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A little common sense about power adapter

This is a common sense question about the power supply voltage, power supply, power supply and load matching, but later found that many friends discussed it. Here are a few words.


I feel that the most typical questions are as follows:

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1. What does the nominal voltage and current of power adapter (hereinafter referred to as power supply) mean?

2. Power supply with the same nominal voltage and different output current. Can it be used on the same notebook?

3. The same machine, other people's warm power supply, I always very hot, why?

4. The nominal voltage of the power supply is much higher than that of my battery. Will nothing happen?

5. Why is the original power supply usually better than the non-original power supply in theory?


Firstly, the nominal voltage of power adapter usually refers to the open-circuit output voltage, that is, no load, no current output voltage value. Therefore, it can also be considered that this is the upper limit of the output voltage of the power supply. Generally speaking, the nominal voltage can be measured directly by using a DC voltmeter with high internal resistance when active voltage stabilizing unit or voltage reference element is used in the power supply. The inexpensive small transformers on the market, such as those used for walkman, are basically the traditional core transformer plus four rectifier diode bridge rectifiers plus a large filter capacitor. In this case, the value measured by the ordinary DC voltmeter will be larger than the nominal voltage, because the output of the bridge rectifier is pulsating DC, which is simply one. The time chain of a sinusoidal voltage signal connected by a positive half-cycle will become flatter after large capacitance filtering, but the ripple coefficient is still very large (ripple coefficient is the ratio of the amplitude of the voltage signal fluctuation to the average voltage value, the more novel it is, the closer the voltage is to DC). The so-called nominal voltage refers to the voltage dividing the time integral by the integral time. It is the average value of time, if measured by a common DC voltmeter, the measured value is very close to the maximum value of the voltage signal, so the measurement is not accurate. At the same time, the output of this kind of power supply will change with the fluctuation of municipal electricity.


Generally speaking, the true no-load voltage of the general power adapter is not necessarily identical with the nominal voltage, because the characteristics of electronic components can not be completely identical, so there are certain errors allowed, and the civil situation can be controlled at about 0.1% to 5% according to the needs of the use. The smaller the error, the higher the consistency requirement for electronic components, the higher the cost in industrial production, and of course the more expensive the price.


Secondly, the nominal current value of the power supply. No matter any power supply has a certain internal resistance, so when the output current of the power supply, there will be a voltage drop inside, which is equal to the output current multiplied by the internal resistance of the power supply. One is to generate heat, equal to the square of the output current multiplied by the internal resistance, so the power supply will be hot, the other is to change the output voltage to nominal voltage minus the internal voltage drop, resulting in a lower output voltage. When the output current reaches this value, the output voltage is reduced to 95% of the nominal voltage or other proportion. Each manufacturer can set a higher or lower proportion according to the different needs of the load products. This current value is the nominal current. For example, the nominal current of 72W IBM 16V power adapter is 4.5A (16*4.5=72, nonsense). If the load resistance is too low, which leads to the output current exceeding the nominal current, two things will generally happen. One is the power damage caused by the burning of individual components because the heat exceeds the heat dissipation capacity. The other is the residual heat dissipation design, which only reflects the further reduction of the output voltage. If the load is reduced too much, it may not work properly.


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