Depending on design parameters, choosing the right thermistor can be difficult. Discover the applications, functions, and design parameters to consider when selecting a thermistor in our helpful guide below.
What is a Thermistor?
Thermistors are a class of devices that change their resistance based on temperature. Thermistors can serve many purposes, and you can find several different types and form-factors available on the market today. But what factors should you consider when choosing one?
Here are a few characteristics to consider as you explore thermistors for your next application.
How Does a Thermistor Work?
1. NTC or PTC A thermistor’s most basic feature may be whether its resistance goes up or down with increasing temperature. Those that lose resistance have a negative temperature coefficient (NTC), while those that gain resistance have a positive temperature coefficient (PTC). Use a PTC device to control current directly based on temperature and employ an NTC device for sensing applications.
2. Nominal Resistance Thermistors generally have a nominal resistance specified at 25°C (77°F), which can vary greatly. You can also find thermistors with a nominal resistance specified at 10kohm, 100kohm, or other values.
Thermistor Design Considerations
3. Sensing or Direct Control Manufacturers design certain thermistors to transmit very little current, and these devices are quite small, feeding resistance information to a separate controller that can react to this information. Other thermistors, as we noted above, can regulate current flow directly based on temperature.
4. Physical Size and Form Factor Thermistors come in a wide range of sizes, some of which are incredibly small – as small as 1mm for the bead diameter. Some thermistors are larger to handle more current directly, while others come in an industrial housing, ready to endure expected exposure without additional shielding.5. Interchangeability Most thermistors come with a certain interchangeability tolerance, with ±.1°C being a standard example. If each thermistor is within an acceptable range, the interchangeability tolerance allows you to change devices as needed without additional calibration.
6. Cost Choosing the components for your application sometimes comes down to price. You can find bare sensing components for well under a dollar, while the larger direct-control components will cost more. Thermistors with integrated housings for industrial or appliance use can be even more expensive, and industrial sensing units can cost $100 or more. Depending on your application, high-quality components may be well worth the price tag, especially if these components can alleviate problems down the road.
How Does a Thermistor Work in Different Applications?
7. Temperature Range Depending on your application, you may need your thermistor to work within a certain temperature range. If you purchase a thermistor with a temperature range of -40 to 100°C (-40 to 212°F), be sure your application will not include temperatures outside that range.
8. Time Constant Depending on your specific application, you may want a near-instantaneous response to changing temperature conditions. You also may want some type of delay, allowing the thermistor to act as a timer or carefully regulate current flow. A thermistor with a time constant offers both features.9. Non-Linearity
A thermistor’s resistance will vary non-linearly throughout its useful range, and how each device behaves will depend on its construction. Learn more about thermistor resistance in a temperature versus resistance chart, modeled via the Steinhart-Hart equation, or the simplified B parameter equation.
A thermistor’s inherent non-linearity may be acceptable for your application if the device has the correct specifications. You may also need to look for a different sensor type, or perhaps additional equipment, to work with this output.