Surface Mount
Thermistors

• Wide Ohmic Value Range
• Accurate & Stable
• Fast Thermal Response Time
• Standard Sizes
• High Sensitivity

Negative Temperature Coefficient (NTC) thermistors are thermally sensitive semiconductor resistors which exhibit a decrease in resistance as absolute temperature increases. Change in the resistance of the NTC thermistor can be brought about either by a change in the ambient temperature or internally by self-heating resulting from current flowing through the device. Most of the practical applications of NTC thermistors are based on these material characteristics.

RTI manufactures precision resistance-temperature SMD thermistors. Standard values from 250 ohms to 150,000 ohms are available. As with RTI's disc and chip thermistors, SMD thermistors are suitable for temperature sensing applications over a wide range of resistance values and temperature coefficients. SMD's operating temperature range is from -55°C to +150°C. Dissipation constant is 2mW/°C, with a maximum power rating of 250mW @ 25°C.

• D.C. - The dissipation constant is the ratio, normally expressed in milliwatts per degree C (mw/°C), at a specified ambient temperature, of a change in power dissipated in a thermistor to the resultant change in body temperature.
  
• T.C. - The thermal time constant is the time required for a thermistor to change 63.2% of the total difference between its initial and final body temperature when subjected to a step function change in temperature under zero-power conditions and is normally expressed in seconds (S).
  
• Alpha () or Temperature Coefficient of Resistance - The temperature coefficient of resistance is the ratio at a specified temperature, T, of the rate of change of zero-power resistance with temperature to the zero-power resistance of the thermistor. The temperature coefficient is commonly expressed in percent per degree C (%/°C).

• Select Req'd. Resistance Value & Temperature Coefficient
• Calculate DEV for R @ T
• Determine Accuracy Req'd.
• Review Power Dissipation
• Review Thermal Time Constant

Most semiconductors and the circuits comprised of them exhibit a positive temperature coefficient. NTC thermistors are well suited for compensating these responses to temperature changes. It is important to match the temperature of the compensating NTC thermistor to that of the component responsible for the temperature response.

RTI's SMD temperature measurement NTC sensors can operate over a wide temperature range (-55 to +150°C). They are stable throughout a long lifetime, and are small and comparatively inexpensive. Typically, they have negative temperature coefficients between -3.3 and -4.7%/°C at 25°C. RTI's SMD style thermistors are used in many applications that require a high degree of accuracy and reliability.

Some of the most popular applications of NTC thermistors include:

• Temperature Compensation
• Temperature Measurement & Control
• LCD Controls
• Power Transistor Stabilization

Selection Considerations for SMD NTC Devices

To determine the nominal resistance value of a thermistor at a specified temperature, multiply its R_T/R₂₅ value for the desired temperature and R-T curve from the table on NTC Resistance/Temperature Conversion Tables by its nominal resistance value at 25°C. As an example, the nominal resistance value at 80°C for a thermistor with the part number KR0805B103K is 10,000 times 0.157, the R_T/R₂₅ value in the R-T Curve "B" in the table under NTC Resistance/Temperature Conversion Tables.

Standard resistance tolerances at 25°C for RTI's SMD thermistors is ±10% and is indicated in its part number by the addition of the suffix K. To determine the resistance value at other than 25°C, add the appropriate DEV value from the NTC resistance/Temperature Conversion Tables to its resistance tolerance at 25°C. For example, the resistance tolerance at 80°C for a thermistor with part number KR0805B103K is ±10% ±3.0%, the DEV value from the R-T Curve "B" Table.

Although standard sizes, resistance values and tolerances are listed on SMD Standard Products and Sizes, custom sizes, resistance values and tolerances are available depending on the application and volume requirements.