-WELD® components are provided with a start-up protection/anti-oxidation protection ex works. This thin transparent layer prevents the start-up or oxidation of copper in the atmosphere and thus ensures that the components remain solderable over a certain period of time. The organic start-up protection fully vaporizes during the process-induced heating of the components and does not influence the soldering process in any way. In order to ensure the shelf life of the components after delivery until processing, they are welded into film pouches and back-gassed with nitrogen in addition to the start-up protection. From this point on, the components can be stored for at least two years. After opening the packaging, the components must be processed within two months.
What maximum drift of the resistance value is to be expected with a current measuring resistor with a tolerance of 1%?
The tolerance of a component describes the maximum production-related deviation of the resistance value, i.e. the real resistance value of the component can be a maximum of ±1 % of the nominal value. Due to the safety factors in the 100-percent final control of the components, the real resistance value in the worst case scenario will still be within this 1 percent range, also taking into account influences such as temperature fluctuations, inductance, copper thermal stress and long-term stresses.
What MSL level do our components have?
For the MSL level please see this document.