Complete Guide to Thermistors

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Complete Guide to Thermistors

The word "thermistor" is a portmanteau of "thermal" and "resistor", and describes a type of resistance thermometer that has a wide range of applications. Thermistors are commonly used in digital thermometers, and they are also used in cars, trucks, and buses for monitoring temperature without regulating it. If the thermistor is damaged, it gives false readings, which may disguise a serious problem with the vehicle. Motorists should learn to identify potential issues, and correct any faults as soon as possible, to keep the vehicle in the best operational condition.


Understanding thermistors

Thermistors are a way to read changes in temperature. They are common in many devices that need heating or cooling protection, and are frequently used because they are inexpensive, sturdy, and accurate. They are very good at reading precise temperatures at a desired base point.


A thermistor is composed of metallic oxides, pressed into a bead, disk, or cylinder, and encapsulated in an impermeable material. This outer material is often glass, but may be an epoxy.


Thermistors react to changes in temperature, becoming more or less resistant. By measuring the increase or decrease in voltage of an electrical current passing through the thermistor, it is possible to accurately gauge temperature.


There are two types of thermistors. With a Negative Temperature Coefficient (NTC) thermistor, an increase in temperature results in a decrease in resistance, and a drop in the temperature results in an increase in resistance. This is the most common kind of thermistor, which has uses in vehicles and digital thermometers.

With a Positive Temperature Coefficient (PTC) thermistor, an increase in temperature results in an increase in resistance. Conversely, when temperature decreases, resistance decreases. This characteristic makes a PTC thermistor useful as a fuse.


Thermistors in vehicles

NTC thermistors are located in cars, trucks, and buses to determine the temperature of oil and air conditioner coolants. While the thermistors do not regulate temperature, they accurately record it, and then transmit the information to indicators on the vehicle's dashboard. When a fault is recorded, the indicator informs the motorist, making it possible to correct the fault before it causes serious damage to the vehicle.

A faulty thermistor does not cause an engine to overheat; however, when a thermistor is not working correctly, there is no early warning system when overheating occurs. A broken thermistor in an air conditioner may cause a malfunction such as overcooling or undercooling.


Fault finding

Even under normal usage, it is possible for a thermistor to develop faults. At first, a fault may not be obviously apparent, but over time the problem gets worse, so motorists should be vigilant for symptoms.

Potential issues

A thermistor with dirt on the connectors may start to provide false readings. In an air conditioner, this leads to undercooling or overcooling, as the system is unable to accurately gauge the current temperature level. An electrical overload may burn out the contacts, in which case the thermistor is unable to take any readings.


It is possible to test the functionality of a thermistor using an ohmmeter, an electrical device that measures electrical resistance. Hooking up the ohmmeter to the thermistor's leads quickly reveals if there is an issue.


How to repair a thermistor depends on the nature of the fault. If the connectors are dirty, wiggling the thermistor may work temporarily, although fluctuations in readings may occur. A better option is to spray the contacts with electrical contact cleaner. If contacts have burnt out, the only option is to purchase a replacement thermistor.


Choosing the right thermistor

It is essential to purchase the correct type of thermistor for the necessary application. PTC thermistors are often used as current-limiting devices for circuit protection, and are therefore similar to fuses. They also work as heating elements in small ovens. When used in a car for monitoring air conditioner coolant or oil temperature, consumers need an NTC thermistor.

Installing the incorrect thermistor on a vehicle may have serious consequences. If in doubt, consumers should consult with a qualified mechanic to ensure they are not doing anything that could result in harm to the vehicle or its occupants.


How to buy thermistors on eBay

Replacement and spare car parts, including thermistors and other components relating to monitoring the performance of the vehicle, are readily available from sellers on eBay. To start searching for the parts you need, enter a search criterion in the field available on every eBay page, including the make and model of the vehicle the components are for. It is also possible to apply filters to refine the list of results, and to organise results by price to make it easier to find bargains.

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