5

Understanding Thermistors

Temperature measurement is one of the most common requirements in electronics. From home appliances and industrial machinery to electric vehicles and embedded systems, countless devices need to monitor temperature accurately and reliably.

One of the simplest and most widely used temperature-sensing components is the thermistor. Despite its small size and low cost, a thermistor can provide remarkably accurate temperature measurements and protection functions.

Thermistors are found in everything from coffee makers and battery packs to HVAC systems, 3D printers, laptops, medical devices, and automotive electronics. Their popularity comes from their sensitivity, simplicity, and ability to respond quickly to temperature changes.

Understanding how thermistors work is essential for anyone interested in electronics, embedded systems, sensors, or temperature control applications.

What Is a Thermistor?

A thermistor is a resistor whose resistance changes significantly with temperature.

The name comes from:

THERMal + resISTOR

Unlike ordinary resistors, which are designed to maintain a stable resistance value, thermistors are specifically engineered so their resistance changes predictably as temperature changes.

This characteristic allows them to function as temperature sensors and protection devices.

Why Thermistors Are Different from Ordinary Resistors

Most resistors exhibit small resistance changes as temperature varies.

For example:

• A 10kΩ resistor might change by only a few ohms
• The change is often negligible

A thermistor, however, may change by thousands of ohms over the same temperature range.

This large change makes temperature measurement practical and inexpensive.

Types of Thermistors

Thermistors fall into two primary categories.

NTC Thermistors

NTC stands for:

Negative Temperature Coefficient

As temperature increases:

• Resistance decreases

As temperature decreases:

• Resistance increases

NTC thermistors are by far the most common type used for temperature measurement.

PTC Thermistors

PTC stands for:

Positive Temperature Coefficient

As temperature increases:

• Resistance increases

As temperature decreases:

• Resistance decreases

PTC thermistors are often used for protection applications rather than precise temperature sensing.

NTC Thermistors Explained

NTC thermistors are extremely popular because they provide a large resistance change over a relatively small temperature range.

Consider a typical 10kΩ NTC thermistor.

At:

• 25°C → 10kΩ

At:

• 50°C → approximately 3.6kΩ

At:

• 75°C → approximately 1.5kΩ

The resistance drops rapidly as temperature rises.

This behavior allows microcontrollers and measurement circuits to determine temperature accurately.

Why NTC Thermistors Work

NTC thermistors are made from semiconductor materials.

As temperature increases:

• More charge carriers become available
• Electrical conductivity rises
• Resistance falls

This semiconductor behavior is the opposite of many metallic conductors.

The result is a highly temperature-sensitive device.

PTC Thermistors Explained

PTC thermistors operate differently.

As temperature rises:

• Resistance increases dramatically

In some designs, the increase can be sudden and substantial.

For example:

• A few ohms at room temperature
• Hundreds or thousands of ohms when heated

This characteristic makes PTC thermistors useful for protection circuits.

Thermistor Resistance Curves

Unlike many sensors, thermistors do not change linearly with temperature.

Their response curve is exponential.

A simplified NTC response looks like:

Resistance
|
|\
| \
|  \
|   \
|    \
|     \____
|
+---------------- Temperature

The steep curve provides excellent sensitivity but requires calculations or lookup tables for accurate measurements.

The Beta Value

One of the most important thermistor specifications is the Beta value.

Often written as:

B = 3950

or

B = 3435

The Beta constant describes how resistance changes with temperature.

Different thermistors use different Beta values depending on their intended application.

Higher Beta values generally indicate greater sensitivity.

Thermistor Temperature Calculation

Temperature can be estimated using the Beta equation.

\frac{1}=\frac{1}+\frac{1}\ln\left(\frac\right)

Where:

• T = Temperature in Kelvin
• T₀ = Reference temperature
• R = Measured resistance
• R₀ = Reference resistance
• B = Beta constant

Most embedded projects use software libraries or lookup tables instead of calculating the equation manually.

The 10k Thermistor Standard

One of the most common thermistors is:

10kΩ @ 25°C

This means:

• Resistance equals 10,000 ohms at 25°C

These thermistors are widely available and supported by numerous electronics platforms.

They are commonly used in:

• 3D printers
• Battery packs
• HVAC controls
• Embedded projects

Reading a Thermistor with a Microcontroller

Thermistors are usually connected in a voltage divider circuit.

3.3V
 |
Resistor
 |
 +---- ADC Input
 |
Thermistor
 |
GND

As temperature changes:

• Thermistor resistance changes
• Divider voltage changes
• ADC reading changes

Software converts the ADC value into temperature.

This technique is widely used with:

• ESP32
• Raspberry Pi Pico
• STM32
• Arduino boards

Why Voltage Dividers Are Used

Microcontrollers measure voltage, not resistance.

The voltage divider converts resistance changes into measurable voltage changes.

This approach provides:

• Simplicity
• Low cost
• Good accuracy

for many applications.

Common Temperature Ranges

Thermistors are available for a wide variety of temperature ranges.

Typical ranges include:

Always verify specifications before selecting a sensor.

Thermistors in Battery Packs

Modern lithium-ion battery packs often contain thermistors.

The thermistor allows the battery management system to monitor:

• Charging temperature
• Discharging temperature
• Overheating conditions

If temperature exceeds safe limits:

• Charging may stop
• Output current may be reduced
• Protection mechanisms activate

This improves safety and battery lifespan.

Thermistors in 3D Printers

One of the most common hobbyist applications is temperature measurement in 3D printers.

Thermistors monitor:

• Hot end temperature
• Heated bed temperature

Accurate temperature control ensures:

• Proper extrusion
• Reliable layer adhesion
• Consistent print quality

Most consumer 3D printers use NTC thermistors.

HVAC and Climate Control

Thermistors are widely used in:

• Air conditioners
• Refrigerators
• Heat pumps
• Smart thermostats

Their low cost and fast response make them ideal for temperature regulation.

Automotive Applications

Vehicles use thermistors extensively.

Examples include:

• Coolant temperature sensors
• Intake air temperature sensors
• Battery monitoring systems
• Cabin climate control

Modern vehicles may contain dozens of thermistors.

Medical Equipment

Medical devices often require accurate temperature monitoring.

Applications include:

• Patient monitoring
• Laboratory instruments
• Medical storage systems
• Diagnostic equipment

Thermistors provide precise measurements at relatively low cost.

PTC Thermistors for Overcurrent Protection

A common PTC application is circuit protection.

These devices are often called:

Resettable Fuses

or

Polyfuses

When excessive current flows:

• Device heats up
• Resistance rises sharply
• Current decreases

Once the fault is removed:

• Device cools
• Resistance returns to normal

Unlike conventional fuses, they automatically reset.

Inrush Current Limiting

NTC thermistors are often used to reduce startup current.

Examples include:

• Power supplies
• Motor drives
• Large transformers

At startup:

• Thermistor is cold
• Resistance is high
• Current is limited

As it warms:

• Resistance falls
• Normal operation begins

This protects components from damaging surge currents.

Thermistors vs RTDs

Resistance Temperature Detectors (RTDs) are another temperature sensing technology.

Thermistors often win on cost and sensitivity.

RTDs are preferred when maximum accuracy is required.

Thermistors vs Thermocouples

Thermocouples are common in industrial applications.

Thermocouples excel at extreme temperatures.

Thermistors excel at normal operating temperatures.

Common Beginner Mistakes

Ignoring the Beta Value

Two 10k thermistors may have different Beta values.

Using the wrong value causes inaccurate temperature readings.

Assuming Linear Behavior

Thermistors are highly nonlinear.

Conversion calculations or lookup tables are necessary.

Poor Thermal Placement

Mounting location greatly affects accuracy.

Poor placement can measure ambient air instead of the intended target.

Self-Heating Effects

Measurement current can slightly warm the thermistor.

Excessive current may distort readings.

Popular Thermistor Specifications

When selecting a thermistor, pay attention to:

• Resistance at 25°C
• Beta value
• Temperature range
• Accuracy tolerance
• Package type
• Response time

These factors determine suitability for a specific application.

Where You Will Find Thermistors

Thermistors appear in:

• Power supplies
• Battery chargers
• Electric vehicles
• HVAC systems
• Medical devices
• 3D printers
• Home appliances
• Solar systems
• Industrial automation
• Embedded electronics

They remain one of the most widely used temperature sensors in the world.

Conclusion

Thermistors are temperature-sensitive resistors that provide a simple and highly effective method of measuring temperature and protecting electronic systems. NTC thermistors decrease in resistance as temperature rises, making them ideal for sensing applications, while PTC thermistors increase in resistance and are commonly used for protection circuits and current limiting.

Their low cost, fast response, and high sensitivity have made thermistors essential components in everything from consumer electronics and automotive systems to industrial controls and medical equipment. Understanding how thermistors work provides a strong foundation for designing reliable temperature-monitoring and protection systems.