Nonlinear Resistor Technology

An ordinary resistor is linear: the current through it is proportional to the voltage across it, so its resistance stays constant. A nonlinear resistor breaks this rule on purpose. Its resistance varies with some condition, such as temperature, applied voltage or light level, which means the relationship between current and voltage is no longer a straight line. This deliberate variation makes nonlinear resistors valuable as sensors and as protective devices rather than as plain current-limiting parts.

Thermistors are nonlinear resistors that respond to temperature. A negative-temperature-coefficient (NTC) thermistor decreases in resistance as it warms, and is widely used for temperature measurement and to limit the inrush current that flows when equipment is first switched on. A positive-temperature-coefficient (PTC) thermistor increases sharply in resistance above a certain temperature, which lets it act as a self-resetting protective element: excessive current heats it, its resistance rises, and the current is limited until the fault clears.

Other nonlinear resistors respond to different stimuli. A varistor, commonly the metal-oxide varistor (MOV), has a resistance that drops steeply once the voltage across it exceeds a threshold. At normal voltages it conducts almost nothing, but a surge drives it into conduction, diverting the energy and protecting the circuit from transient overvoltages. A light-dependent resistor (LDR) changes resistance with illumination, falling as light increases, which suits it to light-sensing tasks such as automatic lighting. Humidity-sensitive resistors work on a similar principle, varying their resistance with the moisture in the surrounding air.

Frequently asked questions

What makes a resistor nonlinear?

Its resistance changes with a condition such as temperature, voltage or light, so the current and voltage are no longer directly proportional as they are in an ordinary resistor.

How is a PTC thermistor used for protection?

Excess current heats it, raising its resistance and limiting the current until the fault clears, after which it cools and resets itself automatically.

How does a varistor protect against surges?

It conducts almost nothing at normal voltage but its resistance collapses above a threshold, diverting the energy of a voltage spike away from the protected circuit.