There are 4 pairs of wires in the standard Cat.5 network cable, while only two pairs of them are used in 10/100Mbps. The following are the two methods allowed in IEEE 802.3af.

1.When the free pin is used to supply power, pin 4 and 5 are connected to the positive terminal and pin 7 and 8 are connected to the negative terminal.

The following diagram shows a 48V power transfer using the free pins(4,5,7,8).

2.The PINs (1,2,3,6) are used to simultaneously transmit the data and power (48V).

The DC power supply is applied to the midpoint of the transmission transformer when applying power to the data pins, in which case wire pairs 1, 2 and 3, 6 can be of any polarity. In this simultaneous transmission, the power won’t be affected by the data transmission as their frequency is different.

It does not allow using the above two methods at the same time in IEEE 802.3af. For instance, the PSE device can only provide one way, while the PD device must support both methods. The standard also specifies the power supply as 48V/13W for usual use. Although it is easy for PD devices to provide voltage conversion from 48V to low voltage, it still needs an insulation safety voltage of 1500V.

10/100/1000MBPS POE SYSTEM

There are no free wires in 10/100/1000Mbps PoE system, all the 4 pairs of wires are transmitting data and power. The voltages and signals from pins 1 and 2, pins 3 and 6 are split through a network transformer. The separated voltages are connected to a bridge rectifier for rectification of the power supply, as shown in the figure.

The following is the mode of PoE switch.

POE POWER SUPPLYING PROCESS

DETECTION

To detect the standard of PD device through a small voltage.

CLASSIFICATION

Classify and evaluate the power loss required for the PD device

POWER UP

Start to supply power to PD devices from low voltage until a 48V DC power supply is provided.

SUSTAINED POWER-UP

Provide a stable and reliable 48V direct power to meet the power consumption of the device, which is not exceeding 15.4W /30W.

DISCONNECTION

If the device is disconnected from the network, the PoE switch will quickly stop powering the device and repeat the detection process to check whether the cable terminal is connected to the device.