Power supply circuit, relay device and power over ethernet system

A relay device for connecting a first network cable with a second network cable, comprising:

• a relay circuit;
• a power supply circuit for powering the relay circuit, comprising at least one rectifier module and a switch control module, wherein, each rectifier module has an input end connected to an input interface of the relay device, a positive output end connected to a positive supply pin, and a negative output end connected to a first end of the switch control module, wherein, the positive supply pin is connected to a positive terminal of the relay circuit;
• and the switch control module has a second end connected to a negative supply pin, and a control end connected to the positive supply pin, wherein, the negative supply pin is connected to a negative terminal of the relay circuit;
• and wherein, the switch control module allows conduction between the first end and the second end when the rectifier module outputs a voltage greater than a preset threshold;
• and
• a first PHY chip connected to the first network cable and a second PHY chip connected to the second network cable;
• wherein, a data transmission interface of the first PHY chip is communicatively connected to a data reception interface of the second PHY chip, a data reception interface of the first PHY chip is communicatively connected to a data transmission interface of the second PHY chip, and the power supply circuit is connected to and powers the first PHY chip and the second PHY chip.