Choosing a Thermistor
NTC Thermistors can be used for a wide variety of tasks and applications. Among other things, NTC thermistors can be used to detect overheating in electronic equipment, to protect circuits from drawing too much electrical current, and to compensate for temperatures in mobile communications equipment. Part of choosing the right thermistor is determining which use best suits your needs.
Essentially, there are two types of NTC thermistors to choose from. The difference between these types lies in the way in which the electrodes have been attached to the ceramic body of the thermistor. The first basic type of thermistor is the bead thermistor. These NTC thermistors all have leadwires made of platinum allow that are sintered directly to the body. This type of thermistor includes: bare beads, glass coated beads, ruggedized beads, miniature glass probes, glass probes, glass rods, and bead in glass enclosures.
The second type of thermistor is one that has metallized surface contacts. These NTC thermistors can come with either axial or radial leads or even without leads to make surface mounting or spring contact mounting easy. This type of thermistor includes: disks, chips or wafers, flakes, surface mounts, rods, and washers.
What to Consider
In order to find the best thermistor for your application, you will need to consider all aspects of each thermistor type. You can begin with the basics. Start with the device size and the desired physical characteristics of the thermistor that you need. You may be able to narrow the potential list of NTC thermistors down greatly using just these basic criteria. Next, think about exactly what the NTC thermistor that you are looking for needs to do. This should allow you to further narrow your list of potentials.
Now, consider the resistance temperature curves that you are looking for. Most manufacturers of NTC thermistors will have various tables available of resistance ratios for their products that you can look at. They may even be able to provide you with the necessary coefficients in order to assist you in your decision. Each material system will also have its own limitations regarding the range of nominal resistance values available that will work.
You should also consider the resistance tolerance for the NTC thermistors that are left on your list of potential candidates for your application. The most cost efficient thermistor will have the broadest tolerance. Beta tolerance and resistance limits must also be considered when selecting your thermistor, as this fixes your lowest and highest resistance values. Keep in mind that the structure and composition of the metal oxides as well as the actual process of manufacturing can affect the beta tolerance.
Depending on the desired use for the thermistor, you may also want to consider the issues of curve matching and the interchangeability of the thermistor. Cost and performance may need to be considered when you are looking these factors. You may want to think about metalized surface contact thermistors and those hermetically glass enclosed when considering these elements.
Finally, calibration is something to consider when looking for the right thermistor type. Luckily, there are NTC thermistors available with precise calibrations that offer both broad tolerance and low cost. Testing and calibration of NTC thermistors can be very tricky, so be sure to find out how the calibration and testing of the thermistor is handled before you make your purchase.
Linear Response Elements vs. Thermistor Probes
Some applications require a thermistor with linear response to temperature change, but there is also sometimes the need for a thermistor element that can stand alone. For this purpose, thermistor probes may be considered. Many of these come enclosed in metal tubes making them more rugged.