WHAT ARE NONLINEAR RESISTORS?

A linear resistor has a constant ratio of voltage to current (Ohm’s Law), so doubling the voltage ideally doubles the current. Nonlinear resistors do not follow Ohm’s Law over their operating range, and current may rise slowly at one region and rapidly in another. Their voltage-current relationship is a curve rather than a straight line. This behaviour makes nonlinear resistors very useful in sensing, protection, and control circuits.

The term “nonlinear resistor” is often used broadly for any two-terminal element with a nonlinear V-I characteristic, including devices whose resistance depends on temperature, voltage, light, or internal state. In circuit analysis, such a device is modelled by its characteristic curve rather than a single fixed resistance value.

Distinguishing feature of nonlinear resistor is that its incremental or effective resistance changes as operating conditions change. A filament lamp draws proportionately much smaller current at higher voltage because heating increases its resistance. In many devices like diodes, V-I curve is not symmetric about origin, and the response differs in forward and reverse conditions.

Nonlinearity in these devices can generate harmonics and distort waveforms in AC circuits. In power electronics and signal systems, nonlinear elements can affect noixe, efficiency and waveform quality. Analysis of such circuits involves approximations, empirical equations, or graphical load-line methods rather than Ohm’s Law based simple calculations.

Types of nonlinear resistors

1) Thermistors: resistance changes with temperature. A fixed resistor keeps nearly the same resistance at all temperatures, while the thermistor’s resistance drops as it heats up, which is why thermistors are ideal for temperature feedback.

NTC: resistance decreases as temperature increases.

PTC: resistance increases as temperature increases.

2) Varistors (VDR): resistance changes with applied voltage, and they are widely used for surge protection.

3) LDRs (photoresistors): Resistance changes with light intensity, decreasing as light increases.

4) Diodes: These are not resistors in the strict sense, though they are not resistors in the strict sense. They are discussed here since they exhibit strongly nonlinear V-I behaviour, have near constant forward voltage drop over their entire range of operation, not dependent on current passing through them.

5) Memristors: These are new class of passive components that came into existence few decades back. Their resistance depends on history of current passing through them, and they retain this value even after current is switched off, till it  is turned on again. They derive their name from ‘Memory+ Resistor’.  These are very useful for memory and neuromorphic computing.

Linear vs Nonlinear Resistors

Property	Linear resistor	Nonlinear resistor
Resistance	Constant	Varies with voltage, current, temperature, light, or history
V-I graph	Straight line	Curve
Ohm’s law	Valid over the normal range	Not valid over the full range
Typical uses	Current limiting, biasing	Sensing, protection, control, memory  etc.

Circuit design with nonlinear resistors

When designing with nonlinear resistors, characteristic of device should be available (datasheet or measured curve). Operating point or range is decided by the device curve and circuit demands. Energy absorption, clamping voltage, response time, and surge endurance will also be factors to be accommodated.

Applications

Nonlinear resistors can sense an environmental quantity or protect a circuit without needing complex active control. This makes them especially useful in compact, low-cost, and high-reliability systems.

Nonlinear resistors are widely used in temperature measurement and compensation, especially thermistors in thermometers, battery packs, and motor windings. Varistors are used across mains and DC bus circuits to clamp overvoltage transients caused by lightning, switching, or inductive load interruptions. LDRs are common in automatic lighting and light-sensitive controls.

Memristors are attracting attention for non-volatile memory, adaptive circuits, and brain-inspired computing because their resistance state can encode information.

Circuit & Surge Protection

• Surge suppression: Varistors absorb dangerous high-voltage spikes from lightning or power grid surges to protect appliances.
• Overcurrent protection: PTC thermistors act as resettable fuses by heating up and rapidly increasing resistance if too much current flows.
• Inrush current limiting: NTC thermistors start with high resistance to safely limit the initial power surge when a device turns on, then warm up and allow normal operation.

Environmental Sensing & Control

• Temperature measurement: NTC thermistors act as highly accurate temperature sensors in digital thermometers, car engines, and home ovens.
• Light detection: Photoresistors (LDRs) detect light levels to automatically trigger outdoor street lights, night lights, and solar-powered garden lamps. [
• Fire alarms: Thermistors detect rapid temperature spikes to trigger building fire and smoke alarms.

Advanced Electronics & Computing

• Volume & signal control: Varistors regulate signal levels in communication equipment to keep audio or data transmission stable.
• Neuromorphic computing: Memristors store memory based on past electrical current, mimicking biological synapses for artificial intelligence hardware.
• Non-volatile memory: Memristors allow computer chips to retain data even when the power is completely turned off.