DISC STYLE
Feaures
  • Wide Ohmic
    Value Range
  • Fast Thermal Response Time
  • Range of Transition/
    Switching
    Temperature
  • High Sensitivity
   STANDARD PTC THERMISTORS

Positive Temperature Coefficient (PTC) thermistors are thermally sensitive semiconductor resistors which exhibit an increase in resistance at a specified temperature. Change in the resistance of a PTC 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 PTC thermistors are based on these material characteristics.

PTC DISC STYLE DEVICES

RTI Electronics manufactures and distributes disc style thermistors in resistance values ranging form 1.0 to 1500 ohms @25°C, and switching temperatures (Ts) of 40 to 120°C. The maximum operating voltage (Vmax) available is 350V DC.

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).
APPLICATIONS

The applications of standard PTC thermistors can be classified in two main groups:

  1. Applications where the temperature of the PTC is primarily determined by the temperature of the surrounding environment. The first group includes applications such as temperature measurement, temperature control, temperature compensation and over-temperature protection.
  2. Applications where the temperature of the PTC is primarily determined by the electrical power dissipated by the device. The second group includes applications such as over-current protection, liquid level detection and time delay
 
Some of the more popular applications
of PTC Thermistors include:
  • Over-Temperature Protection
  • Temperature CompensationArc Suppression
  • Time Delay
  • Liquid Level Sensing
  • Air Flow Sensing
  • Automatic Color TV Degaussing
  • Non-Destructive Fusing
  • Self Regulating Heaters
  • Single Phase Motor Starting
Other
Considerations:
  • Voltage
  • Review Thermal time constant
  • Determine Operating temperature range
  

NOTE: Do not apply voltage exceeding Vmax to the PTC device. Doing so may destroy the thermistor. Although several PTC's may be connected in series for temperature sensing applications, do not connect PTC thermistors in series to obtain higher voltage ratings. Since no two devices are exactly the same, one would tend to heat faster than the others thereby limiting the current flow through the other devices and resulting in the entire voltage available being dropped across the single device.

PTC's may, however, be connected in parallel to increase the current ratings in current limiting applications. RTI Electronics' engineers specialize in the development of state-of-the-art devices for the most demanding applications. If you have a unique application which requires a part not listed in this catalog please feel free to contact our applications engineering department. RTI Electronics is capable of producing many custom PTC thermistor designs to meet your critical demands.

Selection Considerations for PTC Thermistor Disc Devices

The most important considerations necessary when selecting standard PTC thermistors are resistance at 25ºC as well as the transition temperature. 

Base Resistance (R@25°C) - The first consideration in the selection of a standard PTC disc PTC disc thermistor is the nominal Base Resistance value at 25°C (R@25°C). The available values are tabulated in the Standard PTC Thermistor Specifications table.

Switch Temperature (Ts) - The second consideration is to determine the required Switch Temperature (Ts) (sometimes alternatively identified as the Transition Temperatures or Curie Point). RTI's standard PTC disc thermistors are available with Switch Temperatures ranging from 40°C to 120°C. The Switch Temperatures available for each Base Resistance value are show in the Standard PTC Thermistor Specifications table.

Maximum Operating Voltage (Vmax) - Next calculate or estimate the voltage that will be across the tentatively selected thermistor when the device is switched into its high resistance mode (the thermistor's resistance in its high resistance mode will typically be greater by two to three orders of magnitude than its R@25°C value). If the voltage estimated is greater than the Vmax rating of the selected part then another selection with a higher Vmax rating must be made.

Note: Consult RTI's Application Engineering Department for technical assistance in selecting devices with requirements that cannot be found in the Standard PTC Thermistor Specification table.

 

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