Is there a way to tell PoE injector to supply specific voltage?
I use Hex S, which supports 48 V and will use Unifi U-POE-at injector, which is capable of 48 V. I need that voltage, because I’m feeding that voltage via PoE out port to a device that requires 48 V.
Explain better
It seems so (802.3at, output voltage 48VDC @ 0.65A):
https://eu.store.ui.com/collections/operator-accessories/products/u-poe-at
How PoE works:
https://www.versatek.com/poe-negotiation-of-power-power-step-down-of-the-poe-protocol/
https://shop.dbg.co.za/index.php?route=blog/blog&blog_id=47
i think i understand - “active” protocol only assures that
- recepient actually requests poe
- voltage drop on the line is taken into account, up to certain amount.
it does not mean that device will request certain voltage, say 10V, and at will be able to provide it. 48 V is nominal for PoE+, fullstop. is that correct?
No, hEX S will always supply input voltage on PoE out. If you connect hEX S to a 48V power adapter, then it will only output 48V … so better take care of what voltages are supported by devices connected to PoE out port of hEX S.
Also note that hEX S only support passive PoE out … which means that if connected device (PD in 802.3 af/at jargon) requires PoE handshake to occur, it won’t work. However, PDs are mostly not sensible, it’s up to PSE (power providing device) to make sure the other end can handle 48V … and since hEX S supports only passive PoE it’s then up to you to make sure powered device can handle voltage thrown at.
So yes, your understanding outlined in previous post is correct.
Yes, Metod, I’m worried about this “same output as input voltage”. What if active injector decides 44V is enough (and what are criteria for him to do so) ?
You do not understand anything from previous posts and previous links.
Take a look here https://wiki.mikrotik.com/wiki/Manual:PoE-Out
and here https://help.mikrotik.com/docs/display/ROS/PoE-Out
None of Mikrotik PoE PSEs (devices with PoE out) have voltage regulators. So apart from the 802.3 af/at handshake it’s simple power switch … either it passes voltage (supplied to the device) or not.
There are only a few exceptions where things are (only slightly) less trivial: some devices have two power inputs, each taking different voltage (e.g. netPower 16P) where it is possible to select which of supply voltages will be applied to each port (that’s manual selection, no magic involved).
@mkx there are devices such as CRS328 and CRS354 that do support voltage selection ( LOW, HIGH ) and do not need 2 PSUs…
Obviously they do have voltage regulators 
Just fresh at this and don’t want to jump to conclusions. I think you aim at “uni ap at+ injector will provide 48 V & 0.65 A, if client introduces itself as at+ client, else 0 V”, but want to be sure there is no gotchas.
My setup: active PoE (at+) injector → MT hex s → 10m cable → unifi ap (PoE at+ / passive 48 V)
from this link https://shop.dbg.co.za/index.php?route=blog/blog&blog_id=47 I read
802.3af/at also has some ‘smart functionality’, as the PoE source (switch or injector) will automatically negotiate the voltage with the PD.
So, does this mean poe active devices have some means of adjusting voltage and / or current limitation?
My fear is simple - what if mikrotik negotiates voltage, that is too low for my unifi device, because mt is passive on poe out? preferably, i’d like to know criteria used, if this happens.
@Edit: Okay, it seems active PoE client Type (1, 2) depends on a resistor in the input, so no magic there… I guess the at+ injector is then happy and provides its nominal voltage, 48 V in my case. Thanks. Did I got it right or still missing something?
You’re correct. I forgot about devices with AC input … those better don’t apply input voltage to PoE out ports.
My fear is simple - what if mikrotik negotiates voltage, that is too low for my unifi device
From a quick look to some of my production equipment a CAP AC and a wsAP for example, that are capable of af/at at POE in are fed with 52V from a CRS328…
I have to admit that I haven’t actually read the standard. In such cases I tend to trust wikipedia articles more than some blogs on e-commerce sites. So here’s my reference on 803.2 af/at PoE: https://en.wikipedia.org/wiki/Power_over_Ethernet … it doesn’t mention that PSE and PD would actually negotiate voltage, it mentions they negotiate power class (and I’m not sure what exactly that means … possibly it both allows PSE to detect over-current condition and allows power management when PSE total capacity is reached). I guess some PoE PSEs might vary output voltage slightly, but they should still remain within tolerances. But Mikrotik devices don’t do it.
All sources coerce in this case.
For all I read, there is power negotiation between PSE and PD (via LLDP or the resistor I guess) and the PSE valid output is 44-50V.
Which means my fear may be valid - what if Hex negotiates too little power and / or voltage for the device it powers (via passive poe out)?
OK, so you choose not to accept my explanation and chose to interpret information sources in a completely different meaning. Over&out.
Which means my fear may be valid - what if Hex negotiates too little power and / or voltage for the device it powers (via passive poe out)?
Why don’t you just test it ?
I don’t have any of those devices yet and I prefer to verify my setup before buying everything…