BS EN IEC 62812:2019 pdf download

BS EN IEC 62812:2019 pdf download.Low resistance measurements -Methods and guidance.
The raised temperature on the unknown resistor due to self-heating not only affects the specimen, but also spreads the heat to the test assembly or mounting and affects those parts of the measurement circuit as well. Therefore, the raised temperature will be root cause of the variation of resistance with temperature, as discussed In 4.4, and of the thermoelectric e.m.f.. as discussed In 4.5.
Self-heating is decreased by reducing the measuring current i as much as possible while still providing the required voltage for a measurement with the desired accuracy. However. setting the measuring current Is not a common feature with resistance meters. Other options to reduce the self-heating are to activate the measuring current for a short period only, as discussed In Clause 5. and of course to enhance the heat flow from the specimen and the test fixture.
4.4 VarIation of r•sistance with temperatur.
One of the reference conditions prescribed in IEC 60115-1 for measuring the resistance is the reference temperature of 20 C. For practical reasons, however, most tests and measurements are permitted to be executed under standard atmospheric conditions for testing as defined in IEC 60068-1, which includes a permissible range for the ambient temperature from 15 C to 35 C.
if measured with sufficient accuracy, a resistor measured at 15 C or at 35 C will not show the same resistance as when measured at 20 C. In fact, there Is a variation of resistance with temperature for almost every type of resistor, which typically does not follow a linear relationship. The slope and the amount of variation depend substantially on the technology and manufacturing of the resistor and in some cases also on the actual resistance.
EXAMPLE 3 A chip resistot, 541C RR32I6U. may be manufactured with terminations consisting of a 3 iinl nickel layer end a S im tin layer on top, wrapped around the edges With a termination width of 1.6 mm and an effective conductor length assumed to be 1 mm, the Conductor resistance amounts to 7,8 m on cacti side, ow 15.6 mO in total. If the resiStor was, for exempt., specified to be 100 mO. then this termination resistance would already represent about 15,6% of it, and every 64 pm difference in the effective conductor l.nqth would change that contnbution by I %.
If the tesrator was measbred at 35 C, itie co.’nbined conductor re5r5tance anOunla Ia 16,8 mC). wfhcSi ,5 due to an effectiy. TCR of approximately 5400 106!K for both termination layers combined
4.5 Thermoelectric e.rn.f.
Any solid conductor subjected to a temperature gradient features a displacement of charge carriers through thermal diffusion, leading to a movement of electrons towards the cold end. Hence, an electrical field establishes itself from the hot to the cold end.
Two different materials combined to form a loop with both junctions subjected to different temperatures result in the flow of a continuous thermoelectric loop current. Opening the loop within any one of the materials results in a measurable thermoelectric e.m,f,, , as shown in Figure 5.
The thermoelectric e,m,f.. known as Seebeck effect, depends on the two involved materials and on the temperature difference between the two junctions The Seebeck effect is not linear and depends on the actual temperatures, For a limited temperature range, however, it is possible to assume sufficient linearity and therefore to calculate the thermoelectric e,m.f., ET, from the difference of the Seebeck coefficients as of the two joined metals and the temperature difference of their junctions:
Seebeck coefficients can be given as absolute figures or relative to a second material. Formetallic conductors , the coefficients typically are in a range of one to a few tens of microvoltsper Kelvin, while for doped semiconductor materials they are rather in the order of a millivoltper Kelvin.Table 1 gives the relative Seebeck coefficients ag of a number of potentiallyrelevant metals joined with platinum, or joined with copper.