NTC Thermistors – Disc and Chip Style

Temperature Measurement and Control Thermistors

DISC & CHIP NTC
STYLE NTC THERMISTOR
Features

• Wide Ohmic Value Range
• Accurate & Stable
• Fast Thermal
  Response Time
• Tight Tolerances
• High Sensitivity

NTC Thermistors

NTC DISC & CHIP Style Devices

Thermistor Terminology for Temperature Measurement & Control Devices

• 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 or 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).

NTC DISC & CHIP
Selection
Considerations

• Select Req’d. Resistance Value & Temperature Coefficient
• Determine Accuracy Req’d.
• Review Power Dissipation
• Determine Operating Temperature Range
• Review Thermal Time Constant

Thermistor Applications

• Temperature Compensation
• Temperature Measurement & Control
• Fan Motor Control
• Fluid Level & Temperature Sensors

 NTC DISC & CHIP – Selection Process

• Select R Value
• Determine R @ T
• Calculate DEV for R @ T
• Evaluate Power Rating (D.C.)
• Review T.C. Requirements

Selection considerations for NTC DISC & CHIP Devices

• If the power dissipated exceeds the dissipation constant (D.C.) rating of the sensor it is likely that it will exhibit self heating.
• Most thermistors dissipate from 1 to 25 mW/°C nominal. This means that the resistance changes by an equivalent of 1°C for each D.C. rating (mW/°C) for the selected device.
• To maintain a higher degree of accuracy, temperature error caused by self-heating should be an order of magnitude less than the required sensor accuracy. For many applications, this degree of accuracy is not required and a less stringent de-rating may be adequate.
• Several options to reduce the thermistor power are to increase the thermistor resistance, lower the source voltage and/or increase the series resistor in the divider circuit.

• if the D.C. of the thermistor selected is 5 mW/°C and the power dissipated by the device is 20 mW/°C, then a 4°C error is induced due to the effect of self-heating.
• To minimize this effect, a factor can be derived simply by taking the DC rating times 10-1(one order of magnitude lower) and use it in the power equation to produce a good approximation of the maximum allowable power.
• For instance, if the desired accuracy is 1°C, and the rated D.C. of the device selected is 5 mW/°C, adjusting the specified D.C. rating in the power equation to 0.5 mW/°C compensates for self-heating error and effectively predicts the maximum power the device can dissipate without significantly affecting the desired accuracy.
• The resulting maximum power that should be applied would be calculated as 1°C*0.5mW/°C = 0.5mW.

NTC Standard Disc Thermistor Specifications