1. What are Thermistors?
2. How Does a Thermistor Work?
3. Applications of Thermistors
4. Advantages and Disadvantages
5. How Do They Compare to RTDs?
6. Choose the Right Thermistor for Your Application
7. Different Types of Thermistor Probes
8. The Most Common Thermistor Resistances
9. Conclusion

 

Introduction to Temperature Measurement with Thermistors

 

What are Thermistors?

Thermistors are a type of resistor whose resistance varies significantly with temperature. The term "thermistor" is derived from a combination of the words "thermal" and "resistor." They are broadly classified into two categories based on their temperature coefficient:

• Negative Temperature Coefficient (NTC) Thermistors: Their resistance decreases as the temperature increases.
• Positive Temperature Coefficient (PTC) Thermistors: Their resistance increases as the temperature increases.

 

How Does a Thermistor Work?

Thermistors work by utilising the temperature-dependent change in electrical resistance. This change occurs due to the properties of the semiconductor materials used in thermistors. Here’s a step-by-step breakdown of how they function:

• Material Properties: Thermistors are made from ceramic materials composed of metal oxides, such as manganese, nickel, or cobalt. These materials are sintered to achieve the desired resistance-temperature characteristics.
• Temperature Change: When the temperature changes, the intrinsic properties of the semiconductor material alter. In NTC thermistors, increasing temperature causes the charge carriers (electrons and holes) in the material to increase, reducing resistance. Conversely, in PTC thermistors, the material’s structure changes in a way that reduces the number of charge carriers or increases the scattering of charge carriers, thereby increasing resistance.
• Resistance Variation: This temperature-induced change in charge carriers results in a measurable change in resistance. For NTC thermistors, resistance decreases with an increase in temperature. For PTC thermistors, resistance increases with an increase in temperature.
• Measurement and Calibration: The resistance of the thermistor can be measured using an electrical circuit, and this resistance can be correlated to a specific temperature using calibration data or an equation like the Steinhart-Hart equation for NTC thermistors.

 

Applications of Thermistors

Thermistors are widely used due to their sensitivity to temperature changes and their accuracy. Common applications include:

• Temperature Sensing and Measurement: In devices like digital thermometers, weather stations, and home appliances.
• Temperature Compensation: To stabilise the performance of electronic circuits that are affected by temperature variations.
• Inrush Current Limiting: NTC thermistors limit the initial surge of current when electrical equipment is turned on.
• Overcurrent Protection: PTC thermistors act as self-resetting fuses in power supplies and battery packs.

 

Advantages and Disadvantages

Advantages

• High Sensitivity: Thermistors can detect minute changes in temperature.
• Wide Range: Suitable for various applications due to different types of thermistors.
• Compact Size: Small and can be integrated into various devices easily.
• Low Cost: Generally inexpensive compared to other temperature sensors.

Disadvantages

• Non-linear Response: The resistance-temperature relationship can be highly non-linear, complicating the calibration.
• Limited Temperature Range: Certain types may only be suitable for specific temperature ranges.
• Self-Heating: Can cause measurement errors if not properly managed.

How Do They Compare to RTDs?

Resistance Temperature Detectors (RTDs) are another type of temperature sensor that also relies on the change in resistance with temperature, but they differ significantly from thermistors in several ways:

Material and Construction

• RTDs are typically made of pure metals such as platinum, nickel, or copper, while thermistors are made of ceramic or polymer materials.
• Thermistors have a semiconductor base, leading to a more pronounced resistance change with temperature.

Temperature Range

• RTDs are suitable for a wider temperature range, often from -200°C to 850°C.
• Thermistors usually operate effectively within a narrower range, typically from -50°C to 150°C.

Accuracy and Stability

• RTDs provide higher accuracy and stability over a wide temperature range.
• Thermistors offer higher sensitivity but can be less stable and accurate over broad temperature ranges.

Response Time

• Thermistors generally have a faster response time due to their smaller size and lower thermal mass.
• RTDs might have a slower response time but are more stable for long-term measurements.

Linearity

• RTDs exhibit a more linear relationship between resistance and temperature.
• Thermistors have a highly non-linear response, which may require complex calibration.

Cost

• RTDs are typically more expensive due to their materials and construction.
• Thermistors are generally less expensive and easier to produce.

Choose the Right Thermistor for Your Application

Selecting the appropriate thermistor for a specific application involves considering several factors:

• Temperature Range: Determine the range of temperatures the thermistor will be exposed to. NTC thermistors are suitable for a narrower range, while PTC thermistors or RTDs may be necessary for broader ranges.
• Accuracy Requirements: Assess the accuracy required for your application. RTDs offer higher accuracy over a wide range, but if your application demands high sensitivity and moderate accuracy, thermistors might be more suitable.
• Response Time: Consider how quickly the sensor needs to respond to temperature changes. Thermistors typically provide faster response times than RTDs, making them ideal for applications requiring quick temperature readings.
• Environmental Conditions: Evaluate the environmental conditions, such as humidity, chemical exposure, and physical stress. Choose a thermistor with appropriate encapsulation and material properties to withstand these conditions.
• Size and Mounting: Ensure the thermistor's size and mounting method are compatible with your device or system. Thermistors come in various shapes and sizes to fit different applications.
• Cost Constraints: Balance your budget with performance needs. Thermistors are generally less expensive than RTDs, making them a cost-effective choice for many applications.
• Application-Specific Needs: Identify any specific requirements unique to your application, such as inrush current limiting or overcurrent protection, which might necessitate a particular type of thermistor.

Different Types of Thermistor Probes

Thermistor probes come in various forms to suit different applications and environmental conditions. Here are some common types:

• Bead Thermistors: Tiny beads of thermistor material encapsulated in glass, offering a quick response time. They are often used in medical applications and precision instruments.
• Disk and Chip Thermistors: Flat thermistors made by pressing thermistor material into a disk or chip shape. They are used in temperature sensing and compensation in electronic circuits.
• Rod and Cartridge Thermistors: Cylindrical thermistors that provide a rugged and durable option for industrial applications, such as HVAC systems and industrial process control.
• Bolt-on Probes: Thermistors embedded in a bolt for easy attachment to surfaces. They are used in applications where surface temperature measurement is required, such as motor or transformer monitoring.
• Epoxy-Coated Probes: Thermistors coated in epoxy for environmental protection. These are versatile and can be used in a variety of general-purpose temperature sensing applications.
• Glass-Encapsulated Probes: Thermistors sealed in glass, offering excellent stability and reliability at high temperatures. They are ideal for automotive and aerospace applications.
• Surface Mount Devices (SMDs): Tiny thermistors designed for mounting directly onto printed circuit boards (PCBs). They are used in electronic devices where space is limited.
• Insertion Probes: Thermistors housed in a protective sheath, designed to be inserted into liquids or gases. These are commonly used in environmental monitoring and laboratory applications.

At Labfacility, we specialise in manufacturing a diverse range of NTC thermistor temperature sensing probes. Our probes are crafted using high-accuracy, interchangeable elements known for their excellent long-term stability. Below are our range of thermistor sensors available to buy online:

• Surface Measurement Thermistors Sensors
• Wall Mounted Thermistor Sensors
  
• General Purpose Thermistor Probe with PTFE lead
• Hermetically Sealed Wire Thermistor Sensor
• Thermistor Sensor with Exposed Detector
• High Accuracy Thermistors

The Most Common Thermistor Resistances

Thermistors come in various resistance values, optimised for different applications and temperature ranges. Some of the most common resistance values for NTC thermistors at 25°C include:

• 10 kΩ: This is one of the most widely used thermistor resistances, suitable for a variety of temperature sensing applications in consumer electronics, HVAC systems, and automotive sensors.
• 5 kΩ: Often used in applications where a slightly lower resistance is needed while maintaining a good sensitivity range.
• 100 kΩ: Used in applications requiring higher resistance values, such as high-temperature sensors and some industrial applications.
• 2.2 kΩ: Common in applications with a need for moderate resistance values, balancing sensitivity and resistance range.
• 1 kΩ: Typically used in low-temperature applications or where space constraints require a small thermistor.

Choosing the right resistance value depends on the specific requirements of the application, including the temperature range, desired sensitivity, and the electrical characteristics of the measurement circuit.

Conclusion

Thermistors are versatile components for temperature measurement, providing high sensitivity and precision in various applications. Understanding their characteristics and the principles of their operation is essential for effectively incorporating them into temperature-sensing systems. While thermistors offer several advantages, selecting the right type and resistance value is crucial for optimal performance in any given application.

 

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