The function of resistors is to oppose the flow of electric current in a circuit. Therefore their primary parameter is the resistance value. The manufacturing tolerance must be adequately chosen for each specific application. The ultimate resistance value may deviate from the specification because of many reasons. One is the temperature coefficient of resistance, or TCR, which is often specified for precision applications. Stability defines the long term variation of the resistance. After a long duration of electric load, the resistance value may not return to its original value. Electric noise appears in every resistor, and is an important characteristic for low-noise amplifying applications. For high frequency applications, the parasitic inductance and capacitance properties play a role. In addition to the characteristics related to resistance value, the maximum power and voltage can be specified. The maximum power rating is important for power electronics applications, but resistors in many electronic circuit boards typically never reach their maximum power rating. For high voltage circuits, the maximum rated voltage must be taken into account. The quality of a resistor in terms of durability and reliability is more important for some applications than for others. An overview of the most common resistor properties and characteristics is provided below.
The temperature coefficient of resistance (TCR) is dependent on the resistive material and the resistor construction. The temperature dependence of electrical resistivity is determined by the material:
The power rating indicates the maximum dissipation that the component is capable of. The rated dissipation is normally specified at room temperature and decreases at higher temperatures. This is called derating. Typically, derating is specified above 70 °C. Above this temperature, the resistor can only safely operate at a reduced power level. This is illustrated by a derating curve. The designer should not only take the ambient temperature in consideration, but also the ventilation around the component in tight enclosures.
For some resistors it is important to have low noise properties. Resistor noise is primarily dependent on 3 parameters: resistance, temperature and bandwidth. A high-gain amplifier is an example where noise must be low.
The parasitic inductance and capacitance of a resistor become increasingly important when the frequency increases. A resistor has good high frequency properties when, for the required operating frequency range, the parasitic effects are negligible.
Wirewound resistors are enamel insulated (possibly winded with synthetic fiber, silk or cotton) and the oxide layer of the material itself.
If the complete resistor body is covered, for example, with enamel paint, special care has to be taken that all expansion coefficients are approximately equal. If this is not the case, the enamel layer might burst after the baking process. For applications in very hot and humid climates, the resistor may be enclosed in an airtight metal case.
Many different materials and alloys with different values of resistivity are used to create resistors. The resistivity of the material influences the size of the resistor.
If the maximum allowed voltage is exceeded, it may cause a disruptive electrical discharge permanently damaging the wire insulation. Also, a discharge can pass through the solid insulating material and damaging parts that are nearby.
The stability indicates the maximum tolerable change of the resistance value. The resistance value changes in the long term due to mechanical, electrical and thermal loads. In standards, several stability classes are determined. The standards define tests to define the stability classes. Short term tests include exposure to overloading, rapid temperature variations and vibrations. Long term tests include the damp heat test and load life tests (constant 70 °C with a certain electrical load).
|Stability Class||Long term test||Short term test|
|2||± (2 % · R + 0.1 Ω)||± (0.5 % · R + 0.05 Ω)|
|1||± (1 % · R + 0.05 Ω)||± (0.25 % · R + 0.05 Ω)|
|0.50||± (0.50 % · R + 0.05 Ω)||± (0.10 % · R + 0.01 Ω)|
|0.25||± (0.25 % · R + 0.05 Ω)||± (0.05 % · R + 0.01 Ω)|
|0.10||± (0.10 % · R + 0.02 Ω)||± (0.05 % · R + 0.01 Ω)|
|0.05||± (0.05 % · R + 0.01 Ω)||± (0.025 % · R + 0.01 Ω)|
The pulse stability describes the effect on the long term variation of the resistance value when the resistor is loaded with short term pulses instead of a constant load. The pulses can be much higher than the normal power rating, without having an effect on long term stability. Special tests with pulses are defined in standards like the IEC 90115-1, 4.27. To specify a resistor with sufficient pulse stability, the following requirements must be met:
Resistors are manufactured with a certain tolerance. Depending on the application, the tolerance must be specified.
The complete construction must be designed for the planned operating temperature (think for example of heating elements).
Because different materials are used for the mounting wire and the resistor material, the thermo-electric effect causes unwanted electric currents. Precision resistors are carefully manufactured to minimize the thermo-electric effect.