Question Clarifying PoE splitter for Pi4

[CryptoWizard]

OK, so I know something about the Power over Ethernet protocol. By no means an expert.

I'm currently thinking of implementing a Pi4 installed w OpenWrt (Open Router software). OpenWrt does support this. I want to try the Pi4 with a PoE splitter. I now that you can get various PoE Hats that implement PoE on Pis. Thats a possibility, but I would be more comfortable with a splitter.

OK, so I know there are 3 different implementations of PoE: 802.3AF (15.4w/ port), 802.3AT (25w / port), 802.3BT (50-70w / port). My PoE switch for use w the Pi4 is 802.3AT (PoE+, 25w/port). The manuf advertises 30w/port.

Pi w OpenWrt requires a USB/ETH adaptor, so that the Pi has a WAN NIC & a LAN NIC. So at least 1 USB port will be drawing power.

A potential PoE splitter for the Pi is....
https://www.amazon.de/-/en/dp/B0CHW...0U9K0AU2&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

I understand that PoE devices will negotiate the voltage to draw from the PoE switch, so the switch is NOT sending 48V on all its ports. 48volts seems to be MAX that can be sent from each switch port.

Normal AC power supplies for a Pi4 draws quite a lot of juice... ca 2.5a

When calculating how many watts the Pi needs from this switch's port, I assume I use 5v (usb power drawn), not 48v, the theoretical max that can be drawn by the splitter? Will the splitter be drawing (input) MORE volts than its outputting, or does it only need to draw 5v, or a little more than 5v to cover any overheads (step-down transformer perhaps) inside the splitter?

watts = volts x amps.

watts = 5 x 3 = 15 watts.

So, at a stretch, even an older 802.3AF (15w) switch could potentially power the Pi4, & 802.3AT (30w) is supplying PLENTY for the Pi4.

Can someone confirm that I should be correct in my assessment, or have I gone wrong here somewhere??

 

[kanewolf]

OK, so I know something about the Power over Ethernet protocol. By no means an expert.

I'm currently thinking of implementing a Pi4 installed w OpenWrt (Open Router software). OpenWrt does support this. I want to try the Pi4 with a PoE splitter. I now that you can get various PoE Hats that implement PoE on Pis. Thats a possibility, but I would be more comfortable with a splitter.

OK, so I know there are 3 different implementations of PoE: 802.3AF (15.4w/ port), 802.3AT (25w / port), 802.3BT (50-70w / port). My PoE switch for use w the Pi4 is 802.3AT (PoE+, 25w/port). The manuf advertises 30w/port.

Pi w OpenWrt requires a USB/ETH adaptor, so that the Pi has a WAN NIC & a LAN NIC. So at least 1 USB port will be drawing power.

A potential PoE splitter for the Pi is....
https://www.amazon.de/-/en/dp/B0CHW...0U9K0AU2&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

I understand that PoE devices will negotiate the voltage to draw from the PoE switch, so the switch is NOT sending 48V on all its ports. 48volts seems to be MAX that can be sent from each switch port.

Normal AC power supplies for a Pi4 draws quite a lot of juice... ca 2.5a

When calculating how many watts the Pi needs from this switch's port, I assume I use 5v (usb power drawn), not 48v, the theoretical max that can be drawn by the splitter? Will the splitter be drawing (input) MORE volts than its outputting, or does it only need to draw 5v, or a little more than 5v to cover any overheads (step-down transformer perhaps) inside the splitter?

watts = volts x amps.

watts = 5 x 3 = 15 watts.

So, at a stretch, even an older 802.3AF (15w) switch could potentially power the Pi4, & 802.3AT (30w) is supplying PLENTY for the Pi4.

Can someone confirm that I should be correct in my assessment, or have I gone wrong here somewhere??

Based on this -- https://www.pidramble.com/wiki/benchmarks/power-consumption your power usage for a PI4 is off. A standard POE (.af) has plenty of capacity for a PI4. I use the official PI4 POE hat to power one of mine. It has a fan in addition to POE power.
A POE 802.3af switch does handshake to enable POE power. Many POE switches don't have POE on all ports. Most POE switches are managed switches and you can manually disable POE if you are concerned.

 

[CryptoWizard]

Based on this -- https://www.pidramble.com/wiki/benchmarks/power-consumption your power usage for a PI4 is off. A standard POE (.af) has plenty of capacity for a PI4. I use the official PI4 POE hat to power one of mine. It has a fan in addition to POE power.

Appreciate the link, very interesting; esp interested in the power meter.
So.... power consumption IS much lower than I anticipated.

I was just going via the max rating of the power supply I prev had on a Pi4.
Initially I powered a Pi4 via a normal usb adaptor, but I kept having Raspbian throwing up a power warning on the desktop. So I did a little research, & discovered that I was UNDER powering the little device. I think the usb adaptor was rated at 2amp (can't remember). I then bought an official Raspberry usb adaptor, & no more warnings. I seem to remember the official usb power rated at 2.5 amps. Could be wrong. I just upgraded that to 3 amp in the online calc.

1.2 amps is much better.

Anyways, certainly my PoE+ switch will have zero problems, glad to hear that....

Appreciate your feedback.