Given that NTC thermistors come in different types, it can be difficult for engineers who are using NTC thermistors for the first time to select the most suitable thermistor for their needs.
Note that NTC thermistors have specific electrical properties:
- Current-time characteristic
- Voltage-current characteristic
- Resistance-temperature characteristic
These characteristics must be taken into account when a decision has to be made as to the type of NTC thermistor required.
There are also other factors that need to be considered:
Type/Size of Product
The thermistor user usually will know what is needed in terms of size, thermal response and other physical features that go into the configuration of the thermistor. It should be easy to narrow down the choice of NTC thermistors even when data is lacking, but a careful analysis of the intended application of the thermistor must be made. This may be all that is required to determine the most appropriate NTC thermistor to be purchased.
Sales catalogs from manufacturers and suppliers contain a table or matrix of resistance ratios vis-a-vis temperature for each of their NTC thermistor products. Coefficients are also sometimes provided for particular equations to help the user or designer analyze the resistance/temperature data. There is quite a wide range of materials that can be used to manufacture thermistors, but there are limitations involved depending on the size, operating and storing temperature range and nominal resistance values.
Nominal Resistance Value
In deciding which NTC thermistor to use, the next factor to consider is the nominal resistance value at a given temperature. Manufacturers offer an entire range of nominal resistance values for their NTC thermistors. The standard reference temperature is 25 degrees Celsius, but buyers and designers can request different temperatures.
A word of caution: if the desired resistance is not available in the combination of product type and material component, then a decision must be made as to which characteristic takes priority: produce type/size, material preference or resistance ratio.
With today’s available technologies, users and designers cannot request all three features – nominal resistance, product type and size – especially when the combination requested does not fall within established guidelines.
When looking at product specification sheets, standard tolerances are provided by the manufacturer or supplier. Thermistors usually have a zero-power resistance distribution of:
- ± 20% to ± 25% (bead-type thermistors)
- ± 5% to ± 10% (metal-surface thermistors)
- ± 20% and higher (flake-type thermistors)
To save on costs, manufacturers recommend specifications of the broadest possible tolerance that is relevant to intended use. Metal-surface thermistors present an advantage because their shapes can be modified to yield closer tolerances.
The thermistor’s beta tolerance is largely determined by the metal oxides used in its manufacture. Beta tolerances are different for bead-type and metal-surface thermistors. They play a role on resistance limits which must be taken into consideration when determining its specific application. In cases where there are many high and low temperature ranges, it is recommended to specify the nominal resistance at the operating temperature.
Ametherm sells NTC thermistors that are available as radial-leaded disks or as probes optimized for performance. We will be pleased to provide samples of our NTC thermistor probes. Please contact us directly or online.
Alternatively, we would be pleased to customize our thermistors to fulfill your requirements.