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PoE Detection, Classification and Line Losses

POE Switch

Here at Savant, we are working on a project where we need a circuit that can draw power from a PoE (Power Over Ethernet) switch. The switch will only provide power if it detects that a PD (Powered Device) is connected to it. This is a safety precaution so that the switch will not send voltages to a device that is not PoE capable or possibly exposed wires.

The detection process is relatively simple. The PSE (Power Sourcing Equipment) detects the resistance and the capacitance. A valid device has a resistance between 19 kΩ and 26kΩ. It also has less than 150nF of capacitance. Once a valid device has been detected, the PSE moves to the classification stage. It classifies the device by measuring the current flowing through the device. Once the device has been classified, the PSE increases the voltage to the normal operating voltage and provides power until it determines that the device has been disconnected.

We decided to use the TPS2375 from Texas Instruments as the PD controller. It handles all of the detection and classification aspects of PoE and regulates the power that can be used by the circuit.

The PSE has to be capable of providing 15.4 W of power for a class 3 device. However, the power delivered to the PD is less due to losses in the cabling. Cat 5e cable is 24 AWG. The resistivity of copper is approximately 17nΩ per meter. The formula for resistance is given below.

Equation 1 PoE Detection, Classification and Line Losses

If we use the maximum segment length for Cat 5e (which is 100m) and use 0.51 mm for the diameter of the wire, we end up with the following resistance. We divide the 0.51 by 1000 to convert millimeters to meters.

Equation 2 PoE Detection, Classification and Line Losses

The power is delivered on two pairs. That means that the resistance is in parallel. However, you have to take into effect that you have the resistance going to the load and the resistance coming back.

Equation 3 PoE Detection, Classification and Line Losses

In order to calculate the power loss, we multiple the resistance times the current squared. For an example, let’s say that we have a device that is capable of drawing 250mA.

Equation 4 PoE Detection, Classification and Line Losses

In this example, the power loss is only about 5% which is very small. As you decrease the length of the cable, the power loss will eventually be negligible.

About Jason Harris

Jason is a network technician at Savant Ltd. He specializes in wireless communications and server infrastructure.

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