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# What does the voltage and current of the power adapter stand for?

Issue Time:2019-05-10
First, the nominal voltage of the power adapter usually refers to the open-circuit output voltage, that is, the voltage value without any load and no current output. Therefore, it can also be considered that this is the upper limit of the output voltage of the power supply. For the case where an active voltage regulator unit or a voltage reference component is used inside the power supply, it is generally said that a DC voltage meter with a high internal resistance can directly measure the nominal voltage (a more accurate method of using an electromotive force bridge, belonging to the university general physics course) Experiments, not to mention), even if the mains voltage fluctuates, its output is a steady constant value; but for the cheap small transformers on the market, such as those used for the Walkman, basically the traditional magnetic core The transformer plus four rectifier diode bridge rectifiers plus a large filter capacitor is done, so if the value measured with a normal DC voltmeter will be greater than the nominal voltage, the output of the bridge rectifier is pulsating DC. .

Simply put, the time chain of a positive half cycle of a sinusoidal voltage signal is flattened after large capacitance filtering, but the ripple coefficient is still large (the ripple coefficient is the amplitude of the voltage signal fluctuation and the voltage average The smaller the ratio, the closer the voltage is to DC. The so-called nominal voltage refers to the voltage divided by the time and divided by the integration time. The simple understanding is the average value of time. If measured with a common DC voltmeter, the measured value Very close to the maximum value of the voltage signal, so the measurement is not accurate. At the same time, if the mains fluctuates, the output of this type of power supply will also change.

Generally speaking, the true no-load voltage of a common power adapter is not necessarily exactly the same as the nominal voltage. Because the characteristics of the electronic components cannot be completely consistent, a certain error is allowed, and the civilian situation is controlled at 0.1% to 5 according to the needs of the application. %about. The smaller the error, the higher the consistency requirements for electronic components, the higher the cost in industrial production, and the more expensive the price.

Followed by the nominal current value of the power supply. No matter what kind of internal resistance of any power supply, when the power supply output current, it will internally generate a voltage drop, which is equal to the output current multiplied by the internal resistance of the power supply. Two things are caused, 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, and the other is that the output voltage becomes the nominal voltage minus the internal voltage drop, resulting in a lower output voltage. The usual design generally limits a current value after considering the heat dissipation problem. When the output current reaches this value, the output voltage is reduced to 95% of the nominal voltage, or other ratios, which can be set by each manufacturer according to the different needs of the load product. Higher or lower ratio, this current value is the nominal current.

For example, the 72W ibm16V power adapter has a nominal current of 4.5A (16*4.5=72). If the load resistance is too low, causing the output current to exceed the nominal current, two things usually happen. One is that the individual components are burnt due to heat generation exceeding the heat dissipation capacity, causing power supply damage. The other is that the heat dissipation design leaves a margin, which is only reflected as The output voltage is further reduced, and if it is lowered too much, the load may not work properly.

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