In our audio amplifier, in order to achieve good performance indicators, we often use high-power power transformers and multiple large-capacity filter capacitors. Before starting up, these capacitors called "big ponds" are basically It is empty, the electricity stored in the capacitor is zero, and the voltage at both ends of the capacitor is also zero. We know that the voltage between the two poles of the capacitor can not be mutated, and the voltage at both ends of the capacitor is gradually increased from zero to the quota voltage when charging. At the moment of startup, the full voltage output from the transformer is rectified by the rectifier tube and added to both ends of the zero voltage capacitor. At this moment, the charging current output from the secondary coil of the transformer is extremely large, almost short-circuit current. In addition, the power amplifier uses a high-power transformer, and the excitation current at the moment of startup is also very large. These factors often cause the power overload fuse to blow at the moment of startup. In order to solve this problem, we usually add a start-up delay circuit in front of the transformer. At the moment of startup, several thermistors of more than ten ohms are connected in series on the primary incoming line of the transformer to limit the overload current caused by the charging of the large pond during startup. After the startup impact overload phenomenon has passed, the thermistors are short-circuited through the relay contacts that are turned on with delay, so that the AC mains power is directly connected to the primary of the transformer and normal power supply is restored.

Because the current overload is the largest at the moment of starting up in the cold state, it is required that the resistance value of the thermistor connected in series should be the largest at the cold state. With the power-on heating, the temperature increases, but the resistance value decreases, so that the secondary output voltage of the transformer automatically moves closer to the rated value after the moment of starting up, gradually reducing the limit on capacitor charging and speeding up the charging process. At the end of the delay, the relay operates, the thermistor is short-circuited by the contact, and when the power supply returns to normal full voltage power supply, the pond has been charged to a certain extent, and the normal full voltage power supply will not cause large impact current again. In this way, the phenomenon that the power fuse is blown at the moment of starting up is avoided, and we can choose the appropriate smaller fuse to provide sensitive and reliable fuse protection for the normal operation of the power amplifier. Based on the above analysis, we can know that NTC thermistor with negative temperature coefficient should be used in this startup delay circuit (the temperature increases and the resistance decreases.) This is different from the TV degaussing circuit. We must analyze specific problems and correctly select thermistors. Do not scratch the beard and eyebrows. If the thermistor is used incorrectly, it will be counterproductive. Because there are many filter capacitors with super large capacity behind the audio power transformer, NTC thermistors produced by general manufacturers cannot meet the above capacitor charging requirements, and often blow up as soon as the thermistor is powered on. In order to solve this problem, several thermistors have to be connected in parallel to charge the capacitor. Although this can solve the problem that the thermistor is damaged as soon as the thermistor is powered on, there are great disadvantages, this is mainly caused by the unbalanced resistance of the power NTC thermistor (note: the allowable deviation of R25 for the accuracy of the power NTC thermistor is generally 20%, for example, the minimum MF72-10D20 resistance is R25 = 8 ohms and the maximum value is R25 = 12 ohms with a difference of 4 ohms and the maximum value is 50% larger than the minimum value). according to ohm's law, the current with a small resistance is more powerful, that the thermistor temperature is higher, because the negative temperature effect of NTC thermistor leads to the smaller resistance value of small resistance value, the more unbalanced the current will be, which will eventually lead to the damage of NTC thermistor with smaller resistance value. When the NTC thermistor with smaller resistance value is damaged, the current will be concentrated in the thermistor with larger resistance value, thus causing the NTC thermistor in parallel to be damaged one by one from small resistance value to large resistance value due to the imbalance of resistance value, this damage occurs almost instantaneously or in a short period of time.

Huagu Electronics is a professional manufacturer of thermistors. In the past, many customers used multiple NTCs in parallel to suppress the surge current, but this problem has not been solved. In view of the above phenomenon, Huagu Electronics has jointly developed a new generation of surge-suppressing power NTC thermistor with several well-known domestic universities and research institutes, such as Nanjing Southeast University and University of Science and Technology, Nanjing University, Wuhan University of Science and Technology, Nanjing University, Southeast University, Shanghai Jiaotong University and so on, A new generation of anti-surge NTC thermistor produced by new technology and nanomaterials in production has the characteristics of large steady-state current and strong surge current suppression. At present, the maximum products of 1 euro 32A,1 euro 30A,2 euro 23A,2 euro 25A,20 euro 10A, etc. (for details, please ask huaju electronics www.sinochip.net for products that can learn more parameters) have well solved the surge suppression of high-power power supply. At present, it has been widely used in electric vehicle charging station, electric vehicle charging station, electric vehicle charging pile, high-power UPS power supply, high-power ozone generator, high-power power amplifier sound, high-power inverter, motor soft start, high-power transformer power surge suppression and other fields, high-power inductive load power surge suppression.