Household appliances, referred to as household appliances, refer to appliances driven by electric energy (or mechanically operated), which can help perform household chores, such as cooking, food preservation or cleaning. In addition to the family environment, they can also be used in the company or industrial environment. Basically, household appliances are divided into large household appliances (white household appliances, black household appliances) and small household appliances. With the development of the electronic information industry and the popularity of household appliances, the development of refrigerators, washing machines, tape recorders, black-and-white televisions, household air conditioners, telephones, energy-saving lamps, VCDs, DVDs, computers, program-controlled exchanges, etc. has improved the quality of life of every Chinese family since the localization of color televisions in the 1980s. There are more and more requirements for the electronic industry, in order to meet the needs of the development of household appliances in terms of quantity, quality and service. Electronic components have also developed rapidly in the household industry, including fuses, self-healing fuses, disposable fuses, plug-in fuses, chip fuses, chip capacitors, high-voltage chip capacitors, chip resistors, chip inductors, tantalum capacitors, precision resistors, chip aluminum electrolytic capacitors, zener diodes, rectifier diodes, light emitting diodes and other electronic components.
Self recovery fuse
The self recovery fuse is also known as the self reset fuse, which can be automatically restored to use again after troubleshooting after being blown.
Self recovery fuse (PPTC polymer positive temperature coefficient) is a national six and over temperature protection element. The PPTC element can limit the flow of dangerous large current under fault conditions, but it is different from the traditional one-time fuse that can only be used once and must be replaced. The PPTC element can be automatically restored to use after troubleshooting, thus reducing the after-sales cost.
The PPTC self recovery fuse is used in series in the circuit. When the current increases rapidly, the PPTC self recovery fuse changes from the low resistance state to the high resistance state to protect the circuit. This process is called "action". The resistance of PPTC fuse in normal working state is far higher than that of other components in Xiao Yuyu resistance. However, in the case of overcurrent, the resistance of the original will increase (ACT), thereby reducing the current in the circuit to protect other original. This process involves: ² The principle of R is to heat the inside of the original, so that the temperature of the original rises rapidly.
Action principle
The action principle of PPTC fuse is a kind of energy balance. In normal use, the heat generated by the original and radiated to the environment will form a balance under relatively low temperature.
When the external temperature remains relatively balanced, the current flowing through the PPTC fuse increases, and the temperature of the fuse will also increase. When the current or ambient temperature continues to increase, the PPTC fuse element will reach a higher temperature. If the temperature increases at this time, the thermal energy generated will be greater than the thermal energy emitted. Yes, the temperature of the PPTC fuse element continues to increase,
At this stage, a small temperature change will cause a large increase in resistance, which is the normal operation process area of PPTC components. A large change in resistance causes a corresponding reduction in the current in the circuit. This relationship will continue until the element resistance reaches a higher value of the curve. As long as the supply voltage is stable, the element will remain in operation. Once the voltage is reduced to a certain value or the power supply is disconnected, the component will be restored and can continue to be used.
Basic principle of PPTC self recovery fuse
Temperature reduction table corresponding to operating and non operating current
It can be seen that the operating current and the operating current of the PPTC element change with temperature. Each available element can be divided into 3 partial areas. Zone A is the operating state (high resistance) area of PPTC fuse element. Zone B is the area where the PPTC element maintains the normal working state of the circuit. The C region is in the possible action or non action region (depending on the resistance of the element).
Current action time curve
The current action time characteristic curve of PPTC element in still air at 0 ℃ and 75 ℃. These two curves are different because the thermal energy of the original action comes from I ² R heat or ambient temperature. The temperature from the outside at 75 ℃ is much higher than that at 0 ℃, so the additional I required for the action of the self recovery fuse ² R heat is relatively reduced, resulting in a relatively small action current within the same action time (or a reduction in action time under the same action current)
Recovery characteristics of fuse resistance after action
The graph shows the relationship between recovery time and resistance after PPTC fuse action. In the figure, we can see that even after several hours, the resistance of the element is still greater than the initial resistance, and the resistance reduction process will continue for a long time, and finally the resistance will approach the initial value.
However, this time may be several days or several years, but it is possible to expect the PPTC self recovery fuse to recover to the initial value after operation. Therefore, when selecting the PPTC recoverable fuse, the value of r1max must be considered when determining the holding current, that is, the maximum resistance value after 1 hour of action recovery.
Selection guide of recoverable fuse
1、 Determine current parameters
1. Maximum operating ambient temperature
2. Standard service current
3. Maximum operating voltage
4. Maximum interrupting current
2、 Select the PPTC original that can adapt to the maximum ambient temperature and standard operating current of the circuit: select the maximum operating ambient temperature of the circuit, then calculate the reduction rate according to the temperature current reduction table and select the model that is equal to or greater than the standard operating current value of the circuit.
3、 Compare the maximum electrical rating of the selected components with the maximum operating voltage and interrupting current of the circuit: use the electrical characteristics table to verify whether the selected components in the second step will use the maximum operating voltage and interrupting current of the circuit. Referring to the maximum operating voltage (Vmax) and the maximum interrupting current (IMAX) of the element, it is ensured that Vmax and IMAX are greater than or equal to the maximum operating voltage and the maximum interrupting current in the circuit.
4、 Determine the action time: the action time is the time when the PPTC fuse switches to the high resistance state when the fault current occurs. In order to achieve the expected protection effect, it is necessary to specify the action time of PPTC components. If an element that operates too fast is selected, it is easy to cause malfunction or damage. If the component moves too slowly, the protected component may be damaged before the component switches to the high resistance state.
5、 Verify the ambient operating temperature: ensure that the minimum and maximum ambient temperature of the application is within the service temperature range of the PPTC recoverable fuse. The temperature range of most PPTC components is - 40 ℃ ~ 85 ℃, and can reach 125 ℃ under some special conditions.
6、 Verify the outline dimension of PPTC recoverable fuse: use the outline dimension table to compare the outline dimension of the PPTC component you selected with the space price adjustment of the application scenario.
