GLOSSARY :

Ambient Temperature

The ambient temperature is the temperature in the immediate environment of the resistor.

Climate Category

The climatic category indicates the lowest and the highest ambient temperature at which the resistors may be operated continuously. The third indication is the severity of the damp heat steady state test expressed in days.

Critical Resistance

The critical resistance Rcrit is the resistance which can be calculated from the rated dissipation Pv occurring under operating voltage Vmax.

A resistor of critical resistance will exhibit the largest drift in a style, because it is the highest value which may carry the full rated power load.

Current Noise

The current noise voltage E in V is a fraction of the total noise. It results from the current in the resistor and appears in addition to the thermal noise. It is defined by the quotient A of the additional noise and the applied DC voltage and thus given in V/V.

Derating

The power load capability of a resistor is limited by its permissible film temperature. Since the rated dissipation is referred to a specific ambient temperature, higher ambient temperature require a reduced permissible load, i.e. a derating. The derating curve indicates the permissible power load as a function of the ambient temperature.

Drift

Drift is the change of a resistance value from its initial value during a specified period of time under specified operating conditions, e.g. 1% after 1000 hours at P Rated Metal film resistors exhibit positive drift, i.e., resistance increases with operating time.

The amount of change of resistance significantly depends on the temperature of the resistive film and on the resistance value itself. High value resistors change more than lower values because their resistive film is thinner.

Drift doubles with every rise of the film temperature by 30°C, thus operation at a film temperature of 155°C shows twice the drift compared to operation at 125°C. Accordingly, the stability of a circuit can be improved by reducing the temperature of the resistors by reducing their load. For resistors that feature hot spots in the resistive film, the higher temperature of the hot spot is to be considered.

E-Series

The series E24 comprises of 24 values per decade, each of which is derived from the foregoing one by multiplication with 2410 rounded to two digits. In the series E96 and E192, the factor is 9610 or 19210, respectively, rounded to three digits.

Failure Rate

The failure rate indicates the statistically established maximum rate of failures at a level of confidence of 60%. The figures are derived from certified results of standard endurance tests after 1000 hours duration at the rated dissipation.

Film Temperature

The temperature at the resistive film is considered in discussions about power rating and pulse load capability. The film temperature determines the drift and stability of the resistor. For resistors that feature hot spots in the resistive film, the higher temperature of the hot spot is to be considered.

Since most resistors are covered with lacquer or protective coating, only the surface temperature can be measured on the outside. However, the surface temperature is almost as high as the film temperature.

HF Resistors for RF Applications

The special HF resistor products have, by Virtue of a special method of trimming to value, reduced self inductance and therefore improved frequency characteristics, as compared to resistors helically trimmed by standard methods.

The inductance of the trimming helix forms a resonant circuit with the capacitance of the termination caps. This restricts the usability of resistors to frequencies well below the resonance point.

IEC

International Electrotechnical Commission.

Non-Linearity

In deviation from Ohm's law, resistors do not only show linear performance. The result of a pure sinusoidal current passed through on actual resistor is not only a pure sinusoidal voltage. Actually, a composition of the pure voltage with some minor harmonic distortion can be found.

Non-linearity gains importance as it is also a general criterion for the quality of a resistor. Substandard resistors can be identified by their too low values of A3.

For measurement during production, the resistor is driven by a 10 kHz source. The level of the received signal at 10 kHz and the level of the third harmonic at 30 kHz determine the result. Given is the logarithm of the quotient of the two voltages.

Operating Voltage

The limiting element voltage Vmax is the maximum voltage which may be applied continuously to the resistor, provided its resistance value is equal to or higher than the critical resistance. The limit applies to DC voltages and to AC rms voltage of undistorted sinusoidal shape.

Permissible Change

The permissible change of a resistance value is usually specified as a relative deviation. It is indispensable to indicate the test or condition for which the permission change applies. A fixed contribution is often added due to the inherent uncertainty of measuring results on low ohmic resistors.

Pulse Load Capability

The pulse load capability of a resistor is its ability to withstand transient loads, that considerably exceeds the rated dissipation with its peak value. Different limits apply for different pulse appearances:

• Periodic pulse load under steady state conditions

The permissible peak pulse load depends on the shape, width and period of the pulse and possibly on a basic load.

• Single pulse load

A change of resistance indicates the beginning destruction of the resistive film after a few pulses. Tests are normally made with standard pulse shapes according to IEC 60.

Rated Dissipating Pv

The rated dissipating Pv specifies the maximum electrical load that can be dissipated from the resistor through heat without affecting the specified performance parameters. The resistor may be operated with Pv up to the ambient temperature v, typically set at 70°C.

The rated dissipation depends on the heat transition to circuit boards and environment and on the permissible film temperature. The film temperature determines the drift of the resistance and thus the usable lifetime of the resistor.

The occurrence of other heat sources near a resistor is likely to affect its heat transition to the circuit board and to the environment. Therefore, the actual electrical load is possibly limited to a value below the rated dissipation.

Solderability

Solderability is the property of the termination's to accept new solder in a soldering process. Resistors are tested by the solder bath method according to IEC 68-2-20.

Temperature Rise

The temperature rise indicates the thermal resistance which impedes the dissipation of heat from the resistor.

Thermal Resistance Rth

Thermal resistance determines the temperature difference resulting from a heat flow.

with Rth given in K/W, v given in K and P given in W.

The thermal resistance of leaded resistors is specified for a standard mounting situation. It depends on the free length of the leads and to some extent on the copper tracks and pad areas connected.

In a detailed model, the total thermal resistance would be a series connection of inner and outer thermal resistance's, permitting a good estimate of the temperature at the solder joint. Unfortunately, figures for the internal thermal resistance are not generally available.

Temperature Coefficient (TC)

The temperature coefficient indicates the permissible relative and reversible change of resistance due to change in temperature. The actual temperature coefficient may change value and sign, provided it does not exceed the specified limits.

A complete specification of a typical TC limit is 0 ± 50 ppm/°C often abbreviated to 50 ppm/°C, with the TC given in ppm/°C = 10 ppm/°C. The temperature limits specified in the climate category are valid for specification of the TC as well, unless stated otherwise.

Tolerance

The tolerance is the permissible relative deviation of the actual resistance from the nominal resistance value. It only refers to the possible deviation as received. It does not bear any indication whatsoever on possible changes of resistance under operating conditions.

Voltage Coefficient

A resistor has a voltage coefficient if measurements of resistance with different voltages yield different results. The voltage coefficient is the quotient of the relative difference in resistance and the different of the measuring voltage.