If " R" turns out to be 25 ohms, PSE concludes indeed PD is connected. Now the 2nd stage comes"Classification of PD"
During classification, PSE offers 14.5v-20.5 volt( PD is still in off-state), PSE responds by inserting "rclass" resistance.
1)How this rclass resistance helps PSE determine the class of PD?.
It appears to me there is some sort of mapping between current and Power needed by the PD.
My logic is If PSE needs to find the current flowing in the circuit to determine the class of PSE, then we may deduce that there must be some mapping between the current and class(power requirement by PD).
How does PSE determine that current?
Form your reply:
"when the switch decides to power on the port it applies the full voltage 42V.
With an higher voltage a lower current is needed to provide the same power"
Let say PSE apply power 42 watts
For PD, to use that power fully( same power) (42 watts)( ignoring the power loss on the line)
following must be true:
Impedance of PSE= impedance of PD, otherwise , the power will not be transferred fully.
1) PD is not "ON" yet, how can it offer load current?
There are 2 options with PD like cisco IP phone,
First you connect them with external power like your PC.Then you can connect them to switches to which you connect ur PC etc.
Second they are not connecetd to any power source you just connect a RJ 45 cable like you connect ur PC. PSE detect them and provide power and they are ON. Like a switch detects a signal from end device and u see a green light on switch. Difference here the detection protocol is detecting from a non powered device
what is Rclass pin? just dropped it in the link without any explantion?
Not an expert on this topic ,I am also reading about PoE and didn't try to dig the theory behind the process. Maybe someone else can help you on this.
Few links on google from companies who makes that chip,
the question is does switch has to allocate this much voltage (above 42 v) per port, then it can add up as switch could many poE ports.
PoE equipments have power modules with higher power rating than a switch without PoE.
The Cisco Catalyst 3560-E Series access switches have one power supply slot and support the following power supplies. PoE switches require a PoE power supply. Data-only switches can operate with any of the power supplies:
â¢ C3K-PWR-1150WAC: 1150WAC power supply with 740W PoE
â¢ C3K-PWR-750WAC: 750WAC power supply for 24-port switch with 370W PoE
â¢ C3K-PWR-265WAC: 265WAC power supply for 48- or 24-port switch without PoE
â¢ C3K-PWR-265WDC: 265WDC power supply for 48- or 24-port switch without PoE
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