An ideal resistor offers a fixed resistance value, works at any voltage, and dissipates infinite power. But in practice, all types of resistors exhibit non-ideal characteristics. A practical resistor varies in its resistance value, limited to a working voltage, dissipates power as specified by the manufacturer. Again all these parameters vary with operating temperature. In this post, I am going to discuss about the non-ideal behavior of resistors.

Nominal value of resistance

Nominal value of resistance is the value of the resistor when it is purchased from a manufacturer. The nominal value is the value the resistor should have at 20 °C based on its design.

Resistor Tolerance

The tolerance on delivery is the range within which the resistor can deviate percentually from the nominal value at the time of delivery. It is measured at 25°C with no load applied.

Temperature Coefficient of Resistance (TCR)

TCR is the relative change in the resistance value within a given temperature interval. In general this change is nonlinear, but for small temperature changes may be approximated as linear. The units are ppm/°C, meaning parts-per-million per degree Celsius. Another specification is ppm/K, meaning parts-per- million per (degrees) Kelvin.

TCR is a function of temperature. Assume a TC of +150 ppm/°C for a 1K resistor. If the ambient temperature rises from 25 °C to 30 °C, then the resistor’s value will change with
When the temperature drops to 25 °C the resistor’s value returns to 1K.

When a resistor is exposed to large, sudden changes in temperature, minute, and irreversible physical changes occur, and the resistance change permanently. This change can be as high as 2% for carbon composition resistors, and as low as 0.1% or smaller for metal film and wire-wound resistors.

Soldering (2 s @ 230 °C) expose a resistor to a thermal shock that causes minute and irreversible mechanical changes in the body of the resistor, and consequently permanent changes in the resistance value. These changes can be as small as 0.05% for some wire-wound resistors, and as high as 2% for carbon composition resistors.

Power Rating of Resistor

The rated power dissipation is the maximum dissipation the resistor is capable of up to a defined ambient temperature (the rated temperature, typically + 70 ºC). At this loading, the temperatures at the component do not exceed the maximum. Above this temperature the resistor can only utilize a reduced level of power. This is described by a derating curve:

Limitting Element Voltage (LEV)

Limiting Element Voltage (LEV) is the maximum continuous voltage that can be applied to a resistor. For lower values the power rating is exceeded before the LEV is reached, but with higher values the LEV imposes limits on the applied power. For instance, a 470k 0.33 watt 1206-size resistor requires 394 volts to dissipate its rated power; but a commercially available such part has an LEV of 200V, so this can in fact only dissipate (continuously) 85mW.

The LEV limit becomes more significant with pulsed applications of lower-valued resistors where the average power dissipated may be very low, but the peak applied voltage can exceed the LEV.

Voltage Coefficient of Resistance (VCR)

Voltage Coefficient of Resistance is the change in resistance with applied voltage. It is normally associated with carbon composition and carbon film resistors. High value resistors tend to have high voltage coefficients.

Pulse Stability of Resistor

If a resistor is exposed to pulses instead of constant loading, it can accept multiples of its rated loading for short periods without impairing its long-term stability.

Pulse applications may stress the power rating of the resistor as well as the voltage; the average power dissipated is equal to the peak power times the duty cycle of the pulses, but for pulse duration longer than a millisecond or duty cycles in excess of 10 or 20, the average power permissible has to be derated from the theoretical. Different types of resistor construction suffer in different ways. For instance wirewound or film types must dissipate all the applied power in the conductor itself, rather than letting the heat out through the body of the resistor, since it takes a finite time for the heat to pass from the conductor into the body. As the mass of the wire or film is low, the energy handling capability is also low and the de-rating is significant. Some manufacturers publish curves to allow this de-rating to be calculated.

For a resistor to possess sufficient pulse suitability for a specific application, the following criteria must be met:

  • The average load must not be greater than the rated loading at the required ambient temperature.
  • The permissible pulse loading as a function of the pulse duration must not be exceeded
  • The pulse voltage at the resistor must be lower than the permissible pulse peak voltage.

Noise in Resistor

There are two kinds of noise: the thermal noise and the current noise by the applied voltage. Noise generated by any conductor is known as thermal or Johnson noise. This is caused by the Brownian motion of carriers in the conductor, and is related to temperature as well as the resistance value.

Thermal noise increases with increasing temperature and resistance, and has well-defined statistical properties.

Thermal noise remains constant over a wide frequency range (white noise), current noise declines with rising frequency.

High frequency characteristics of Resistor

In addition to the resistance value, as the operating frequency increases, the parasitic properties become noticeable. This includes the inductivity of the windings in cylindrical resistors and the capacity between component terminations.

A resistor has good high frequency characteristics when the parasitic elements in the frequency range in question are negligible.

Special trimming processes for film resistors and winding processes for wire-wound resistors substantially improve the high frequency characteristics of these components.

Resistor Stability and Drift

The resistance value can change under thermal, electrical, or mechanical influence. Stability classes indicate the maximum permissible change. Stability is tested by procedures defined in standards. Short-term tests include overloading, mechanical sturdiness of the terminations, resistance to soldering heat, rapid temperature changes, and vibrations. Long-term testing includes criteria such as climate sequences, damp heat, long-term exposure to the maximum permissible temperature and long-term exposure at + 70 °C ambient temperature with cyclic electrical load (load life).

In addition to all the environmental and electrical factors listed above, resistors experience a long-term drift that may be as high as 2% per year for carbon composition resistors. Metal, oxide, metal film, and wire-wound resistor are typically 10 times more stable.

Resistor Aging

Normally all types of resistors are guaranteed for few years of life, by their manufacturing technology. And the life of resistors also depends on their working environment. The life of resistors can be extended if they are operated at below their rated temperature, rated power, rated voltage, etc.

With aging, the value of resistor may change slightly. Amount of change caused by aging, is depends on the manufacturing technology, resistor working environment.

All the above discussed non-ideal characteristics of resistors shall be considered during the selection of resistors. For more details about resistor selection, you can read Resistor Selection for Analog Circuit Design