I’m planning to power my MikroTik RB5009UPr+S+IN using a 57V passive PoE injector. I understand that the device supports passive PoE-in, but I have a few questions:
Is it safe to use a 57V passive PoE injector via the Mode B ethernet ports on this?
What is the maximum amperage I can expect with this setup?
Are there any specific considerations or limitations I should be aware of when using this power configuration?
Any insights or experiences with similar setups would be greatly appreciated!
The RB5009UPr+S+IN accepts (per specs) 18-57 V so 57 V is OK.
The Mode A vs. Mode B is one of the mysteries of Mikrotik, generally and specifically for this device.
Specs are:
“PoE-in: 802.3af/at (ether1), Mode B (ether2-ether8)”
which - translated - means that port ether1 will support active PoE (and Mode A) and the other ports passive PoE (and Mode B).
This is not fully 802.3af/at compliant, see also: http://forum.mikrotik.com/t/rb5009upr-poe-management/164558/1
Anyway both PoE modes will allow to power the device that in itself is specced at 16W, at 57V that would be 16/57=0.28A
The PoE out is another issue, each port is limited to 440mA in the 30-57V, so at 57V that is 570.44=25.08W (as well not fully compliant with 802.3af/at, with the PoE out as PSE it should have 30W).
BUT there is a further limitation at total output current 2.28A (57V2.28=129.96W) and total output power 130W.
This more or less means that you can draw the max current on only 2.28/0.44=5.18=5 ports concurrently.
AND you cannot expect to power it with 802.3af/at (the at is max 25W) and power from it anything but a single low power device, let’s say a low power AP.
Now, if you power it through one of ports ether2-8 with passive PoE (Mode B) logically you cannot inject in it much more than the 440 mA the single port can output, so it doesn’t change the result.
After all if we go back to 802.3at with 2 pairs mode (no matter if mode A or mode B) the max power delivered by the PSE is 30W and to be able to deliver 60 or 90 W you need the all 4 pairs mode.
PoE (active, 803.3af/at, two pairs) has power limits. the max power a PSE can provide:
af=15.40 W
at=30 W
and current:
af=350 mA per pair
at=600 mA per pair
This means for the higher spec at that 30 W / 57 V= 526 mA max
The Mikrotik reduces this (on PoE out ports) to 440mA.
These same PoE out ports can be used as PoE in ports with Passive PoE (Mode B), but there is no reason to presume that the accepted current in will in will be more than 440 Ma.
On the other hand, the higher bt specs have 90 W and raise the allowed Amperes on a pair to 960 mA, so pushing 1A might be fine (in the sense that the cable will be able to carry that current without overheating).
So, to your questions:
Mode B means that the + is on the pair 4,5 and the - is on pair 7,8, this is what the Mikrotik will accept, what is put on the line depends on your power supply/injector.
You can push as many A as you want, but if - as I believe - the 440 mA limitation is bi-directional they won’t get through, BUT you can put a PoE splitter (passive), and extract the power from the ethernet cable and feed it to the RB5009 jack or two pin DC input (NOT on one among ether2-8).
Thanks for the explanation. I was mainly hoping to avoid an additional point of failure on the router side by adding a PoE splitter but looks like that is the safest way to go.
Unless someone from Mikrotik support says otherwise regarding the max current rating.
With all the devices you have in play, a splitter, which is essentially passive has (IMHO) very low probabilities of being a point of failure, and - even if it fails - it is the least expensive item to replace.
Even if the original Mikrotik Is 8$ list price, similar “no-name” ones can be found (though It has to be seen , most are 10/100, not Gb) for less.
Personally I would put the power supply as #1 possible point of failure, the more likely issues are peaks/surges on mains and those will affect the first item on the chain, i.e. the power supply (or the UPS if you have one).
Only as a curiosity, and only if you wish to share this information, what “role” will have the RB5009?
In your network layout that place seems more suitable for a switch than for a router.
