• Many friends have asked many times whether the poe power supply is stable? What is the best cable for poe power supply? Why use the poe switch to power the camera still no display? and so on, in fact, these are related to the power loss of the POE power supply, which is easy to ignore in the project.
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 making any changes to the existing Ethernet Cat.5 cabling infrastructure. At the same time, it can also provide DC power supply technology for such devices.
PoE technology can ensure the normal operation of the existing network while ensuring the security of the existing structured cabling, and minimize the cost.
A complete PoE system includes two parts: power supply equipment and power receiving equipment.

Power Supply Equipment (PSE): Ethernet switches, routers, hubs or other network switching devices that support POE functions.
Powered device (PD): In the monitoring system, it is mainly the network camera (IPC).
2. POE power supply standard
The latest international standard IEEE802.3bt has two requirements:
￼The first type: One of them is that the output power of the PSE is required to reach 60W, the power reaching the power receiving device is 51W (it can be seen from the above table that this is the lowest data), and the power loss is 9W.
The second type: the PSE is required to achieve an output power of 90W, the power reaching the power receiving device is 71W, and the power loss is 19W.
From the above criteria, it can be known that with the increase of power supply, the power loss is not proportional to the power supply, but the loss is getting bigger and bigger, so how can the loss of PSE in practical application be calculated?
3. POE power loss
So let’s take a look at how the loss of conductor power in junior high school physics is calculated.
Joule’s Law is a quantitative description of the conversion of electrical energy into heat by conduction current.
The content is: the heat generated by the current passing through the conductor is proportional to the square of the current, proportional to the resistance of the conductor, and proportional to the time it is energized. That is, the staff consumption generated in the calculation process.
The mathematical expression of Joule’s law: Q=I²Rt (applicable to all circuits) where Q is the power lost, P, I is the current, R is the resistance, and t is the time.
In actual use, since the PSE and PD work at the same time, the loss has nothing to do with time. The conclusion is that the power loss of the network cable in the POE system is proportional to the square of the current and proportional to the size of the resistance. Simply put, in order to reduce the power consumption of the network cable, we should try to make the current of the wire smaller and the resistance of the network cable smaller. Among them, the significance of decreasing the current is particularly important.
Then let’s take a look at the specific parameters of the international standard:
In the IEEE802.3af standard, the resistance of the network cable is 20Ω, the required PSE output voltage is 44V, the current is 0.35A, and the power loss is P=0.35*0.35*20=2.45W.
Similarly, in the IEEE802.3at standard, the resistance of the network cable is 12.5Ω, the required voltage is 50V, the current is 0.6A, and the power loss is P=0.6*0.6*12.5=4.5W.
Both standards have no problem using this calculation method. However, when the IEEE802.3bt standard is reached, it cannot be calculated in this way. If the voltage is 50V, the power of 60W must require a current of 1.2A. At this time, the power loss is P=1.2*1.2*12.5=18W, minus the loss to reach the PD The power of the device is only 42W.
4. Reasons for POE power loss
So what is the reason?
Compared with the actual requirement of 51W, there is 9W less power. So what exactly is causing the calculation error.

Let’s look at the last column of this data graph again, and carefully observe that the current in the original IEEE802.3bt standard is still 0.6A, and then look at the twisted pair power supply, we can see that four pairs of twisted pair power supply are used (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 whole circuit is 1.2A, but the total loss is twice that of the two pairs of twisted pair power supply,
Therefore, the loss P=0.6*0.6*12.5*2=9W. Compared with 2 pairs of twisted-pair cables, this power supply method saves 9W of power, so that the PSE can make the PD device receive power when the output power is only 60W. The power can reach 51W.
Therefore, when we choose PSE equipment, we must pay attention to reducing the current and increasing the voltage as much as possible, otherwise it will easily lead to excessive power loss. The power of the PSE equipment alone can be used, but it is not available in practice.

A PD device (such as a camera) needs 12V 12.95W to be used. 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, the PD device cannot receive the power transmitted 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 large enough to be used, but the loss is not counted. As a result, the camera cannot work normally due to insufficient power supply, and the reason cannot always be found.
5. POE power supply resistance
Of course, what is mentioned above is the resistance of the network cable when the power supply distance is 100 meters, which is the available power at the maximum power supply distance, but if the actual power supply distance is relatively small, such as only 10 meters, then the resistance is only 2Ω, correspondingly The loss of 100 meters is only 10% of the loss of 100 meters, so it is also very important to fully consider the actual use when selecting PSE equipment.
The resistance of 100 meters of network cables of various materials of super five types of twisted pairs: