Power Over Ethernet (PoE) technology enables Ethernet cables to deliver both power and data to connected devices, simplifying installations and reducing the need for separate power supplies. However, before the PoE system can distribute power, it must undergo a crucial process known as electronic handshaking. This process ensures that power is only delivered to devices that require it, and it occurs in three distinct stages: Detection, Classification, and Under-Voltage Lockout (UVLO).
1. Detection
The first stage in the electronic handshaking process is Detection. When a Power Sourcing Equipment (PSE) unit powers on, it begins by outputting a ramp voltage across the Ethernet cable’s wire pairs. The primary goal at this stage is to detect whether there is a load impedance connected to the wire pairs. A Power Device (PD) that needs power will exhibit a resistance of 25k Ω on the wire pairs, which the PSE can detect. If the PSE identifies this impedance, it signifies the presence of a PD that requires power.
Conversely, if the PSE does not detect any load impedance, it concludes that no PD is present and therefore does not deliver power. In this scenario, the PSE functions solely as a standard Ethernet switch, handling data transmission without powering any connected devices.
2. Classification
Once the PSE determines that a PD requires power, it proceeds to the Classification stage. Here, the PSE supplies the PD with a voltage ranging between 15.5 to 20.5 volts and monitors the current flowing through the cable. This process allows the PSE to ascertain the power requirements of the PD.
Each PD is equipped with an internal power classification resistor that informs the PSE of its power needs. These classification values are predefined and help the PSE in determining how much power to allocate to each PD. For example, if the PSE has a total power output capacity of 100 Watts, and each device is classified under Class 0, the PSE can theoretically support approximately 7 devices (100 Watts / 12.95 Watts per device).
3. Under-Voltage Lockout (UVLO)
The final stage in the handshaking process is Under-Voltage Lockout (UVLO). If the voltage supplied by the PSE drops below a certain threshold, the UVLO mechanism ensures that power delivery is stopped to protect the PDs from operating under insufficient voltage conditions. This safeguard helps in maintaining system stability and prevents potential damage to the connected devices.
In summary, the electronic handshaking process in PoE systems ensures that power is efficiently and safely distributed to devices that need it, optimizing both power management and system performance. By following the stages of Detection, Classification, and UVLO, PoE technology supports seamless integration of power and data over Ethernet, benefiting a wide range of applications from IP cameras to network switches.
This technology allows the PSE to characterize the power requirements of each PD and manage the power distribution.
Note: A PSE does not force power to a PD; the PSE will only deliver power if the PD requires it.
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Part 1 of 3: Particle Counters and Power Over Ethernet Series
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