Temperature Sensing Type

Reed switches in deep freezers — Temperature Sensing Type Application in Deep Freezer

Normally, Reed Switches are used in collaboration with magnets in all the other types of applications. A Thermal Reed Switch is not actuated with a magnet but is constructed in such a way that it is used for temperature sensing. When a Thermal Reed Switch is used in an application to check if a set temperate as been met, such applications can be classified as Temperature Sensing Applications.

Normally, temperature is sensed using a Thermocouple, which uses two dissimilar metals to detect a change in temperature by means of a potential difference produced at the output. This needs to be electronically compensated to allow for more accurate readings. For this, additional calibrated controllers are required, which monitor the temperature and then trigger outputs at set the sensed temperature. This is not only expensive, but is not required for simple temperature sensing applications.

Simpler temperature cut-outs such as bimetallic switches are also constructed using two dissimilar metals that undergo differential expansion and break contact at the sensed temperature. The disadvantage is that depending on the control circuit current, the set point also varies. These types of switches, being mechanical in nature, wear out and cause changes in the sensing temperature. They are also prone to corrosion. Thermistors, on the other hand, do have a fast response time, but the power lines require shielding, and their resistance-temperature characteristics are not linear. Also, if the control circuit current is not low, it tends to heat up the device, and alters the temperature sensing point.

The Thermal Reed Switch overcomes all these problems. It consists of a reed switch, two permanent magnet, and a special alloy that conducts the magnetic flux lines between the two permanent magnets, at temperatures lower than the Thermal Ferrite’s Curie Point. This keeps the reed switch closed. When the ambient temperature crosses the Thermal Ferrite’s Curie Point, the flux no longer passes between the exterior magnets, and the reed switch contact opens. This is a highly repeatable temperature cut-out that works for the rated wattage of the reed switch which also means that thermal reed switches can be constructed for various loads.

Thermal reed switches find applications in over-heat protection and in precise temperature switching. Thermal reed switches are also available in cut-in cut-out differentials ranging from 3°C to 8°C. This wider temperature differential helps cooling fan circuits in over-heating applications to get turned off after sometime, and the narrow temperature differential helps in triggering alarms which can go off as soon as the temperature drops by 2-3 degrees.

Thermal reed switches can also sense sub-zero temperatures as low as -30°C and can also used to maintain the temperature of a bath for example, at a set temperature. Functionality of Level sensors can also be improved by adding a temperature cut-out. Needless to say, that Thermal Reed Switch can replace expensive thermistor, bimetallic switches at a fraction of the cost. Some examples of Temperature sensing applications are listed below.