How Thermistors Work - The Learning Circuit

Summary

In 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.