48V or 57V power supply for hEX PoE?

Hardware MikroTik hardware general
1

I need to power a PoE router that can power my two TP-Link EAP653 access points. The specs for this AP says:

Interface: 1× Gigabit Ethernet (RJ-45) Port (> supports IEEE802.3at PoE> )
Power Supply: 48V Passive PoE

I’m a bit confused because Mikrotik sells both a 48V PSU and also a 57V PSU. If I use the 57V PSU, will I get 57V at the PoE-out ports, or will the voltage be negotiated?

The PoE Wikipedia page says, for 802.3at:

Voltage range (at PSE): 50.0–57.0 V
Voltage range (at PD): 42.5–57.0 V

where: PSE = Power Sourcing Equipment, PD = Powered Device.

Looking at those numbers, I’m sure you can see where the confusion originates. What voltage should I use to power the hEX PoE then? Are 48 V power supplies used for 802.3af PSEs and 57 V power supplies for 802.3at PSEs?

2

The 57V are 57V (minus losses on the cable), the 48V are 48V (minus losses on the cable) there is no voltage conversion.
The negotiation is only about providing the “full” voltage available.
Very basically the negotiation goes something like (it depends if using LLDP or not):
Q(PSE): Hi, is there a device on the other end of this cable
A(PD): Yes, I am here
Q(PSE):Good, which kind of device are you?
A(PD): I am an 802.3at device.
Q(PSE):That’s what you say, let me make sure, are you a 802.3at device?
A:(PD):Yes, I am an 802.3at (class 4) device.
A:(PSE):Ah, ok, I am giving you all the voltage I have.

There may be further issues, PoE is a mess.
There are two “standards” for PoE, mode A and mode B, the first uses to transmit power the same four wires used for data (1,2,3,6) in 100 Mb cables, the second uses the “unused” ones (4,5,7,8).
Actually compatible with 802.3/at PSE (Power Sourcing Equipment) devices can provide and PD (Powered Devices) should accept both Mode A and Mode B, but there are devices that are not fully compliant and require either the one or the other, while passive only devices are usually either Mode A or Mode B.
Mikrotik devices use Mode B.

In theory, 48V at PSE is not 802.3at compliant (not in the range 50-57V) while it is 802.3af compliant (in the range 44-57V), but what really counts is voiltage at the PD, unless you have terrible or very long cables you should loose no more than 2 or 3 V, so, even in the worst case you will have 45 V at the PD which is compliant with both.(37-57 and 42.5-57).

From the specs the EAP653 can be powered BOTH with 802.3at and with passive 48V PoE, as always there are not enough details, but it seems like you are in one of those cases where everything should just work, the note about passive 48V very likely means that it can accept also non-negotiated power supplies (this is not AFAIK mandated in any standard).

The specs call the device 802.3at, while stating max power at less than 14W, so you won’t have issues with the 450 mA the Hex Poe can deliver at either voltage:
48V x 0.45A=21.6W
57V x 0.45A=25.65W
even in the worst condition, 45V x 0.45=20.25, you have plenty of slack for a 14W AP.

3

Such a detailed analysis—Thank you! Agreed, PoE is a bit of a mess, too many “standards” and manufacturers mix and matching bits of each standard even into the same product, whilst providing ambiguous information.

I’ve been testing one EAP653 with a 48 V Unifi PoE injector over a 30 m CAT5 run (to be shortened to about 20 m once properly installed) for a week and so far I haven’t noticed any issues, so voltages are probably within the EAP653’s requirements. I might measure it just for peace of mind anyway but, as you said, it looks like anything will just works (if 48 V works 57 V should work too) so is there any good reason to use 48 V vs a higher voltage (e.g. versatility, stability…)? Are there any trade-offs, or should I just get whichever is cheaper?

4

The idea is that the higher voltage “travels better” on long distances.

In the case of a Mikrotik router/switch used as a PSE there is the additional issue that the output Amperage is low, so to declare the device actually 802.3at compliant they (IMHO) stretch a bit the definitions.

The standard says that:

  1. the PSE should provide up to 30.0 W
  2. the power available at the PD should be 25.5W

So, the limit (Hex PoE) of 450 mA means that the PSE is not fully compliant even at 57V (57V x 0.45A=25.65W<30) let alone at 48V (48V x 0.45A=21.6W<30W).

In theory if you have a 25.499W PD (fully 802.3at compliant) and you attempt to power it from a Hex Poe, it will work only until the cable is long enough to loose enough voltage/power, you can play a little with this calculator:
https://www.poetexas.com/poe-calculator/

Since your device is only 14W or so you don’t really need the 57V, the 48V would be enough on 30 m or so distance.

The 48V is more “common” than 57V, so it can be reused more easily in different setups, if it is a fixed/stable setup, it makes no difference, get the cheaper one (though it will probably be still the 48V).

5

Thanks. Right, so the Mikrotik 48V2A96W adapter costs around 40 € where I live. I’m sure it’s just a generic AC to DC adapter but damn… I’ve seen used 5-port managed PoE+ switches going for less, with a PSU included, and I still would need to buy a hEX PoE to connect my PoE APs.

While I was typing this post I found a used Netgear GS308EPP (8-port gigabit with a 123 W power budget) un-opened for 55 €. I couldn’t resist. Even if get something else later, I can sell this switch at a profit anyway. If Mikrotik later releases a hEX S PoE+ (or whatever they name it) I can buy it then. TBH for a short while I was sure I’d be getting a RB5009UPr+S+IN since it was the only Mikrotik that fit my needs, except it is a lot bigger, more power-hungry, overkill and considerably more expensive. Now I’m set for 1/6 of the price. There’s Mikrotik’s missed opportunity.

6

Actually, the last step is rather

A:(PSE): Ah, ok, I am giving out 30W max and the voltage I have.

Different PD categories mean different max power and if PD wants more than PSE can provide, powering up may actually fail (PSE rejects to provide power).

7

A fully compliant 802.3at PSE will probably “say” that, a Mikrotik Hex Poe could:

  1. Lie, blatantly, and say that nonetheless.
    or:
  2. Not mention Watts at all as in my example.
    or :wink: :
  3. Ah, ok, I am giving out all the Watts I have max and the voltage I have, if my Watts (currently 25 or so) when they arrive to you are not enough, bad luck, pal, your problem. not mine. :laughing:
8

I’m using a 48V psu to a hEX S to feed a camera. works great.