Varistor and Thermistor: What Sets Them Apart

　　Varistors and thermistors are two specialized resistors that break the mold of their traditional counterparts. While standard resistors offer constant opposition to current flow, varistors and thermistors are dynamic. Varistors act as voltage guardians, changing resistance to safeguard circuits from harmful surges. Thermistors, on the other hand, are temperature detectives, their resistance fluctuating with heat.

　　This blog explores the unique functionalities of varistors and thermistors, highlighting the key characteristics that differentiate them within the vast world of electronic components.

　　Varistor: The Voltage Dependent Resistor

　　A varistor, also known as a Voltage Dependent Resistor (VDR), is a special type of resistor whose resistance varies depending on the applied voltage. Under standard operating voltage, a varistor exhibits high resistance, acting like an open circuit. However, the varistor's resistance drops dramatically when a voltage surge exceeds a specific threshold. This sudden change allows excess current to flow through the varistor, diverting it away from sensitive circuit elements and protecting it from damage.

　　Varistors' common applications include protecting AC power lines and safeguarding electronic devices from transient voltage spikes caused by lightning strikes or power grid fluctuations.

　　Thermistor: The Temperature Sensitive Resistor

　　In contrast to varistors, thermistors are all about temperature. These components are essentially temperature-sensitive resistors (TSRs) whose resistance value changes significantly with fluctuations in temperature. There are two main types of thermistors:

　　PTC (Positive Temperature Coefficient): As the name suggests, the resistance of a PTC thermistor increases with rising temperature. These are often used in resettable fuses or self-regulating heating elements.

　　NTC (Negative Temperature Coefficient): Conversely, NTC thermistors exhibit a decrease in resistance as the temperature climbs. These are commonly employed in temperature sensors, fever thermometers, and temperature control systems.

　　Thermistors offer a wide range of applications in various industries. From monitoring engine coolant temperature in automobiles to regulating battery temperature in laptops, NTC thermistors play a vital role in ensuring optimal operation. On the other hand, PTC thermistors are used in circuit protection scenarios where a surge in temperature can trigger a safety response.

　　Differences Between Varistors and Thermistors

　　While both varistors and thermistors are crucial components, their functionalities and operating principles differ significantly:

　　Electrical Properties: Varistors exhibit non-linear resistance, dramatically changing based on voltage. Thermistors, on the other hand, display a more linear relationship between resistance and temperature.

　　Material Composition: Varistors are typically made of metal oxide ceramics, while thermistors can be constructed from various materials like ceramic semiconductors, polymers, or even thermistor beads.

　　Response to Environmental Factors: Varistors react to voltage spikes, while thermistors are sensitive to temperature variations.

　　Application Areas: Varistors excel in transient voltage protection, while thermistors shine in temperature sensing and control applications.

　　Varistor vs. Thermistor: When to Use Which?

　　Choosing between a varistor and a thermistor depends on the specific needs of your circuit:

　　Voltage Protection: A varistor is an ideal choice if your circuit requires protection from voltage spikes and transients.

　　Temperature Sensing: When accurate temperature measurement or control is crucial, NTC or PTC thermistors are the preferred components.

　　For example, a power supply circuit would likely utilize a varistor to safeguard against potential lightning strikes. At the same time, a battery pack might incorporate an NTC thermistor to monitor internal temperature for safety purposes.

　　Comparing Varistor and Thermistor Circuits

　　A simple circuit demonstrating a varistor application could involve connecting it in parallel with a sensitive electronic device across the power supply. In the event of a voltage surge, the varistor's resistance drops, diverting the excess current and protecting the device.

　　On the other hand, a thermistor circuit for temperature sensing might involve placing it in series with a current source. As the temperature rises, the thermistor's resistance decreases, causing a change in the current flow, which can be measured to determine the temperature.