A little common sense of the power adapter!


Contact Special Network has encountered several discussions about power supply voltage, power supply power, and whether the power supply and load are matched or not in the past year. Originally this was a common sense problem, but later I found that many friends discussed it, so I will say a few words here.

The most typical problems I feel are as follows:

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

2. They are all power supplies with the same nominal voltage and different output currents. Can they be used on the same notebook?

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

4. The nominal voltage of the power supply is much higher than the voltage of the battery of my laptop, so there will be no accident, right?

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

  and many more.

Next, I will talk about my personal understanding, and please be more correct.

First of all, 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 as the upper limit of the output voltage of the power supply. For the case where an active voltage stabilization unit or a voltage reference element is used inside the power supply, generally speaking, a DC voltmeter with high internal resistance can be used to directly measure the nominal voltage (more accurately, the method of electromotive force bridge should be used, which belongs to the general physics of the university). Course experiments, not to repeat), even if the mains voltage fluctuates to a certain extent, its output is a stable and constant value; but for cheap small transformers on the market, such as those used for walkmans, they are basically traditional magnetic The core transformer plus four rectifier diodes, bridge rectifier and a large filter capacitor are all done. In this case, if the value measured by an ordinary DC voltmeter will be greater than the nominal voltage, the reason is that the output of the bridge rectifier is pulsating. DC, simply put, is a time chain connected by the positive half cycles of a sinusoidal voltage signal. After filtering by a large capacitor, it will become flatter, but the ripple coefficient is still very large (the ripple coefficient is the amplitude of the voltage signal fluctuation that is the same as the voltage The ratio of the average value, the smaller the voltage is, the closer the voltage is to DC). The so-called nominal voltage refers to the integration of this voltage over time and then divided by the integration time. A simple understanding is the average value of time. If it is measured with an ordinary DC voltmeter, The measured value is very close to the maximum value of the voltage signal, so it is not accurate. At the same time, if the mains power fluctuates, the output of this type of power supply will also change accordingly.

Generally speaking, the real no-load voltage of ordinary power adapters is not necessarily exactly the same as the nominal voltage, because the characteristics of electronic components cannot be completely consistent, so a certain error is allowed. %about. The smaller the error, the higher the requirements for the consistency of electronic components, the higher the cost in industrial production, and of course the more expensive the price.

The second is the nominal current value of the power supply. Any power supply has a certain internal resistance, so when the power supply outputs current, a voltage drop will be generated internally, which is equal to the output current multiplied by the internal resistance of the power supply. It causes two things, one is to generate heat, which is equal to the square of the output current multiplied by the internal resistance, so the power supply will heat up, and the other is that the output voltage becomes the nominal voltage minus the internal voltage drop, causing the output voltage to drop. 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 proportions. Each manufacturer can set it according to the different needs of the load products. Higher or lower ratio, this current value is the nominal current. For example, the nominal current of a 72W ibm16V power adapter is 4.5A (16*4.5=72, nonsense). If the load resistance is too low, causing the output current to exceed the nominal current, two things will generally happen. One is that the individual components burn out due to the heat exceeding the heat dissipation capacity, causing the power supply to be damaged. The other is that the heat dissipation design has a margin, which is only reflected as The output voltage is further reduced, and if it is reduced too much, the load may not function properly.

After understanding the above concepts, you can judge the second question of this article. Power supplies with the same nominal voltage have different output currents, can they be used on the same notebook? The basic principle is that a power supply with a large nominal current can replace a power supply with a small nominal current. Some friends have misunderstandings and think that a power supply with a large nominal current will burn out the notebook because the current is too large. In fact, the current depends on the load when the voltage is the same, that is, the working conditions of the books. When the books are running at high load, the current is larger, and when the books are in standby, the current is smaller. In short, the current is equal to adding to the books. The voltage on it divided by the equivalent resistance of the books. A power supply with a large nominal current has sufficient current headroom to prevent overheating or low output voltage after replacing a power supply with a small nominal current. On the contrary, replacing the high-current power supply with a small-current power supply has the above-mentioned danger. However, some friends use a 56w power supply instead of a 72w power supply without any problem. The reason is that the design of the power adapter usually leaves a certain margin, and the load power must be smaller than the power supply, so this replacement is feasible in general use. , but the remaining power margin is very small. Once your notebook is connected to a lot of peripherals, such as two usb hard drives, then the cpu runs at full speed, and there is a base with an optical drive on it to read the disk at full speed, plus At the same time charging the battery, it is estimated that it is dangerous. You should always touch your power supply to see if it can cook eggs. The extra 16w of the 72w than the 56w adapter is to deal with this situation. Therefore, it is best not to replace the high-current power supply with a low-current power supply. If you really want to do it, friends who have the conditions can find a way to measure the actual current required by your notebook when it consumes the most. The measurement method is very simple, a socket, a plug, an ammeter, a few wires, and a soldering iron. If it is within the nominal current of your power supply, of course it is safe.

After answering the second question, the third question is very simple. Don't suspect that there is a problem with your power supply, first look at what your notebooks are doing, whether it is like the two usb hard drives mentioned above, the cpu is running at full speed, the hard disk is reading and writing frantically, the optical drive is reading the disk at full speed, while charging the battery, playing loudly It is very important to make good use of power management and reasonably adjust the working state of the notebook according to the task. Many people don't like to use power management. I even see many friends who don't want any kind of management mode to be enabled after installing the power management software, so why install him? Don't you just want to see how much percent of the battery is left?

As for the fourth question, the person asking the question may be a layman on electronic circuits. The power supply to the notebook from the power supply is different from the power supply to the notebook from the battery. First of all, the battery is powered. The output of the battery is pure DC, which is very clean. It can be directly connected to the DC transformer module. The voltage of the battery is neither possible nor required to be designed. Very high, the voltage requirements of analog signals and digital signals in microelectronic circuits are basically 5V at present, except for the efficiency and pressure difference requirements of the transformer module, 10.8V is enough, and the chemical electromotive force of the lithium battery determines a section The output voltage of the cell can only be around 3.6V, so many batteries are connected in three-stage series, and 10.8V has become a very popular battery voltage. The nominal value of some batteries is slightly larger than the integer multiple of 3.6V, such as 3.7V or 11.2V, etc., in fact, to protect the battery (very few manufacturers will set the battery's discharge termination voltage as 3.7V) or calculate the battery The tricky behavior when designing capacity. If the power supply is used, the situation is more complicated. First of all, it is necessary to further stabilize and filter the added voltage to ensure stable operation under the condition that the power supply performance is not very good. Power supply, the other way to charge the battery, the part that supplies power to the notebook is the same as when the battery is powered, and the part that charges the battery needs to be added to the battery cell through the battery's charging control circuit, and the control circuit can be very complicated. , In short, it should include primary voltage regulator, precision tunable voltage regulator, thyristor regulated pulsating output, voltage regulator output, current feedback, chip charging process recording and operation, charging program self-feedback adjustment parameters, etc., so the power supply voltage must be greater than Only the cell voltage has sufficient differential pressure margin to be supplied to each unit of the charging control circuit. In the end, the voltage really added to the cell is by no means 16V, please rest assured that it is the friend who is worried.

Let's talk about why the original power supply is usually better than the non-original power supply. I want to say that in theory the original power supply is relatively better, but in practice, you may not feel the difference. The input voltage allowed by the usual load has a safe range, such as the nominal value plus or minus 5%. An example is the input requirement of many 2.5-inch hard drives is 5v plus or minus 5%. The same is true for notebooks. If the input voltage is too large or too small, the protection circuit will act and stop working. However, before the protection circuit operates, the voltage regulator circuit inside the notebook has been biased towards the upper or lower limit of operation, which in principle has a certain impact on the life of the device, but from today's point of view, the reliability of electronic components is quite good, as long as the design is Within the range, there will be few problems, and the lifespan will not be shorter than the lifespan of the books, so this is not the main problem. The more important problem may be the data security of the notebooks. It is a terrible thing for the computer to stop working suddenly after automatic protection, especially for many friends who do not need batteries. Sometimes inexplicable restarts of the computer are also related to this. For the original power supply, the manufacturer knows what kind of load is to be connected, so the nominal voltage and nominal current of the power supply can be easily calculated, that is, the nominal voltage of the power supply should meet the following two conditions. The first is When the maximum current is output, the nominal voltage minus the voltage drop of the power supply itself should be greater than 95% of the voltage required by the load. The second is that the design voltage minus the voltage drop of the power supply itself should be less than 105% of the voltage required by the load when the minimum current is output. However, if a non-original adapter is used, such as a general-purpose transformer, the above problems cannot be seriously considered. This means that the user can only try to find a way to obtain compatibility from the power parameters, but the internal resistance of each adapter is different. The allowable error of the nominal voltage may be different, the definition of the voltage variation range under the nominal current output may also be different, and even whether the output ripple coefficient is small enough, if not carefully measured the output power and output voltage. In principle, there is a certain risk, but this risk is very small, because as long as you choose a power supply with a large enough nominal current, there will be no problem of excessive internal resistance, and the ripple coefficient can also be adjusted by the internal voltage regulator of the notebook. reduced, so I'd say that the original power supply is usually better than the non-original power supply. This is the difference between the original power supply and the ordinary power supply.


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