Thermistors, Photoresistors, and Varistors: Specialty Resistors and Their Real-World Uses Standard resistors hold a steady value, and that is exactly what makes them predictable. Specialty resistors do the opposite. Their resistance changes on purpose in response to temperature, light, or voltage, which lets a simple two-terminal part sense the environment or protect a circuit. Thermistors, photoresistors, and varistors are the three most common examples. They appear in thermostats, street lights, power strips, battery packs, and countless other products. This guide explains how each one works, where it is used, and how to choose between them. If you’re sourcing parts for a project, you can browse the Engineered Parts Supply product catalog.
What Makes a Resistor “Specialty”? A conventional fixed resistor is designed to resist change. Its resistance should stay stable across temperature, voltage, and lighting. Specialty resistors flip that goal. They are built from materials whose resistance depends strongly on one physical quantity: ● Thermistors respond to temperature. ● Photoresistors respond to light. ● Varistors respond to voltage. If you want a refresher on conventional parts first, the guide to the types of resistors covers carbon film, metal film, and wirewound designs. You can also see how adjustable parts compare in this article on fixed vs. variable resistors.
Thermistors: Resistance That Follows Temperature The word thermistor combines “thermal” and “resistor.” These parts are made from ceramic or polymer materials, and their resistance changes sharply and predictably as temperature moves. The thermistor is one of the most widely used temperature sensors because it is small, inexpensive, and highly sensitive. NTC Thermistors A negative temperature coefficient (NTC) thermistor lowers its resistance as temperature rises. This is the most common type. Its response is nonlinear, so designers often use the beta parameter or the Steinhart-Hart equation to convert a measured resistance into an accurate temperature reading. Typical NTC applications include: ● Temperature sensing in thermostats, appliances, and automotive coolant monitoring ● Battery pack monitoring to prevent overheating during charging ● Inrush current limiting in power supplies, where a cold NTC starts with higher resistance, then drops as it warms
● Temperature compensation in circuits whose other components drift with heat PTC Thermistors A positive temperature coefficient (PTC) thermistor increases its resistance as temperature rises. Many ceramic PTC parts show a sudden jump in resistance past a certain switching temperature, which makes them self-regulating. Typical PTC applications include: ● Resettable fuses (polymeric PTC devices) that trip under overcurrent and recover when they cool ● Self-regulating heaters, such as those used in hair dryers, automotive heating elements, and small appliances ● Motor start circuits in some compressors ● Overtemperature protection in electronics Choosing a Thermistor Check these parameters before selecting one: ● ● ● ● ●
Nominal resistance at 25 °C, commonly 10 kΩ for sensing applications Beta value or resistance-temperature table Tolerance of both resistance and beta Operating temperature range Self-heating and dissipation constant, which affect accuracy if too much current flows through the sensor
Because thermistors are sensitive to the current passing through them, keep the measurement current low. Too much current heats the part itself and skews the reading.
Photoresistors: Resistance That Follows Light A photoresistor, also known as a light-dependent resistor (LDR) or photocell, changes resistance with illumination. In bright light its resistance is low, and in darkness it is high, often in the megohm range. Most common photoresistors use cadmium sulfide (CdS) as the light-sensitive material. How a Photoresistor Works Light striking the semiconductor material frees charge carriers, which makes it more conductive. The more light that hits the surface, the more carriers are available and the lower the resistance falls. Real-World Uses ● ● ● ●
Automatic street lights and outdoor fixtures that switch on at dusk Night lights and garden lights Camera light meters in older and simple designs Alarm and security systems that detect a beam being interrupted
● Display brightness control in simple consumer devices Strengths and Limitations Photoresistors are cheap, simple, and easy to interface with a basic voltage divider. They also have drawbacks. Response time is slow, typically tens to hundreds of milliseconds, so they are poorly suited to fast optical signals. Their output varies from part to part and shifts with temperature, so they are better for rough light detection than for precise measurement. In addition, cadmium is restricted in many regions under RoHS rules, so lead-free and cadmium-free alternatives such as photodiodes and phototransistors are often chosen for new designs. If you’re building a divider to read a photoresistor, you’ll rely on accurate fixed resistors. This guide on how to read resistor color codes helps you confirm the values you’re using.
Varistors: Resistance That Follows Voltage A varistor is a voltage-dependent resistor. At normal operating voltage its resistance is very high, so it draws almost no current. When the voltage rises above a threshold, its resistance drops sharply, and it conducts the excess energy away. This makes it an effective surge protector. Metal Oxide Varistors (MOVs) The most common type is the metal oxide varistor, built from zinc oxide grains and other additives. Its nonlinear current-voltage curve works much like a pair of back-to-back diodes, so it clamps both positive and negative voltage spikes. Where Varistors Are Used ● Power strips and surge protectors guarding against lightning-induced and switching transients ● Power supply inputs in appliances, chargers, and industrial equipment ● Motor drives and relays to absorb inductive spikes ● Telecommunications and data lines where transient protection is required Key Varistor Specifications ● Varistor (clamping) voltage: the voltage at which the part starts to conduct significantly ● Maximum continuous operating voltage (AC and DC) ● Energy rating in joules, showing how much surge energy it can absorb ● Peak surge current rating ● Response time, which is typically in the nanosecond range Important Caveat: Varistors Wear Out Each large surge slightly degrades a MOV. Over time its leakage current rises and its clamping ability weakens. A heavily stressed MOV can overheat, which is why many designs
pair it with a thermal fuse or a series fuse. For critical equipment, plan for replacement after major surge events.
Quick Comparison ● Thermistor ○ Responds to: temperature ○ Main role: sensing, limiting, and protection ○ Typical uses: thermostats, battery packs, heaters, inrush limiting ● Photoresistor ○ Responds to: light ○ Main role: ambient light detection ○ Typical uses: street lights, night lights, simple alarms ● Varistor ○ Responds to: voltage ○ Main role: surge suppression ○ Typical uses: power strips, power supplies, motor drives
How to Choose the Right Specialty Resistor Start with the physical quantity you need to sense or control: 1. Temperature? Use an NTC for sensing and compensation, or a PTC for protection and self-regulating heat. 2. Light level? Use a photoresistor for basic, low-cost detection. Choose a photodiode if you need speed or precision. 3. Voltage spikes? Use a varistor, sized for your operating voltage and expected surge energy. Then verify the environment and reliability needs. Consider operating temperature, mounting style, response time, and lifespan. Where the application is critical, as in industrial control, medical, or aerospace equipment, traceable components from a quality-focused supplier are essential. You can see how Engineered Parts Supply approaches this on its quality page.
Using Specialty Resistors in Circuits A few practical tips apply across all three types: ● Use a voltage divider with a fixed resistor to convert a thermistor or photoresistor into a voltage a microcontroller can read. ● Calibrate where accuracy matters, since part-to-part variation is significant. ● Limit current through sensing elements to prevent self-heating. ● Add protection such as fuses with varistors, and consider the worst-case surge, not the average one. ● Follow derating guidance from the datasheet for temperature and power. For deeper background on the specifications that matter in any resistor, see this resource on choosing the right resistor.
Conclusion Thermistors, photoresistors, and varistors turn a simple resistor into a sensor or a protective device. Thermistors track temperature and protect against overheating, photoresistors react to light for basic detection and automation, and varistors guard circuits from damaging voltage surges. Match the part to the quantity you need to measure or control, check the ratings that matter for your design, and plan for the limitations of each type. If you need help sourcing specific components or bulk quantities, submit a request for quote and get the parts your project requires.