Shulou(Shulou.com)06/01 Report--

First of all, take a look at figure 3-1, which is the USB interface and power supply circuit seen in the last class.

Figure 3-1 USB interface and power supply circuit

In the picture on the right, after passing the fuse, there is a capacitor C16 of 470uF, and the picture on the left is connected with a capacitor C19 of 100uF and a capacitor C10 of 0.1uF in parallel. The influence of C16 and C19 is the same, and the influence of C10 is different from their two. Let's first introduce these two larger capacitors.

The capacitance with a larger capacitance can actually be understood as a water tank or a pool, and at the same time, everyone can directly understand the current as a current. In fact, the truth of all things in nature is similar.

Influence one, buffer influence. When the power is on, when the current flows down from the power source, it does not fluctuate and easily impact electronic devices. Adding a capacitor can cushion and affect. It is as if we directly irrigate the ground with the water of the faucet and easily wash out the flowers and plants. We only need to add a pool at the faucet to let the water flow slowly into the grass after passing through the pool, so that it will not wash out the flowers and play an ineffective maintenance role.

Influence two, fluctuation influence. Our whole set of circuits, the power of the later electronic devices are all different, and the size of the current required for the normal task of the device is not intact. For example, when there is a device in the later stage that does not have a task, the current consumption is 100mA, and suddenly it intervenes in the task, and the current increases sharply to 150mA. If there is not a water tank at this time, the voltage (water level) in the circuit will drop suddenly, for example, our 5V voltage will suddenly drop to 3V. And some electronic components in our bits and pieces must be higher than a certain voltage in order to perform a normal task. If the voltage is too low, the task will not be done directly. At this time, there will be fewer water tanks. The capacitor will release the electricity stored in it at this time, fluctuating the voltage, and then, of course, the previous current will fill the tank in real time.

With this capacitor, it can be said that our voltage and current will fluctuate and will not be greatly shaken. The commonly used capacitors are shown in figure 3-2, figure 3-3, and figure 3-4:

Figure 3-2 Aluminum electrolytic capacitor

Figure 3-3 Tantalum Capacitor figure 3-4 Ceramic Capacitor

These three kinds of capacitors are the three most commonly used, of which the first is large, occupies a large space, the unit capacity is the cheapest, the second and the third are small, take up less space, and the function is slightly better than the first, but the price is also high. Of course, in addition to the price, there are also some special parameters, which also need to be considered in places with high communication requests, not to mention here. We now use the first kind on the board. Under the voltage withstand value and capacitance value of the strange matching premise, the capacitance of the first kind of 470uF is less than one dime, while the second and the third kind can cost you 1 yuan.

In the selection of capacitors, the first parameter is the consideration of voltage withstand value. We use 5V bits and pieces, the voltage withstand value of the capacitor is higher than 5V, ordinary referrals can be 1.5 times to 2 times, and some places can be slightly higher. We use 10V voltage withstand on the board. The second parameter is the capacitance value, which is selected on the basis of experience. When it is selected, it depends on the whole set of piecemeal power consumption affected by the capacitor. If the piecemeal power consumption is larger and the shaking can be comparable, then the capacitance value should be larger, otherwise it can be smaller.

Students just began to design circuits to simulate others, and they used as much as others, and gradually accumulated. For example, when we talk about capacitive influence II above, when the current suddenly increases from 100mA to 150mA, in fact, even with this capacitor, the voltage will waver slightly, for example, from 5V to 4.9V, but as long as the device on our board is above 4.9V, this waver is allowed, but if it is not added or added very small, the voltage shake is larger than that. The task of some devices will not be normal. However, if the addition is too large, it takes up space and the price is high, so the selection of this central capacitor has more reference experience.

Let's take a look at another capacitor C10 in figure 3-1, which has a smaller capacitance, 0.1uF, that is, 100nF, which is used to filter out high-frequency signal interference. Such as ESD,EFT and so on. We learned the characteristics of capacitance in junior high school-- it can be switched and separated by DC, but the influence of capacitance parameters on the disturbance of different frequency bands is different. The capacitance of this 100nF is a value summed up by our elders according to the frequency of the disturbance, according to the parameters of the board, and according to the parameters of the capacitance itself. In other words, when everyone is designing a digital circuit in the future, the decoupled high-frequency capacitor at the power supply can be used directly with this 0.1uF, and there is no need to calculate and consider too much.

Another point is that everyone can take a closer look at our KST-51 pioneer board, and a large capacitor will be added near the devices that require a large current supply in the circuit, such as C18 in the upper left corner of the 1602 liquid crystal, VCC close to the single chip microcomputer and VCC in the 1602 liquid crystal backlight, which can affect the fluctuating voltage, while the practical position of the C19 in figure 3-1 is also placed near the motor and buzzer in the upper left corner. Because the current required by them is relatively large, and the current wavers greatly during the task. In addition, between the VCC and GND of all IC devices, the city puts a high-frequency decoupling capacitor of 0.1uF, especially when it is on the cloth board, this 0.1uF capacitor should be as close to IC as possible, and try to connect smoothly with the VCC and GND of this IC. Everyone understands this first, and the details will be commented and debated later.

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