The POE power supply and the actual requirement of 51W have reduced 9W of power. So what exactly caused the calculation error.

Let's look at the last column of this data chart again, and take a closer look at the current in the original IEEE802.3bt standard is still 0.6A. Looking at the twisted pair power supply, we can see that it uses four pairs of twisted pair power supply (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 the two pairs of twisted pairs are powered, so the loss is P = 0.6 * 0.6 * 12.5 * 2 = 9W. Compared with 2 twisted pairs, this power supply method saves 9W of power, so that the power received by the PD device is only 60W when the output power is only 60W. Able to reach 51W.

Therefore, we must pay attention to minimize the current and increase the voltage when selecting PSE equipment, otherwise it will easily lead to excessive power loss. The power feeling of PSE equipment alone can be used, but it is not available in practice.

For example

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 larger the current, the greater the loss. Most of the power has been consumed. Obviously, this 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 there is no loss calculation. As a result, the camera cannot work normally due to insufficient power supply, and the cause is always not found.

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