Resistors and Resistive Components

A resistor is a passive electronic component that opposes the flow of electric current, its resistance measured in ohms. Resistors are among the most common components in any circuit, used to limit current, divide voltages, set the gain of amplifiers and load other stages. Fixed resistors have a single value marked by a colour code or printed figure, and they are made from carbon film, metal film or wound wire, each construction offering different precision, power handling and stability.

A potentiometer is a variable resistor with a movable contact, or wiper, that can be adjusted to set any resistance up to its maximum. Used with three terminals it acts as an adjustable voltage divider, and with two it acts as a simple variable resistance. Potentiometers serve as volume and tone controls and as user-adjustable settings, while small trimming potentiometers are set once during calibration and then left in place.

Several components are resistors whose value deliberately changes with a physical quantity, making them useful as sensors. A thermistor changes resistance with temperature: a negative-temperature-coefficient (NTC) type falls in resistance as it warms, while a positive-temperature-coefficient (PTC) type rises. A light-dependent resistor (LDR), or photoresistor, drops in resistance as the light falling on it increases, allowing it to sense brightness. A varistor, such as the metal-oxide varistor (MOV), has a resistance that falls sharply above a threshold voltage, so it conducts harmlessly at normal levels but clamps damaging voltage surges, protecting equipment from spikes.

Frequently asked questions

What does a resistor do in a circuit?

It opposes current flow, which lets it limit current, divide voltages and set operating conditions for other components; its value is measured in ohms.

How does an NTC thermistor differ from a PTC thermistor?

An NTC thermistor's resistance falls as temperature rises, while a PTC thermistor's resistance increases with temperature; both are used to sense or respond to heat.

What is a varistor used for?

A varistor conducts strongly only above a threshold voltage, so it clamps voltage surges and protects circuits from spikes while drawing little current at normal levels.