Causes of the thermistor burnout_thermistor's role in the power supply circuit

October 10, 2021

Causes of the thermistor burnout_thermistor's role in the power supply circuit

How do I know the thermistor is broken

Select the appropriate resistance gear for the multimeter, touch the two ends of the thermistor respectively, pinch the thermistor with your hands or heat it by other methods. If the resistance changes linearly, it proves to be good. If there is no change, The description has expired.

There are several reasons for the thermistor burnout:

1. The instantaneous current of the thermistor is too large and the resistance coil is broken down;

2, the thermistor's resistance wire insulation protects from wear and tear to form a short circuit between the coils;

3. The line voltage is unstable and fluctuates greatly, and the instantaneous voltage exceeds the safety index of the thermistor.

The above three points are just the cause of general problems. The specific situation still depends on various factors such as your use environment.


What to do if the thermistor of induction cooker is damaged

The thermistor of the induction cooker is damaged and needs to be stopped and repaired and replaced in time. Induction cooker thermistor damage function: The thermistor in the middle of the induction cooker coil is used to detect the temperature of the pot to prevent the pot from drying out and to control the temperature of the food in the pot. When this thermistor is damaged, the induction cooker will stop power output (not heating) and display a fault code.

Thermistor's role in the power circuit
The role of NTC PTC thermistor in the power supply circuit is introduced in the form of question and answer below.

Question 1: What is the main role of the NTC resistor in series in the AC circuit? How does it work?

Question 2: What is the main function of the varistor in parallel in the AC side circuit? How does it work! What will be the impact if there are no above two components?


The NTC resistor is connected in series in the AC circuit mainly to play the role of "current insurance".


The varistor is connected in parallel in the AC side circuit mainly to "limit the ultra-high voltage". In order to avoid the surge current generated in the electronic circuit at the moment of turning on, a power NTC thermistor is connected in series in the power circuit, which can effectively suppress the surge current when turning on, and after the suppression of the surge current is completed , Due to the continuous action of its current, the resistance value of the power NTC thermistor will drop to a very small extent, and the power it consumes can be ignored and will not affect the normal operating current, so in the power loop The use of power NTC thermistors is the simplest and most effective measure to suppress the surge when starting up to protect the electronic equipment from damage.

The working principle of varistor: For example, a "nominal 300V" varistor is working at 220V, and suddenly 220V rises to 310V! At this time, the varistor is broken down, and the fuse is blown through a large current, which protects the subsequent circuit, and then the varistor returns to its original state.


Elderly: According to what you said, the varistor should be placed behind the fuse when it is designed, so when the varistor is turned on, will there be no harm to the power grid? The fuse is generally slow broken! It's NTC. When there is no power, the resistance of the NTC is high. When the power is turned on, the resistance is still high, which limits the inrush current. As the current flows through the NTC, the temperature increases, and the resistance drops to a very low value, which can be ignored.

But in this case, when the current is small during normal operation, the resistance value is small, then a sudden surge current, or the section of the circuit makes the current increase, then it will not be able to protect it, that is to say, it can only be used Is it protected against surges during power-on?

There is basically no inrush current after normal operation, right? Only surge voltage. If there is a surge current, for example, the power supply is short-circuited, because the NTC has been turned on, there is nothing to do with it, only the fuse can work. Remember that NTC is just for boot protection. Imagine if the circuit has been powered up normally and the NTC has low resistance, it is powerless to encounter high-voltage NTC at this time.

It’s a good idea. After the power supply has been working for a period of time, frequent switching on and off will cause damage to the power supply, because at this time, the temperature of the NTC rises and the resistance decreases, and the ability to suppress the surge is extremely limited.

That's right, using NTC to suppress the power-on surge of the power supply equipment can not be switched on and off frequently. You need to wait for the NTC to cool down and return to its cold resistance value before it can be turned on again. Otherwise, there is no point in installing NTC.

NTC does not generate much heat for low power supply current, so it has a certain effect.

I know that NTC resistors are used. If you use ordinary resistance + relay or thyristor, is it possible?

Very good, much stronger than simply using NTC resistance, NTC will lose its inhibitory effect when it is powered off and on immediately.

So if you switch the machine frequently, NTC is invalid

But the bias circuit of the thyristor does not work with a resistor alone, and it is estimated that it will not work on a high-power power supply, so the loss must be a bit large. PTC is the insurance function, and the NTC is to limit the surge current.

NTC: Negative temperature resistance, the higher the temperature, the smaller the resistance, used to string in the input loop to limit the startup surge current. It generates heat during normal operation, and the resistance is reduced, which does not affect the work, but it consumes energy, and the power consumption cannot be ignored. NTC can also be used for temperature measurement.

PTC: Positive temperature resistance, series in the input circuit, also known as: self-recovery fuse. When overcurrent occurs, it will generate heat and the resistance will increase, which will be equivalently disconnected from the input. After the cooling is confirmed, the resistance will decrease, and it can continue to work without replacement. It is often used together with varistors and TVS.

Varistor: Similar to the avalanche effect of regulated DIODE, the current increases rapidly after the clamping voltage is exceeded, but it will not be short-circuited, which is different from the discharge tube.

PTC has many uses, such as the degaussing circuit of color TV, the starting circuit of refrigerator compressor and so on.

Over-temperature protection sometimes uses PTC string in the loop PTC, NTC may be used, but PTC is equivalent to the function of a fuse, NTC is used to limit the startup current.


Varistor (surge absorber) used


NTC (negative temperature coefficient) means that the temperature becomes higher and the resistance value becomes smaller. (PTC) thermistor (positive temperature coefficient) is the opposite. The two functions are completely different. NTC is connected in series with the L line, while PTC is connected in parallel with the L and N lines. Above, the role of NTC plays a buffering role, that is, the instantaneous impulse current at startup is very large, so a series of NTC can reduce the instantaneous impulse current at startup. (Causing low efficiency) Its working conditions are as follows: the moment it is turned on, due to normal temperature, the impedance is large. At this time, it is equivalent to stringing a resistor in the circuit. It is equivalent to a short circuit, that is, the startup can suppress the instantaneous current, and the loss can be small (almost zero loss) during normal operation. It can’t be used as a fuse. If you want to blow up the NTC, I’m afraid the PCB will be completely black. PTC is a high-voltage suppression function, and it can also be called an anti-detonator. When it comes to anti-detonator, maybe everyone is familiar with it. The standard voltage is AV2500V, and it works. The principle is similar to that of a Zener tube, that is, when the voltage of the two legs reaches the breakdown voltage, the two legs are equivalent to a short circuit, and the current can range from more than ten A to hundreds of A, and the working voltage also depends on the value. 7D471K/271K. There is also a discharge tube 200, the high voltage can reach AC4000V. But you may think that lightning strikes the input end, then how can the PTC be connected to the input line for lightning protection? If you want to explain this, then I have to say a lot better, so other netizens answer this question.

If the power supply blows up the varistor, what might be the cause of it? And how to choose a varistor when designing the circuit?

What is the stable current of the SCK057 thermistor? I started to get hot when the current was 1A in 220AC, and it was already very hot at 3A. Now there is a good 10A on the 220AC circuit, what should I do?

Is it okay to put the thermistor on the zero line? Does it have to be on the live line? For the 2PIN line, the AC input is actually the same for all lines

There are still requirements for 3PIN, right? Are there any safety requirements? For example, is there a requirement for the distance between the two legs of the thermistor that the copper cannot be used, and is there a requirement for the body to be elevated?

Things are dead, people are alive. Understanding how it works, understanding your own needs, and flexible use are the key

What is the selection principle of switching power supply and thermistor?

In the case of satisfying the steady-state current, the resistance value measured at a temperature of 25 degrees Celsius should be: R》=1.414*E/Im E: input voltage Im: surge current, it is mentioned that generally in the switch In the power supply, the surge current is 100 times the steady-state current

Use 100k thermistor. For emergency use, a 100k ordinary resistance can be used as a temporary substitute. But don't put this resistor on the thermistor bracket, you can find an open space inside the induction cooker shell and fix it with tape.