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NTC Thermistors

General

The NTC thermistor is a temperature dependent semiconductor resistor according to DIN 44070 or IEC 60593, whose resistance value decreases with increasing temperature. Negative Temperature Coefficient (NTC) is about -2 to -6% per Kelvin and thus about ten times larger than for metals. NTC thermistors are therefore well suited for temperature measurements. 

Thermistors consist of oxides and doping supplements. These are processed into a powder mass and, after the addition of a binding agent, sintered at temperatures above 1000°C. Thereafter, the ceramic elements are contacted with metal and cured by means of a special heat treatment to enhance long term stability.

Application

The change of resistance in operation can occur by temperature change in the environment (external heating) or by self-heating due to excessive measuring current. Therefore, it is very important to comply with the specified maximum performance of the thermistor. While monitoring of a specified temperature is firmly specified by Nominal Response Temperature NRT of PTC thermistors,  the resistance of a NTC has to be measured as a non-linear characteristic at different temperatures.

Functional principle

Thermistors (NTC) are mainly used to monitor resistance changes over a wide temperature range. The characteristic of a thermistor is showing a  non-linear behaviour compared to platinum sensors (as Pt100) due to the temperature dependency of the resistance. 

For correct measurement one has to consider the tolerance in the base resistance value as well as the tolerance in the curve slope (so-called B-value). Further it is essential to note that , that NTC thermistors from different manufacturers or of different types cannot be compared or exchanged due to  different ceramic compositions, different production process conditions and due to differences in the non-linear characteristics from element to element.

Instruction

Because of the flexibility in their design NTC thermistors can be put to use in a wide range of environments with demanding design requirements. New miniature thermistors can be used in monitoring points only 1.5 mm wide. 
Therefore a variety of possible types, configurations and different characteristics are available.  For cost-efficiency reasons in production NTC thermistors will be manufactured only in quantities of at least 500 units.
For this reason, EPHY-MESS will need to know your technical requirements and quantities for your up-coming projects in order to prepare an individual and cost-effective solution.  

The following information is for required:

  • operating temperature range
  • basic resistance value +25°C (in general)
  • resistance tolerance
  •  mechanical requirements on design or size

 

Based on this information, EPHY-MESS will create a characteristic curve (R/T) produced in its own measurement laboratory, so that you can view the nominal resistance values in advance. For motor protection applications, e.g. the wired type K227 with resistance values predominantly of 1.8 kΩ at 100°C or alternatively 10 kΩ at 100°C are used. In addition to the shrinkable tube version with Teflon connection wires other assembling options in metal or ceramic sleeves are available on request.

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