Daily Archives: January 12, 2023

What is Thermorite?

Kemet has developed and patented a type of ferrite material that is sensitive to temperature. They use this material to make various types of thermal sensors. The electronic industry uses thermal sensors and switches for monitoring temperature and maintaining various applications in stable modes of operation. A reed switch inside the thermal sensor makes or breaks the current flow.

Kemet offers three basic types of thermal sensors. The first type is a Kemet sensor using a reed switch inside a thermos-ferrite body. The second type is a thermal sensor using a bi-metal switch. The third type is thermistors.

The Kemet thermos-ferrite and reed switch combination works by sensing the temperature of the surrounding environment. The thermos-ferrite has a curie point and as the temperature crosses this curie point, the magnetic reed switch switches on or off.

The bi-metal switch contains two metal plates that have different thermal expansion coefficients. As the temperature changes, the two metals deform and disconnect at a particular temperature.

Thermistors are semiconductor sensors and may have a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). They change their resistance in accordance with changes in temperature.

Thermorite is a ferromagnetic material with soft magnetic characteristics when under curie temperature. As the temperature goes up, the saturated magnetic flux density of the material decreases, and the material becomes paramagnetic. That is, the material loses its magnetic property rapidly as its temperature reaches the curie point, becoming close to zero. Time does not affect curie temperature as it is a function of the compounding ratio of the material. That means Kemet can make Thermorites with different curie points by changing the material’s permeability.

Kemet offers two types of thermos-ferrite thermal sensors—the Break type and the Make type. The Break type thermal sensors consist of a reed switch surrounded with a Thermorite jacket around the switch part and two permanent magnet cylinders on each end. The Break type switch remains ON when the operating temperature is lower than the trigger temperature. The switch opens as the temperature rises and reaches the trigger temperature or crosses it. When the temperature goes down, the switch closes again only when the temperature goes below the recovery temperature.

When the temperature is below the trigger temperature, the Thermorite jacket is magnetic, and it generates an annular magnetic field. The magnetic field induces the N tip and the S tip of the reed switch to touch due to magnetic attraction. This turns the switch to its ON position. As the temperature rises and reaches the trigger temperature, the Thermorite jacket loses its magnetic flux. This allows the tips of the reed switch to pull apart, and the switch turns OFF.

Construction of the Make type thermal switch is similar to the Break type. The only difference is the former has two Thermorite jackets with a gap in between. The two Thermorite jackets create annular magnetic fields when the temperature is below curie point. This causes the reed switch poles to stay apart, and the switch remains OFF. As the temperature rises to the curie point, the Thermorite jackets lose their magnetic flux. This allows the two reed switch tips to close, and the switch turns ON.