The resistor is a passive electrical component to create resistance in the flow of electric current. In almost all electrical networks and electronic circuits they can be found. The resistance is measured in ohms. An ohm is the resistance that occurs when a current of one ampere passes through a resistor with a one volt drop across its terminals. The current is proportional to the voltage across the terminal ends. This ratio is  represented by Ohm’s law:

Resistors are used for many purposes. A few examples include delimit electric current, voltage division, heat generation, matching and loading circuits, control gain, and fix time constants. They are commercially available with resistance values over a range of more than nine orders of magnitude. They can be used to as electric brakes to dissipate kinetic energy from trains, or be smaller than a square millimeter for electronics.

Resistor Values (Preferred values)
In the 1950s the increased production of resistors created the need for standardized resistance values. The range of resistance values is standardized with so called preferred values. The preferred values are defined in E-series. In an E-series, every value is a certain percentage higher than the previous. Various E-series exist for different tolerances.

Resistor applications
There is a huge variation in fields of applications for resistors; from precision components in digital electronics, till measurement devices for physical quantities. In this chapter several popular applications are listed.

Resistors in series and parallel
In electronic circuits, resistors are very often connected in series or in parallel. A circuit designer might for example combine several resistors with standard values (E-series) to reach a specific resistance value. For series connection, the current through each resistor is the same and the equivalent resistance is equal to the sum of the individual resistors. For parallel connection, the voltage through each resistor is the same, and the inverse of the equivalent resistance is equal to the sum of the inverse values for all parallel resistors. In the articles resistors in parallel and series a detailed description of calculation examples is given. To solve even more complex networks, Kirchhoff’s circuit laws may be used.

Measure electrical current (shunt resistor)
Electrical current can be calculated by measuring the voltage drop over a precision resistor with a known resistance, which is connected in series with the circuit. The current is calculated by using Ohm’s law. This is a called an ammeter or shunt resistor. Usually this is a high precision manganin resistor with a low resistance value.

Resistors for LEDs
LED lights need a specific current to operate. A too low current will not light up the LED, while a too high current might burn out the device. Therefore, they are often connected in series with resistors. These are called ballast resistors and passively regulate the current in the circuit.

Blower motor resistor
In cars the air ventilation system is actuated by a fan that is driven by the blower motor. A special resistor is used to control the fan speed. This is called the blower motor resistor. Different designs are in use. One design is a series of different size wirewound resistors for each fan speed. Another design incorporates a fully integrated circuit on a printed circuit board.