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Why is the Poe power supply unstable because you do n’t understand its power
Many friends have repeatedly asked whether the power supply of the poe is stable? What cable is good for poe power supply? Why there is no display and so on why the camera is powered by the poe switch, in fact, these are related to the power loss of the POE power supply, and it is easy to ignore in the project. There just happened to be a professional friend who did poe-related contributions to us, which very well explained the power loss problem in poe power supply. Let's take a look at it together.
1. What is poe power supply
PoE refers to the transmission of data for some IP-based terminals (such as IP phones, wireless LAN access point APs, network cameras, etc.) without any changes to the existing Ethernet Cat.5 cabling infrastructure. At the same time, it can also provide DC power supply technology for such equipment.
PoE technology can ensure the safety of the existing structured cabling while ensuring the normal operation of the existing network, reducing costs.
A complete PoE system includes two parts: power supply end equipment and power receiving end equipment.
Power Supply Equipment (PSE): Ethernet switches, routers, hubs, or other network switching devices that support POE.
Power receiving device (PD): In the monitoring system, it is mainly the network camera (IPC).
2. Poe power supply standard
The international standard IEEE802.3af established in 2003 requires PSE to achieve an output power of 15.4W, while the power reaching the powered device is 12.95W and the power loss is 2.45W.
The international standard IEEE802.3at established in 2009 requires that the PSE can reach an output power of 30W, the power reaching the powered device is 25.5W, and the power loss is 4.5W.
With the passage of time, the power of these two standards can no longer meet the power supply requirements of the new PD with higher power. Therefore, the international standard IEEE802.3bt has two requirements:
IEEE International Standard Parameter Table
The first type: one of them is to require the PSE to reach 60W of output power, the power to reach the powered device is 51W (from the table above, this is the lowest data), and the power loss is 9W.
The second type: requires that the PSE can reach an output power of 90W, the power to reach the powered device is 71W, and the power loss is 19W.
It can be known from the above standards that as the power supply increases, the power loss is not proportional to the power supply loss, but the loss is getting larger and larger. So how can the loss of PSE in practical applications be calculated?
3. Poe power loss
So let's take a look at how to calculate the loss of conductor power in junior high school physics.
Joule's law is a law that quantifies the conduction current to convert electrical energy into thermal energy.
The content is: the heat generated by the current through the conductor is proportional to the square of the current, proportional to the resistance of the conductor, and proportional to the time of energization. That is the staff cost in the calculation process.
The mathematical expression of Joule's law: Q = I²Rt (applies to all circuits) where Q is the power loss P, I is the current, R is the resistance, and t is the time.
In actual use, because PSE and PD work simultaneously, the loss is independent of time. It is concluded that in the POE system, the power loss of the network cable is proportional to the square of the current and proportional to the resistance. In short, in order to reduce the power consumption of the network cable, we should try to make the current of the wire as small as possible, and make the resistance of the network cable smaller. The significance of the decrease in current is particularly important.
So let's take a look at the specific parameters of international standards:
In the IEEE802.3af standard, the resistance of the network cable is 20, the required PSE output voltage is 44V, the current is 0.35A, and the power loss P = 0.35 * 0.35 * 20 = 2.45W.
Similarly, in the IEEE802.3at standard, the resistance of the network cable is 12.5, the required voltage is 50V, the current is 0.6A, and the power loss P = 0.6 * 0.6 * 12.5 = 4.5W.
There is no problem with these two standards using this calculation method. But when the IEEE802.3bt standard is reached, it cannot be calculated in this way. If the voltage is 50V, the power of 60W needs to be 1.2A, and the power loss is P = 1.2 * 1.2 * 12.5 = 18W, minus the loss to reach the PD device. The power is only 42W.
4. Reasons for power loss of poe
So what is the reason?
Compared with the actual requirement of 51W, there is 9W less power. So what exactly caused the calculation error.
Let's take a look at the last column of this data chart, and carefully observe that the current in the original IEEE802.3bt standard is still 0.6A. Looking at the power supply twisted pair, we can see that it is powered by four twisted pairs (IEEE802.3af, IEEE802. 3at is powered by two pairs of twisted pairs) In this way, this method can be regarded as a parallel circuit, the current of the entire circuit is 1.2A, but the total loss is twice the loss when two pairs of twisted pairs are powered,
Therefore, the loss P = 0.6 * 0.6 * 12.5 * 2 = 9W. Compared with 2 pairs of twisted pairs, this power supply method saves 9W of power, so that the PSE can make the PD device receive when the output power is only 60W. The power can reach 51W.
Therefore, we should pay attention to reducing the current and increasing the voltage as much as possible when selecting PSE equipment, otherwise it will easily cause excessive power loss. The power sense of the PSE equipment is usable, but it is not available in practice.
Examples
A PD device (such as a camera) requires 12V 12.95W to use. If a 12V2A PSE is used, the output power is 24W.
In actual use, when the current is 1A, the loss P = 1 * 1 * 20 = 20W.
When the current is 2A, the loss P = 2 * 2 * 20 = 80W,
At this time, the greater the current, the greater the loss, and most of the power has been consumed. Obviously, this PD device cannot receive the power sent by the PSE, and the camera will have insufficient power supply and cannot work normally.
This problem is also common in practice. In many cases, it seems that the power supply is very large and sufficient, but there is no loss. As a result, the camera cannot work normally because of insufficient power supply, and it can't find the reason.
5. Poe power supply resistance
Of course, the above is the resistance of the network cable when the power supply distance is 100 meters. This is the available power at the maximum power supply distance, but if the power supply distance in actual use is relatively small, such as only 10 meters, the resistance is only 2, corresponding The loss is only 10% of the loss of 100 meters, so it is also important to fully consider the actual use when selecting PSE equipment.
100m resistance of various types of network cables of super five types of twisted pairs:
1. Copper clad steel network cable: 75-100
2. Copper clad aluminum network cable: 24-28
3. Copper-clad silver network cable: 15
4. Copper-clad copper network cable: 42
5. Oxygen-free copper network cable: 9.5
It can be seen that the better the cable, the smallest the resistance. According to the formula Q = I²Rt, that is, the least power is lost during the power supply process, so this is why you should use a good cable. It is recommended to use Category 6 cable for more stability. .
As we mentioned above, the power loss formula, Q = I²Rt, requires the least loss between the PSE power supply terminal and the PD power receiving device for the Poe power supply. It requires the minimum current and minimum resistance to make the entire power supply process ideal.