Principle of thermal fuse

A thermal fuse or thermal cutoff is a safety device which open circuits against overheat. It detects the heat caused by the over-current due to short circuit or component breakdown. Thermal fuses do not reset themselves when the temperature drops like a circuit breaker would. A thermal fuse must be replaced when it fails or is triggered.
Unlike electrical fuses or circuit breakers, thermal fuses only react to excessive temperature, not excessive current, unless the excessive current is sufficient to cause the thermal fuse itself to heat up to the trigger temperature.We will take thermal fuse as an example to introduce its main function, working principle and selection method in practical application.
1. The function of thermal fuse
The thermal fuse is mainly composed of fusant, melting tube and external filler. When in use, the thermal fuse can sense the abnormal temperature rise of electronic products, and the temperature is sensed through the main body of the thermal fuse and the wire. When the temperature reaches the melting point of the melt, the fusant will automatically melt. The surface tension of the melted fusant is enhanced under the promotion of special fillers, and the fusant becomes spherical after melting, thereby cutting off the circuit to avoid fire. Ensure the safe operation of electrical equipment connected to the circuit.
2. Working principle of thermal fuse
As a special device for overheating protection, thermal fuses can be further divided into organic thermal fuses and alloy thermal fuses.
Among them, organic thermal fuse is composed of movable contact,fusant, and spring.Before the organic type thermal fuse is activated, current flows from one lead through the movable contact and through the metal casing to the other lead. When the external temperature reaches the preset limit temperature, the fusant of the organic matter will melt, causing the compression spring device to become loose, and the expansion of the spring will cause the movable contact and one side lead to separate from each other, and the circuit is in an open state,then cut off the connection current between the movable contact and the side lead to achieve the purpose of fusing.
Alloy type thermal fuse consists of wire, fusant, special mixture, shell and sealing resin. As the surrounding (ambient) temperature rises, the special mixture begins to liquefy. When the surrounding temperature continues to rise and reaches the melting point of the fusant, the fusant begins to melt, and the surface of the melted alloy produces tension due to the promotion of the special mixture,using this surface tension, the melted thermal element is pilled and separated to both sides, to achieve a permanent circuit cut. Fusible alloy thermal fuses are capable of setting various operating temperatures according to the fusant of the composition.
3. How to select thermal fuse
(1) The rated working temperature of the selected thermal fuse should be less than the temperature resistance grade of the material used for electrical equipment.
(2) The rated current of the selected thermal fuse should be ≥ the maximum working current of the protected equipment or components/current after reduction rate. Assuming that the working current of a circuit is 1.5A, the rated current of the selected thermal fuse should reach 1.5/0.72, that is, more than 2.0A, to ensure the reliability of the thermal fuse fusing performance.
(3) The rated current of the selected thermal fuse’s fusant should avoid the peak current of the protected equipment or components. Only by satisfying this selection principle can it is ensured that the thermal fuse will not have a fusing reaction when a normal peak current occurs in the circuit.In particular, if the motor in the applied circuit system needs to be started frequently or braking protection is required, the rated current of the fusant of the selected thermal fuse should be increased by 1 ~ 2 levels on the basis of avoiding the peak current of the protected device or component.
(4) The fusant’s rated voltage of the selected thermal fuse shall be greater than the actual circuit voltage.
(5) The voltage drop of the selected thermal fuse shall conform to the technical requirements of the applied circuit.This principle can be ignored in high voltage circuits, but for low voltage circuits, the influence of voltage drop on fuse performance must be fully evaluated when selecting thermal fuses because voltage drop will directly affect circuit operation.
(6) The shape of the thermal fuse should be selected according to the shape of the protected device. For example, the protected device is a motor, which is generally annular in shape,the tubular thermal fuse is usually selected and inserted directly into the gap of the coil to save space and achieve a good temperature sensing effect.For another example, if the device to be protected is a transformer, and its coil is a plane, a square thermal fuse should be selected, which can ensure better contact between the thermal fuse and the coil, so as to achieve a better protection effect.
4. Precautions for using thermal fuses
(1) There are clear regulations and limitations for thermal fuses in terms of rated current, rated voltage, operating temperature, fusing temperature, maximum temperature and other related parameters, which need to be flexibly selected under the premise of meeting the above requirements.
(2) Special attention must be paid to the selection of the installation position of the thermal fuse, that is, the stress of the thermal fuse should not be transferred to the fuse due to the influence of the position change of the key parts in the finished product or vibration factors, so as to avoid adverse effects on the overall operation performance.
(3) In the actual operation of the thermal fuse, it is necessary to install it in the case that the temperature is still lower than the maximum allowable temperature after the fuse is broken.
(4) The installation position of the thermal fuse is not in the instrument or equipment with humidity higher than 95.0%.
(5) In terms of installation position,the thermal fuse should be installed in a place with good induction effect.In terms of installation structure, the influence of thermal barriers should be avoided as much as possible,For example, it shall not be directly connected and installed with the heater, so as not to transfer the temperature of the hot wire to the fuse under the influence of heating.
(6) If the thermal fuse is connected in parallel or is continuously affected by overvoltage and overcurrent factors, the abnormal amount of internal current may cause damage to the internal contacts and adversely affect the normal operation of the entire thermal fuse device. Therefore, the use of this type of fuse device is not recommended under the above conditions.
Although the thermal fuse has high reliability in design, the abnormal situation that a single thermal fuse can cope with is limited,then the circuit cannot be cut off in time when the machine is abnormal.Therefore, use two or more thermal fuses with different fusing temperatures when the machine is overheated, when a faulty operation directly affects the human body, when there is no circuit cutting device other than a fuse, and when a high degree of safety is required.