A There is a huge variation in fields of applications for resistors; from precision components in digital electronics to measurement devices for physical quantities. A few popular uses are described below:
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 connections, the current through each resistor is the same and the equivalent resistance is equal to the sum of the individual resistors. For parallel connections, the voltage across each resistor is the same. The inverse of the equivalent resistance is equal to the sum of the inverse values for all the parallel resistors. The articles resistors in parallel and resistors in series provide detailed introduction to these concepts and calculation examples. 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 to set the current. These are called ballast resistors and passively regulate the current in the circuit.