How Thermistors Work - The Learning Circuit

element14 presents

10 Mar 202108:12

Summary

TLDRIn this episode of 'The Learning Circuit,' Karen introduces thermistors, temperature-sensitive resistors that vary their resistance with temperature changes. NTC and PTC types are discussed, with NTCs decreasing resistance as temperature rises and PTCs increasing resistance with temperature. The script explains how thermistors work, their non-linear resistance changes, and their applications in temperature sensing and as fuses in various appliances. The video also touches on alternative temperature sensors like RTDs and thermocouples, comparing their capabilities with thermistors.

Takeaways

🌡️ Thermistors are variable resistors that change resistance with temperature changes, derived from 'thermal' and 'resistor'.
↔️ There are two types of thermistors: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient).
📉 NTC thermistors decrease in resistance with an increase in temperature, while PTC thermistors increase in resistance with temperature.
🔬 The resistance change in NTC thermistors is non-linear and can vary significantly across different temperature ranges.
🏭 PTC thermistors come in three main types: cylisters, polymer PTCs (resettable fuses), and switching PTCs made from doped polycrystalline ceramic.
⚖️ The beta value is used to calculate the curve of NTC thermistors and is determined using readings at two different temperatures.
🛠️ Thermistors are used in various applications such as temperature sensing, inrush current limiting, and as fuses in circuits.
🏠 Everyday appliances like digital thermometers, toasters, and refrigerators often use NTC thermistors for temperature sensing.
🔒 Encapsulation in epoxy or glass makes thermistors waterproof, rugged, and stable against environmental factors.
📊 Thermistors, while not linear, can be plotted on a curve using specific equations and are chosen based on the required temperature range and resistance.
🌡️ Other temperature sensors like RTDs, thermocouples, and temperature sensor chips offer different advantages and are suitable for various applications.

Q & A

What is a thermistor?
-A thermistor is a type of variable resistor whose resistance changes with temperature changes in its environment. It is derived from the combination of the words 'thermal' and 'resistor'.
What are the two main types of thermistors?
-The two main types of thermistors are NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) thermistors.
How does the resistance of an NTC thermistor change with temperature?
-In NTC thermistors, an increase in temperature causes a decrease in resistance, and a decrease in temperature causes an increase in resistance.
What is the resistive response of PTC thermistors to temperature changes?
-For PTC thermistors, as the temperature increases, the resistance also increases, and as the temperature decreases, the resistance decreases as well.
How do semiconductors in thermistors respond to temperature changes?
-When an NTC thermistor gets warmer, the electrons become more excited and move faster, increasing the current and decreasing resistance due to Ohm's law. Conversely, when it gets colder, the electrons slow down, increasing resistance.
What is the difference between a fixed resistor and a thermistor in terms of temperature response?
-Fixed resistors maintain a consistent resistance until they reach a high enough temperature where their resistance begins to degrade linearly. Thermistors, however, experience a non-linear change in resistance with temperature.
What are the three main types of PTC thermistors and how do they respond to temperature changes?
-The three main types of PTC thermistors are cylisters, polymer PTCs (resettable fuses), and switching PTCs. Cylisters have a near-linear temperature resistance curve, polymer PTCs increase resistance as they heat up due to the expansion of plastic pushing carbon grains apart, and switching PTCs show slight NTC behavior at low temperatures but dramatically increase resistance at a critical temperature.
How is the beta value of an NTC thermistor calculated and where can it be found?
-The beta value of an NTC thermistor is calculated using readings at two temperatures, with temperature measured in Kelvin. It can be found on the thermistor's data sheet within certain temperature ranges, often listed at zero to 50, 25 to 100, and 25 to 85 degrees Celsius.
What are some common applications of thermistors?
-Thermistors are used in various applications such as temperature sensing in digital thermometers, toasters, coffee makers, refrigerators, motor oil monitoring, 3D printer hot ends, and as inrush current limiters in devices like motors and transformers.
What are the advantages and disadvantages of thermistors as temperature sensors?
-Advantages of thermistors include being cheap, durable, precise, easy to waterproof, and able to work at any voltage. Disadvantages include requiring an ADC to interpret temperature values, operating within a limited temperature range, inability to withstand extreme temperatures, and potential for self-heating and damage when handling high currents.
How do thermistors compare to other temperature sensors like RTDs and thermocouples?
-Thermistors are made of semiconductor materials and change resistance with temperature. RTDs and thermocouples, on the other hand, are made of pure metals and produce a temperature-dependent voltage. While RTDs and thermocouples can measure more extreme temperatures, they are less accurate, more expensive, have slower response times, and require an amplifier.

Outlines

00:00

🔍 Understanding Thermistors

This segment introduces thermistors as variable resistors whose resistance changes with temperature. It explains the difference between NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) thermistors. NTC thermistors decrease in resistance with an increase in temperature, while PTC thermistors increase. The segment also discusses how semiconductors in thermistors respond to temperature changes, affecting the flow of electrons and thus resistance. It contrasts thermistors with fixed resistors, which maintain consistent resistance until a critical temperature is reached. The non-linear resistance change in thermistors is highlighted, with NTC thermistors showing varying resistance changes over different temperature ranges. The segment also covers the three main types of PTC thermistors: cylisters, polymer PTCs (resettable fuses), and switching PTCs, each with a unique temperature resistance curve. The use of the beta value equation to calculate the curve of NTC thermistors is mentioned, along with the importance of considering the Celsius to Kelvin conversion. The segment concludes with a brief on the physical protection of thermistors through encapsulation and their applications in various devices for temperature sensing and as fuses.

05:03

🔌 Applications and Limitations of Thermistors

This part of the script delves into the practical applications of NTC thermistors as inrush current limiters in circuits with inductive devices like motors and transformers. It explains how the resistance of NTC thermistors can restrict high inrush current and how resistive heating works. The segment also touches on the use of PTC thermistors as current limiting devices or fuses due to their increasing resistance with temperature. The script compares thermistors with other temperature sensors like Resistance Temperature Detectors (RTDs) and thermocouples, noting their differences in material composition and temperature measurement capabilities. It points out the advantages of thermistors, such as affordability, durability, precision, and ease of waterproofing, but also their limitations, including the need for an ADC to interpret temperature values, limited temperature range, and susceptibility to self-heating errors under high currents. The segment advises on the selection of thermistors based on temperature application needs and the temperature resistance of different coatings. It ends with an invitation to the Element14 community for further discussion and a teaser for the next video on a fun thermistor project.

Mindmap

Keywords

💡Thermistor

A thermistor is a type of resistor whose resistance varies with temperature. In the video, thermistors are introduced as variable resistors that change their resistance based on the temperature of their environment. They are crucial for applications requiring temperature sensing or control. The script explains that thermistors come in two types: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient), which behave differently with temperature changes.

💡NTC (Negative Temperature Coefficient)

NTC stands for Negative Temperature Coefficient, referring to a type of thermistor where an increase in temperature results in a decrease in resistance. The video script uses NTC thermistors as an example to explain how they behave when the temperature rises, causing electrons to move faster and thus decreasing resistance, which is a key concept in understanding their application in temperature sensing.

💡PTC (Positive Temperature Coefficient)

PTC, or Positive Temperature Coefficient, thermistors are those where resistance increases with an increase in temperature. The script contrasts this behavior with NTC thermistors, highlighting that PTC thermistors are used in applications like fuses or current limiting devices, where a rise in temperature leads to an increase in resistance to protect circuits.

💡Ohm's Law

Ohm's Law is a fundamental principle in electrical engineering that states the relationship between voltage, current, and resistance in a circuit. The video script references Ohm's Law to explain how the resistance of an NTC thermistor decreases as the current increases with temperature, illustrating the practical application of this law in understanding thermistor behavior.

💡Semiconductor

Semiconductors are materials that have electrical conductivity between that of a conductor and an insulator. The script discusses how semiconductors, with their free-floating electrons in n-type regions and holes in p-type regions, are fundamental to the operation of thermistors. The movement of these particles in response to an electric current is key to the thermistor's temperature-dependent resistance changes.

💡Resistance Temperature Detector (RTD)

An RTD is a type of temperature sensor distinct from thermistors, made of pure metals. The video script compares RTDs with thermistors, noting that while RTDs can measure a broader range of temperatures, they are less accurate, more expensive, and require an amplifier for readings, unlike thermistors.

💡Thermocouple

A thermocouple is another type of temperature sensor mentioned in the script. It operates on the principle of generating a temperature-dependent voltage from two dissimilar metals. The video contrasts thermocouples with thermistors, highlighting their ability to measure extreme temperatures but also their slower response times and need for amplification.

💡Inrush Current

Inrush current refers to the high initial current drawn by an electrical device when it is first powered on. The script explains how NTC thermistors can be used as inrush current limiters in circuits with inductive devices like motors and transformers, protecting them from the potentially damaging effects of high initial currents.

💡Resistive Heating

Resistive heating is the process by which the passage of an electric current through a resistor produces heat. The video script mentions that electrical current passing through a device can cause resistive heating, which is a form of energy loss. This concept is important in understanding how thermistors can be used to manage and protect against overheating in electronic devices.

💡Voltage Drop

A voltage drop occurs when there is a decrease in electric potential between two points in a circuit, often due to resistance. The script discusses voltage drop in the context of energy lost to heat, which is a critical consideration in circuit design and the use of thermistors for temperature management.

💡Beta Value

The beta value is a parameter used to characterize the temperature sensitivity of an NTC thermistor. The video script explains that the beta value can be calculated using readings at two different temperatures and is typically provided on a thermistor's data sheet. This value is essential for predicting and calculating the resistance of an NTC thermistor at various temperatures.

Highlights

Thermistors are variable resistors whose resistance changes with temperature.

NTC thermistors decrease in resistance with an increase in temperature, while PTC thermistors increase.

NTC stands for Negative Temperature Coefficient, and PTC stands for Positive Temperature Coefficient.

Semiconductors in thermistors have free electrons or holes that respond to temperature changes.

NTC thermistors exhibit a non-linear change in resistance with temperature.

Fixed resistors maintain consistent resistance until a high temperature is reached, unlike thermistors.

Cylisters, a type of PTC thermistor, have a near-linear temperature resistance curve.

Polymer PTCs, also known as resettable fuses, change resistance as the plastic expands with heat.

Switching PTC thermistors show slight NTC behavior before a critical temperature is reached, after which resistance increases dramatically.

The beta value of an NTC thermistor is used to calculate its resistance curve and can be found on data sheets.

Thermistors come in various shapes and are typically encapsulated with epoxy or glass for protection.

NTC thermistors are used in common appliances for temperature sensing, such as digital thermometers and toasters.

PTC thermistors are used as fuses and current limiting devices in circuits.

Thermistors are compared to other temperature sensors like RTDs and thermocouples, which use different principles.

Thermostats and RTDs can measure extreme temperatures but are less accurate and more expensive than thermistors.

Temperature sensor chips offer analog or digital output options and are similar to thermistors.

Thermistors have limitations such as requiring an ADC for temperature interpretation and operating within a limited temperature range.

Epoxy-coated thermistors can withstand temperatures between -50°C and 150°C, while glass-coated ones can go up to 300°C.

Thermistors are a cheap, reliable, and durable solution for sensing temperature in circuits.