The definition of the nominal voltage and current in the power adapter

Power adapter of nominal output voltage is usually refers to the open circuit voltage, also is not connect any load, there is no current of the output voltage value. So can also think this is the power supply output voltage limit. For internal using active voltage regulator power supply unit or voltage reference element, typically use high internal resistance of the dc voltmeter can be directly measured nominal voltage, even if the mains voltage in a certain fluctuation, the output is constant value of the firm. But small transformer for cheap on the market, basically is a traditional magnetic core transformer with four rectifier diode bridge rectifier, coupled with a large, filter capacitor, so if you are using a common dc voltmeter measured value will be greater than the nominal voltage, the reason is that the output of the bridge rectifier for pulsating dc, simply say a a sinusoidal voltage is the positive half cycle to connect into a chain of time, after a large capacitor filter will become flat, but still significant ripple coefficient ( Ripple coefficient is voltage signal wave amplitude with the ratio of the average voltage, the smaller the closer it gets to the dc voltage) , the nominal voltage is the voltage of integral divided by integral time, simple understanding is on the average of the time, if use common dc voltmeter measurement, measured values are very close to the maximum of voltage signal, so the uncertainty. At the same time, if the mains fluctuation, the power supply output changes. In general ordinary real no-load voltage power adapter does not necessarily completely consistent, and nominal voltage because of the characteristics of electronic components can't completely consistent, have certain error, so allow civil case according to the need of the usage control in 0. About 1% to 5%. Error, the smaller the higher consistency requirements for electronic components, the higher the cost of industrial production, the price is more expensive, of course. The second is the power of the nominal current value. Regardless of any power supply has a certain amount of resistance, so when the power supply output current, can produce the pressure drop in the internal, internal resistance is equal to the output current is multiplied by the power. Lead to two things, 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 one is the output voltage to nominal voltage minus the internal pressure drop, results in the decrease of the output voltage. Usually after finished design considering the heat dissipation problem, generally limit a current value, when the output current reaches the worth, decreased to 95% of the nominal voltage, output voltage or other proportion, each manufacturer according to the different need of load products can be set higher or lower, the proportion of the current value is the nominal current. Such as the 12 w 12 v power adapter from the nominal current is 1 a ( 12 * 1 = 12) 。 If the load resistance is too low, which leads to the output current more than nominal current, generally two things happen, one is the individual elements burned due to fever more than the heat dissipation capacity caused power supply is damaged, the other is a thermal design have allowance, only to further reduce the output voltage, if reduce too much can lead to can't normal work load. Ordinary power adapter of the nominal voltage is likely is not equal with the real no-load voltage, this is mainly due to the nature of electronic components to make the two numerical can't completely equal, must exist between the certain error.