What properties do resistors have?

Depending on the application, electrical engineers specify different properties of resistors. The main purpose is to limit the current; therefore the key parameter is the resistance value. The manufacturing accuracy of this value is expressed in the resistor tolerance percentage. Many other parameters that affect the resistance value can be specified, such as long-term stability or temperature coefficient. The temperature coefficient, which is usually specified in high-precision applications, is determined by the resistor material and the mechanical design.

In high-frequency circuits, such as in radio electronics, capacitance and inductance can cause undesirable effects. Foil resistors usually have low parasitic reactance, while wirewound resistors are the worst. For precision applications such as audio amplifiers, electrical noise must be as low as possible. This is usually specified as microvolts of noise per volt of applied voltage for a 1 MHz bandwidth. Power ratings are important for high-power applications This specifies the maximum operating power that the component can handle without changing properties or being damaged. Power ratings are usually specified in free air at room temperature. Higher power ratings require larger sizes and may even require a heat sink. Many other characteristics can play a role in the design specification. For example, maximum voltage or pulse stability. This is an important feature in situations where high-voltage surges may occur.

Sometimes not only electrical properties are important, but designers must also consider mechanical robustness in harsh environments. Military standards sometimes provide guidance for defining mechanical strength or failure rates.

In Sectional Characteristics, a complete overview of the main properties of a specified resistor is given.

Resistor Standards

Many standards exist for resistors. Standards describe methods for measuring and quantifying important characteristics. Other specifications exist for physical dimensions and resistance values. Probably the best known standard is the color code marking of axial lead resistors.

Resistor Color Codes

5.6kOhm Four-Band Resistor, Color Code and 2% Tolerance

According to the marking code IEC 60062, the resistor is 5600 Ohm with a tolerance of 2%.

The resistance value and tolerance are indicated with several colored bands around the body of the component. This marking technique for electronic components was already developed in the 1920s. Printing technology was still not well developed, which made it very difficult to print numeric codes on small components. Today, most axial resistors still use color codes up to 1 Watt.

Resistor Color Code Calculator

The color code can be easily decoded with this calculator. It not only provides the resistance value, but also indicates when the value belongs to the E series.

SMD resistors

SMD resistors on circuit boards (331)For SMD (surface mount device) resistors, numerical codes are used because the components are too small for color coding. SMD resistors - only as leaded variants - are mainly available in preferred values. The dimensions (length and width) of the components are also standardized and are referred to as resistor packages.

Resistor values (preferred values)

In the 1950s, the increase in the production of resistors created a need for standardized resistor values. The range of resistor values was standardized with so-called preferred values. The preferred values are defined in the E series. In the E series, each value is a certain percentage higher than the previous value. Various E series exist for different tolerances.

Resistor applications

The application areas of resistors vary greatly; from precision components in digital electronics, to measuring devices for physical quantities. Several popular applications are listed in this chapter.

Resistors in series and parallel

In electronic circuits, resistors are often connected in series or in parallel. For example, a circuit designer can combine several resistors with standard values (E series) to achieve a specific resistance value. For a series connection, the current through each resistor is the same and the equivalent resistance is equal to the sum of the individual resistances. For a parallel connection, the voltage through each resistor is the same and the reciprocal of the equivalent resistance is equal to the sum of the reciprocal values of all the resistors in parallel. In the article Parallel and Series Resistors a detailed description of the calculation examples is given. To solve more complex networks, Kirchhoff's circuit laws can be used.

Measuring Current (Shunt Resistors)

The current can be calculated by measuring the voltage drop across a precision resistor with known resistance, which is connected in series with the circuit. The current is calculated using Ohm's law. This is a type of resistor called an ammeter or shunt resistor. Usually this is a high-precision manganese copper resistor with a low resistance value.

LED Resistors

LED lights require a specific current to operate. Too low a current will not light up the LED, while too high a current may burn out the device. For this reason, they are often connected in series with a resistor. These are called ballast resistors and passively regulate the current in the circuit.

Blower Motor Resistors

In a car, the air ventilation system is driven by a fan, which is driven by a blower motor. A special resistor is used to control the fan speed. This is called a blower motor resistor. Different designs are being used. One design is a series of different sized wirewound resistors for each fan speed. Another design has a fully integrated circuit on a printed circuit board.