Thermister

A thermister is a Capteur of temperature liability. It consists of a material Semi-conducteur. Its resistance varies according to the Température.

Types of thermisters

One distinguishes two types of thermisters: CTN and CTP.

CTN

The CTN (Negative Temperature coefficient, in English NTC, Negative Temperature Coefficient ) are thermisters whose resistance decreases in a uniform way with the temperature.

When the Joule effect (heating due to the passage of the current) is negligible, it is noted that the resistance of the CTN varies with the Température according to the law:

$R\; (T)\; =\; R\; (T\_0)\; \backslash ,\; e^\; \{(\backslash \; frac\; \{\backslash \; alpha\}\; \{100\}\; \backslash \; cdot\; (T\_0\; +\; 273.15)\; ^2\; \backslash \; cdot\; \backslash \; left\; (\backslash \; frac\; \{1\}\; \{T+273.15\}\; -\; \backslash \; frac\; \{1\}\; \{T\_0+273.15\}\; \backslash \; right))\}$

where:

• $R\; (T)$ is the value of the strength of the sensor to the temperature $T$;
• $R\; (T\_0)$ is the value of the strength of the sensor to the temperature of reference $T\_0$;
• the temperatures $T$ and $T\_0$ are expressed out of C (Celsius);
• $\backslash \; alpha$ and $T\_0$ is constant characteristics of the component.

The CTN are manufactured containing metal oxides of transition (manganese, cobalt, copper and nickel). These oxides are semiconductors.

The CTN can be used in a broad beach of temperatures, of -200°C with + 1000°C, and they are available in various versions: glass shots, discs, bars, pastilles, discs, chips etc nominal resistances go from a few ohms to a hundred kohms. The response time depends on the volume of material used.

The CTN are used for measurements and the control of the temperature, the suppression of transitory impulses, the measurement of flow of liquids.

CTP

The CTP (Positive Temperature coefficient, in English PTC, Positive Temperature Coefficient ) are thermisters whose resistance strongly increases with the temperature in a beach of temperature limited (typically between 0 and 100°C), but decreases apart from this zone.

The CTP are manufactured containing titanate of Baryum. They are like CTN , available in various alternatives and values.

The CTP can be used:

• Like temperature sensor, to protect from the components (Driving S, Transformer S) against an excessive rise in the temperature;
• Like protection against overcurrents;
• Like detector of level of liquid: the temperature of the CTP and thus its resistance, will be different when the sensor is in the air or plunged in a liquid.

The Thermisters has negative temperature coefficient (CTN) are resistance whose resistance has null dissipation (without passage of current), decreases when the temperature increases. The temperature Varies in function:

• of the passage of a current in the element;
• of the variation of the room temperature;
• of the combination of these two means

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