1. Self-recovery fuse is an overcurrent electronic protection component, which is made of high molecular organic polymer under the conditions of high pressure, high temperature and vulcanization reaction, and after adding conductive particle materials, it is processed by a special process. The traditional overcurrent protection of the fuse can only be protected once, and it needs to be replaced if it is blown out, while the self-recovery fuse has dual functions of overcurrent and overheat protection and automatic recovery.
Working principle of self-recovery fuse:
When an abnormal overcurrent passes through the self-recovery fuse, the heat generated will expand the high-molecular organic polymer, and the conductive particles wrapped in the high-molecular organic polymer will separate, thereby cutting off the conductive channel of the PTC and making the PTC The resistance rises to reduce the abnormal overcurrent; when the abnormal overcurrent fault is cleared, the high-molecular organic polymer of the PTC shrinks to the original shape and reconnects the conductive particles, the conductive channel is restored, and the PTC resistance returns to the original low-resistance state .
2. Self-recovery fuse selection experience:
(1). Determine the following parameters of the circuit
a, maximum working environment temperature
b, standard working current
c, maximum working voltage (Umax)
d, maximum fault current (Imax)
(2). Select a self-recovery fuse element that can adapt to the maximum ambient temperature and standard operating current of the circuit.
Use the table of temperature reduction {operating current (A) at ambient temperature (°C)} and select the temperature that best matches the maximum ambient temperature of the circuit. Browse the column for a value that is equal to or greater than the standard operating current of the circuit.
(3). Compare the maximum electrical rating of the selected component with the maximum operating voltage and fault current of the circuit
Use the electrical characteristics table to verify that the component you selected in step 2 will adopt the maximum operating voltage and fault current of the circuit . Check the maximum operating voltage and maximum fault current of the device. Make sure that Umax and Imax are greater than or equal to the maximum operating voltage and maximum fault current of the circuit.
(4). Determining the action time
The action time is the amount of time it takes for the element to switch to a high resistance state when a fault current appears on the entire device. It is important to know the operating time of resettable fuse elements in order to provide the intended protection function. If you select a component that moves too quickly, there will be erratic or unwanted movement. If the element acts too slowly, the protected component may be damaged before the element switches to a high resistance state.
Use the typical operating time curve at 25°C to determine whether the operating time of the resettable fuse element is too fast or too slow for the circuit. If yes, return to step 2 to reselect the spare component.
(5). Verify the ambient operating temperature
Make sure that the minimum and maximum ambient temperature of the application is within the operating temperature range of the resettable fuse element. Most resettable fuse elements have an operating temperature range of -40°C to 85°C.
(6). Verify the external dimensions of resettable fuse elements
Use the external dimension table to compare the external dimensions of your selected resettable fuse with the space conditions of the application.
3. Chip resettable fuse (PPTC: polymer resettable fuse) is a polymer thermistor with positive temperature coefficient, which is used for overcurrent protection and can replace the current fuse. When the circuit is working normally, its resistance value is very small (the voltage drop is very small). When the circuit is over-current and its temperature rises, the resistance value increases sharply by several orders of magnitude, so that the current in the circuit is reduced below the safe value. In this way, the following circuits are protected, and the low resistance value is automatically restored after the overcurrent disappears. Its effect is similar to that of a switching element, but with a slower response.
The action principle of the self-recovery fuse is a dynamic balance of energy. The current flowing through the self-recovery fuse generates heat due to the relationship between the fuse. temperature. The temperature during normal operation is low, and the heat generated and dissipated reach a balance. The self-recovery fuse is in a low-resistance state and does not act. When the current flowing through the fuse increases or the ambient temperature rises, but if the balance between the generated heat and the dissipated heat is reached, the self-recovery fuse still does not act. When the current or ambient temperature increases again, the self-recovery fuse will reach a higher temperature. If the current or ambient temperature continues to increase at this time, the heat generated will be greater than the heat dissipated, causing the temperature of the self-recovery fuse to increase sharply. Here At this stage, a small temperature change will cause a substantial increase in the resistance value. At this time, the self-recovery fuse is in a high-impedance protection state. The increase in impedance limits the current, and the current drops sharply in a short period of time, thereby protecting the circuit equipment from damage. As long as the heat generated by the applied voltage is sufficient for the heat dissipated by the resettable fuse, the element can remain in the active state (high resistance) under the changing state. Resettable fuses reset automatically when the applied voltage disappears.
4. According to the packaging structure, PTC resettable fuses can be divided into chip resettable fuses and plug-in resettable fuses. The voltage values that can be achieved by the two are different.
Among the technical indicators of self-recovery fuses, there is a technical indicator Vmaxi, which indicates the maximum voltage that the protector can withstand in the blocking state. That is to say: in the circuit where the fuse is connected in series, when the current of the circuit is abnormal, the fuse will jump from low resistance to high resistance within a certain time range, thereby preventing the flow of abnormally large current and protecting the subsequent circuit. It is not damaged by large current, and the voltage of the circuit is almost all added to the fuse at this time. If the voltage applied to the fuse exceeds Vmaxi at this time, it is easy to cause damage to the fuse and cause permanent damage, making the fuse irrecoverable.
Chip fuses are mainly used in electronic products such as lithium batteries, digital cameras, inverters, LED drivers, notebooks, backlights, LCD driver circuits, power tools, and electric toys. From traditional glass tube fuses to miniature fuses and SMD self-recovery fuses, due to differences in product technology, their selection emphases are also slightly different.
5. What should be paid attention to when using the self-recovery chip fuse?
(1). The selection of chip resettable fuses involves the following factors: ED drivers, notebooks, backlights, LCD driver circuits, electric tools, electric toys and other electronic products. From traditional glass tube fuses to miniature fuses and chip fuses, due to differences in product technology, their selection focuses are also slightly different. Pulse, inrush current, surge current, inrush current and circuit transients. SMD fuses pay special attention to this point. Due to the technical reasons brought by the small size, the impact resistance of SMD fuses is much smaller than that of glass tube fuses with the same rated current or other larger fuses.
(2). The magnitude of the overload current of the circuit and the shortest and longest time that the overload current exists. It is usually necessary to use the combination of oscilloscope test and theoretical calculation to judge the magnitude of the overload current. The basic requirement for a fuse is that it cannot be broken when it should not be broken (such as when a surge current occurs), and it must be broken within an appropriate time when it should be broken (such as when an overload current that needs to be cut off occurs).
(3). The ambient temperature of the fuse. When the chip fuse is applied to portable equipment, the temperature rise of the fuse should be properly considered, that is, the reduction of the rated current of the fuse should be considered. The ambient temperature when the fuse is working should be within the specified working temperature range. When the ambient temperature around the fuse exceeds 25°C, it should be degraded according to the temperature reduction curve.
(4). The external voltage applied to the fuse. Usually chip resettable fuses are used in portable equipment, and the circuit operating voltage is generally not high. As long as the rated voltage of the chip resettable fuse is higher than the circuit operating voltage, you can choose it with confidence.
(5). Product certification. For example, exporting to North America must have UL or CSA certification. Products currently exported to Europe also need to comply with the EU's RoHS directive, which is commonly referred to as SGS and other environmental certification.
6. Relevant characteristics of self-recovery fuses:
(1). Sensitive to current and temperature, the resistance increases with the increase of temperature and current; (
2). The response speed is slow, usually tens of milliseconds or even seconds, which is related to the magnitude of the current ;
(3) Self-recovery characteristics, reusable within the rated range;
(4). In the normal state of the circuit, the PTC is in a low-resistance state, which does not affect the circuit; (but there is a certain voltage drop)
(5). When applied in series in the circuit;
Detailed explanation of self-recovery fuse parameters:
holding current (Ihold): the highest current that does not trigger resistance breakthrough in a still air environment at 25°C.
Trigger current (Itrip): The minimum current at which the PTC polymer resettable fuse turns from low impedance to high impedance in a still air environment at 25°C.
Maximum voltage (Vmax): The maximum working voltage of the PTC polymer resettable fuse.
Maximum current (Imax): The maximum current that the PTC polymer resettable fuse can withstand.
Operating power (Pdtyp.): The power consumption of the PTC polymer resettable fuse in the operating state at an ambient temperature of 25°C.
Operating time (Ttrip): The maximum operating time at 5 times the holding current.
Selection Guide for Resettable Fuses:
Resettable fuses are widely used in overcurrent protection for power lines, communication lines and I/O ports of communication, security, industrial, automotive, consumer and other electronic products. So, in the actual circuit protection, how to choose a suitable type of resettable fuse to achieve the best protection effect?
第1步：确定被保护电路正常工作的标准工作电流、最大工作电压、最大故障电流、最大工作环境温度、动作时间等参数；
第2步：根据被保护电路或产品的特点，确定是插件PTC还是贴片PTC；
第3步：根据最大工作电压选出耐压等级大于等于最大工作电压的PTC产品
第4步：根据电路中最大环境温度和标准工作电流，对照PTC温度折减率选出合适的产品规格；
第5步：根据该型号PTC的动作时间曲线图确认选出的产品是否符合要求动作保护时间；
第6步：根据PTC规格书中的数据，确定尺寸要求；
在实际过程之中，很多被保护的电路要求极其复杂，PTC具体型号的选取，还是要根据实验测试后最终做定夺。