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RB750P-PBr2 4 node POE switch

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nc8q's picture
RB750P-PBr2 4 node POE switch

Solved, finally.

I would like to add nodes to a limited access site where it is awkward to add additional ethernet cables.
I bought a Mikrotik RouterBoard RB750P-PBr2 Power Box over 16 months ago.
Today I discovered how to configure this box to be an outdoor POE powered 5 port node switch with pass-through POE.
I am using the POE port (ether1) as DtD downlink with 4 ports for nodes.
I have 4 devices POE powered and DtD connected to a NODE port of a GS108Ev2 managed switch.
To simulate a real world environment to test voltage drop I have
Ubiquiti 24 volt 1 amp (gigabit) POE injector
50 feet of CAT5E ethernet cable to the Power Box
50 feet " to a Nanoloco M2
100 feet " Nanostation M9
100 feet " Nanostation M3
50 feet " Nanostation M3

This appears to be working. YMMV.

Configuration of the Power Box is a 1 line, 7 word, 74 character file.
Starting with a reset of the Power Box:
I ftp'ed the 1 liner to the /flash directory of the Power Box.
(I used Filezilla. Flash memory is retained after a 'reset'.)
I hooked a computer to port 5 of the Router Box.
I viewed on the web browser of the computer
The browser auto-logs-on with no 'admin' password (default).
Click on the 'Terminal' button near the top right of the screen.
Enter "import flash/1.txt" (without the double quotes).



File Attachment: 

What resultant voltage is the Mikrotik reporting with all 4 nodes powered on?

I've used the PowerBox quite a bit (it's just a HeX POE in an outdoor case) but have always given it power on the DC jack if more than one downstream device. Good someone decided to test the limits!


nc8q's picture
Voltage measurements

The stock supply is 24 volts @ 2.5 amps.
I didn't find the stock power supply and used an old laptop supply, 19 volts @ 4.74 amps.

Power supply -> 118' CAT5 -> Power Box, then 50 or 100 feet CAT5 feeding the nodes:
With 4 nodes, NS-M9, NS-M3, NS-M3, locoM2; the 19 volts drops to 15.4 volts (at the Power Box) with total draw ~970 ma.
I assume the Power Box pulls 100 ma so the total draw on the PS is ~ 1070 ma.

(This setup fails with the Ubiquiti 24 volt, 1 amp POE injector; Power Box reporting 'short circuit'.
Ditto with 18.5 volt old laptop supply.
Only the locoM2 and one of the NS-M3s would work with these supplies.)

Apparent milliamp draw increases with lower power supply voltage.
This may be a 'switching power supply' issue.
Need to test this setup with stock power supply.



nc8q's picture
Found original power supply

Labeled: 24 volts at 2500 ma
No-load: 24.3 volts
Loaded : 21.1 volts at ~830 ma, ~730 ma (4 nodes) + ~100 ma Power Box, with above cabled setup.

Port 1

I'm not sure what port 1's max current rating is. The specs are kind of hazy but people on the forums say around 1A. It looks like you are right at the limit of "normal" PoE of 15.4W. To get full power to all ports you can use the round DC port, but I know you are trying to avoid extra cable. If the RJ45 circuitry can't handle it I guess you could split off the PoE and feed it into the DC jack.

In any case, I would look at an adjustable 24V supply to offset that 20% voltage drop and lower the current. I use models like this in various settings: (27.6V max) (29V max)


nc8q's picture
Just for grins I added 2 more nodes.

Just for grins I added 2x NS-M5s.
Stock power supply rated 24vdc @ 2500ma -> 118' CAT5 -> Router Box ether1
ether2 -> 100' CAT5 NS-M3
ether3 -> 50' CAT5 NS-M3
ether4 -> 50' CAT5 NS-M5 -> 6' CAT5 -> locoM2
ether5 -> 50' CAT5 NS-M5 -> 6' CAT5 -> NS-M9

NS-M3, NS-M3, NS-M5, NS-M9 set for minimum TX power, locoM2 WiFi client, NS-M5 WiFi AP.
Sum of 4 ethernet ports draw ~ 1000ma for 6 nodes and are sent 20.3 vdc from Power Box.

CAT6E may have less voltage drop and lower apparent current.


nc8q's picture
Regulated by voltage at load

I suggest that, if one wishes to provide 24vdc to the Power Box with an external power supply,
get one with separate terminals for 'sense remote voltage'.
(I've used these in an industrial setting back in the second millennium. Are they still being made?)
Use one pair from ethernet cable (4,5)(7,8) to send power and another pair to return actual voltage at the Power Box.
This would provide regulated 24vdc at the Power Box under varying load.

Next, find one we want to afford.


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