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The role of NTC thermistor in the power circuit

Q: What is the main function of NTC resistance series connection in AC circuit? How does it work?

Question: What is the main function of the varistor parallel connection in the AC side circuit? How does it work? If there are not the above two components, what will happen?

A: NTC resistance series connection in AC circuit mainly plays the role of "current insurance".

The varistor is connected in parallel in the AC side circuit mainly to "limit the voltage is super high".
In order to avoid the surge current generated in the electronic circuit at the instant of start-up, a power NTC thermistor is connected in series in the power circuit, which can effectively suppress the surge current at start-up. After the suppression of surge current is completed, the resistance value of power NTC thermistor will decrease to a very small extent due to the continuous action of surge current. It consumes negligible power and does not affect the normal working current. Therefore, the use of power NTC thermistor in power supply circuit is the most simple and effective measure to suppress the surge when starting, so as to ensure that electronic equipment is not damaged.

The working principle of varistor: For example, a "nominal 300V" varistor in 220V operation, suddenly 220V rise to 310V! At this time, the varistor is broken down. After fusing the fuse through a large current, the circuit behind the fuse is protected, and then the varistor restores its original state.

Q: According to what you mean, it is better to put the varistor in the back of the fuse, so that the varistor will not cause any harm to the grid when it is turned on?
The fuses are generally slow!
A: Yes, NTC is correct.
NTC has a high resistance when not energized. Once energized, the resistance is still high, limiting the inrush current.
As the NTC has a current flowing through it, the temperature increases and the resistance drops to a very low level, which can be ignored.

Q: I understand, but in this case, when working normally, the current is small and the resistance is small. Then suddenly a surge current, or the circuit that makes the current increase, then it can not play a protective role, that is to say, can only use the surge when the power is off? 
A: There is basically no surge current after normal work. Only surge voltage.
If there is a surge current, such as a short circuit in the power supply, because the NTC is already turned on, it can't do anything about it. Only by the fuse. Remember that NTC is only for boot protection.

Q: Imagine if the circuit has been powered up normally, NTC has low resistance, and when it encounters high voltage, NTC is powerless.
A: It is good to say that after the power supply works for a period of time, it will cause damage to the power supply by switching on and off frequently. Because at this time, due to the temperature rise of NTC, the resistance value decreases, and the ability to suppress surge is limited.
To be honest, NTC is used to suppress power surges that are booting up, and it is not possible to switch on and off frequently. It is necessary to wait for NTC cooling and return to its cold state resistance before it can be turned on again. Otherwise, the meaning of installing NTC is gone.
For small power supply current, small NTC does not heat up very much, so it has a certain effect.
Q: I know that I use NTC resistors. If you use ordinary resistors + relays or thyristors, I wonder if it is OK?
A: Very good, much stronger than simply using NTC resistors. NTC will lose its inhibition when it is powered off and immediately powered on.
So if you switch on and off frequently, NTC will not work.
Q: But the thyristor's bias circuit does not work with resistors alone, and it is estimated that the high-power power supply will not work, so the loss is a bit big.

A: PTC is an insurance function, NTC is to limit the inrush current.
NTC protection power circuit design
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