What Is a Thermal Cutoff?
Thermal cutoffs are components that detect abnormal heat generation in electronic equipment caused by internal failures or errors, and protect against overheating by melting themselves to cut off the current in the electronic circuit.
Thermal cutoffs generate almost no heat themselves, but rather they melt when the ambient temperature rises, thereby interrupting the current flow. When a thermal cutoff detects a temperature abnormality, it immediately interrupts the circuit current.
This can prevent smoke and fire from occurring in household equipment and automobiles. Once thermal cutoffs detect abnormal heat generation and shut down, they do not automatically resume conduction even if the ambient temperature decreases.
Applications of Thermal Cutoffs
Thermal cutoffs are used to prevent hazardous conditions when the object to be measured becomes hot due to abnormal heat generation, etc., because they do not automatically return to normal operation once they detect a blown thermal cutoff. Specific applications are as follows:
- Large Household Appliances
Air conditioners, refrigerators, washing machines, fan heaters, hot-water washing toilet seats, gas water heaters, etc. - Small Home Appliances
Coffee makers, electric kettles, home bakeries, rice cookers, hot plates, electric stoves, irons, hair dryers, humidifiers, etc. - Office Equipment
Copy machines, printers, fax machines, etc. - Automobiles
Car air conditioners, seat heaters, engine cleaners, etc.
In the automotive field, thermal cutoffs are used to protect the surface of coils in transformers and motors that may become hot due to circuit malfunctions, and in resistors used to prevent inrush currents in power circuits.
However, it should be noted that once a thermal cutoff blows, it does not automatically recover, so the electronic equipment cannot be used again.
Principle of Thermal Cutoffs
There are two types of thermal cutoffs: fusible alloy-types using a fusible alloy as the thermosensing element and pellet types using a thermosensing element.
1. Fusible Alloy Type
When the thermal cutoff reaches the melting point of the low-melting-point alloy, the alloy changes from a solid to a liquid. The fusible alloy that has become liquid separates into two spheres, thus interrupting the fuse‘s conduction path.
2. Thermosensitive Pellet Type
When the ambient temperature of the fuse rises, the temperature-sensitive pellet melts and liquefies, forcing a distance between the contact electrode and the lead wire, thereby interrupting the fuse’s conduction path.
Most fusible alloy-type thermal cutoffs have a rated current from 0.5A to several A. Most temperature-sensitive pellet-type thermal cutoffs have a rated current from several A to 10A.
Other Information on Thermal Cutoffs
1. Nominal Operating Temperature
Nominal operating temperature is the temperature at which thermal cutoffs will blow when measured by the method defined in safety standards.
International IEC standards specify an error of plus or minus 0°C, plus or minus 10°C. In most cases, the nominal operating temperature is indicated on the body of the thermal cutoffs.
2. Holding Temperature
The ambient temperature at which thermal cutoffs can withstand a minimum of 168 hours without melting while the rated current continues to flow through them. Thermal cutoffs should be selected based on the current and life expectancy of the equipment in which they will be used.
3. Operating Temperature Limit
The maximum temperature at which a thermal cutoff will not conduct again after it has blown. If a thermal cutoff continues to be used when the ambient temperature is above the operating temperature limit, the thermal cutoff may be destroyed.
4. Shape
There are two types of thermal cutoffs: axial and radial. Axial components have leads coming out from both ends of the component and are supplied with both ends of the leads taped. Radial components have leads coming out from one side of the component and are supplied with the leads coming out in one direction taped together.