NTC thermistor.NTC temperature Thermistor

　　NTC thermistors have a negative temperature coefficient, and their resistance decreases as the temperature increases. As shown in the figure, the NTC thermistor suppresses the surge circuit, which is the application circuit for suppressing the surge in the PC switching power supply. The NTCR1 in the circuit is the NTC thermistor, which is used to suppress the surge current when turning on.

　　1. Causes of surge suppression

　　When the PC is cold started, a large surge current will be generated, that is, the AC 220V mains will give the PC a large power-on inrush current. This large surge current may burn the power supply and the internal circuit of the host. For this reason It is necessary to set up a circuit for suppressing surge, so that when the PC is powered on, the surge current is small, and after booting, it can restore the normal 220V power supply state.

　　2. NTC thermistor location

　　Open the PC's switching power supply case, you can find a disc-type ceramic capacitor on the outside, usually olive green. It is an NTC thermistor used to suppress inrush current, as shown in the figure.

　　3. Circuit Analysis

　　It can be seen from the circuit that NTCR1 is connected in series in the 220V power supply loop as part of the 220V AC mains load. When the PC is cold-started, since the resistance value of NTCR1 is large at normal temperature (zero power), this limits the current of the 220V loop when it is turned on, so that it cannot be too large.

　　After power-on, current flows through NTCR1, its temperature rises, and the resistance begins to drop. After the PC is turned on for a short time, the temperature of the NTCR1 resistor rises to the working range, and its resistance value drops to a very low value, which can be ignored. At this time, the 220V mains power supply enters the normal state, and the resistance value of NTCR1 is very small at this time, and it will not generate too much power consumption.

　　This surge suppression circuit also has drawbacks. For example, when the power is turned off and then restarted quickly, the thermistor has not completely cooled down, and some of the surge suppression function will be lost, which is why briefly turning the power off and on is harmful. reason for the operation.

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