The role of thermistors in power supply circuits

This article introduces the role of thermistors in power supply circuits. By reading this article, you can have a clearer understanding of the role of thermistors in power supply circuits, so that you can better choose thermistors. If you require a thermistor chip, please check the stock on Quarktwin’s official website.

The following question-and-answer format describes the role of NTC PTC thermistors in power supply circuits.

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

Question 2: What is the main role of a varistor connected in parallel in an AC circuit? How it works! What would be the effect without the above two components?

What is the role of the thermistor in the power circuit?

The NTC resistor is connected in series in the AC circuit mainly to act as a “current fuse”.

The varistor is connected in parallel in the AC side circuit mainly to “limit voltage overload”. To avoid electronic circuits in the moment of power-on inrush current, in series with a power type NTC thermistor in the power supply circuit, can effectively suppress the inrush current at power-on, and after the completion of the inrush current suppression, due to the continuous effect of the current through its power type NTC thermistor resistance will drop to a very small degree, it consumes negligible power, will not The normal working current will not be affected, so the use of power type NTC thermistor in the power circuit is the most simple and effective measure to suppress the surge when the power is on, to ensure that electronic equipment from damage.

The working principle of the varistor: for example, a “nominal 300V” varistor in the work of 220V, 220V suddenly rose to 310V!

According to what you mean is that the varistor design is best placed behind the fuse so that the varistor will not do any harm to the grid. And the fuse is generally a slow break! NTC is correct. Not energized, the NTC resistance value is high, an energized moment, the resistance value is still high, limiting the inrush, with NTC current flow, temperature increases, and the resistance value drops to very low, which can be ignored.

But then, the normal operation, the current is small, the resistance value is small, then suddenly comes to an inrush current, or the circuit that section of the road makes the current increase, that will not play a protective role, right, that is, can only be used to prevent the surge when the power is on?

After normal operation, there is no inrush current, right? Only the inrush voltage. If there is an inrush current, such as a short circuit in the power supply, because the NTC has been on, it can not do anything about it, only the fuse to play a role. Remember that the NTC is only the power-on protection that can be. Imagine if the circuit has been normally powered on, NTC has been low resistance, this time encountered high voltage NTC is powerless.

Good point, after a period of the normal operation of the power supply, and then frequent switching, will cause harm to the power supply, because at this time, due to the NTC temperature rise, and resistance drop, the inhibition of surge capacity has been it’s limited.

Right, the use of NTC to suppress the power surge power supply equipment, is not to be able to frequently switch on and off. Need to wait for the NTC to cool down and return to its cold resistance value before powering on again. Otherwise, the installation of NTC is meaningless.

For low-power power supply, the current small NTC is not much heat, so there is some role.

I know is to use NTC resistors. If you use ordinary resistance + relay or thyristor, I wonder if it can?

Very good, much stronger than simply using NTC resistors, NTC will lose inhibition when the power is off and immediately on the power.

So frequent switching, NTC will be ineffective

But the bias circuit of the SCR single resistor does not work ah and the estimated high-power power supply does not work, which must be a little loss, ah PTC is the role of insurance, and NTC is to limit the inrush current.

NTC: negative temperature resistance, the higher the temperature, the smaller the resistance, used to string in the input circuit to limit the inrush current. In normal operation of the heat, the resistance decreases and does not affect the work, but it is energy consumption, and power consumption can not 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.

Overcurrent heat, resistance increases, and the input equivalent disconnect, cold sure after the resistance is reduced, can continue to work, do not need to replace, often used at the same time with varistors, TVS.

Varistor: similar to the avalanche effect of voltage stabilization DIODE, more than the embedded voltage, the current increases rapidly, but will not short circuit, this is different from the discharge tube.

PTC uses a lot, such as color TV’s demagnetization circuit, refrigerator compressor start circuit, etc.

Over-temperature protection sometimes also uses PTC series in the circuit PTC, NTC may be used, but PTC is equivalent to the role of fuse, NTC is to limit the power-on current.

With varistors (surge absorber)

NTC (negative temperature coefficient) that the temperature becomes high resistance value becomes small, (PTC) thermistor (positive temperature coefficient) is the opposite, the two roles are very different, NTC series on the L line, and PTC parallel on the L, N line, the role of NTC play a buffer role, that is, the instantaneous shock current is very large, so a series of NTC can reduce the instantaneous shock current, (a series of resistors in the circuit can also be) This effect, but there is a certain loss in the resistance, resulting in low efficiency) it works as follows: just turn on the instant, due to room temperature, then the impedance is large, at this time equivalent to a series of resistance in the circuit, when the circuit work, the current flow through the NTC, the temperature rises, the impedance becomes smaller, at this time equivalent to a short circuit, that is, the boot can suppress the instantaneous current, and normal work can be a small loss (almost zero loss).

PTC is a high-voltage suppression role, can also be called a lightning protection tube, speaking of lightning protection tubes may not be unfamiliar with the standard voltage AV2500V, the working principle is similar to the voltage regulator, that is, the voltage of the two feet to reach the breakdown voltage, the two feet is equivalent to a short circuit, the current can reach a dozen A to hundreds of A, and the working voltage also depends on the 7D471K/271K. There is also a put voltage tube 200, high voltage up to AC4000V. But you may think, lightning hit at the input, then in the input line connected to the PTC how to play the role of lightning protection?

If you want to explain, then I have to say a lot more, so this question other netizens answer it

If the powerbomb varistor, it may be caused by those circumstances. And how to choose the varistor when designing the circuit?

SCK057 thermistor stable current is how big? I string in the 220AC current in 1A when it starts to get hot, to 3A has been hot, now there is a good 10A on the 220AC circuit what to do?

Can the thermistor be placed on the zero line, is it necessary to be placed on the fire line ah? For 2PIN line, the AC input is the same

For the 3PIN, there are still requirements, right? Are there any safety requirements? For example, between the two legs of the thermistor, there are no copper distance requirements, the body has no requirements for the height of the rack.