Hi
I recently buy a SXTsq5ax, all works ok, but at powering, if I connect directly to my RB5009, router show “voltage_too_high” and if I force POE I see that it want 48V. On datasheet say:
Number of DC inputs 1 (PoE-IN)
It’s normal?
Is SXT broken?
Where exactly are you seeing that?
If you are powering the RB5009 at 48V, the PoE out is at 48V.
The SXTsq5ax wants instead 24V.
If you force PoE out on (@48V) the SXT won't like it.
You either power the RB5009 @24V or you use a separate power injector/PSU for the SXT.
Thanks @jaclaz for your attention.
I asume that 48V is what SXT want, but you are on right, other devices like wAP and cAP get 48V too.
I powered RB5009 with the official AC/DC convert to 48V. Do you know if I change de power supply for a 24V, wAP and cAP will work?
Thank for your help!
Work all @24V
Why? Especially when it says https://mikrotik.com/product/sxtsq_5ax#product_specification
There are several different models of wAP and cAP, so better check.
Generally speaking the cAP's and wAP's have wide PoE in specs (11-57V or 18-57V), so they will work @24V just fine (though - depending on distance between them and the RB5009 using 24V might be not the smartest idea).
There is also the solution of using an inline 802.3AF/AT to 24V Passive converter. Then you can power it from the RB5009 and its original supply and it will get the correct voltage.
MikroTik RBGPOE-CON-HP or Ubiquiti Instant 802.3af Adapter (INS-3AF-I-G)
"The datasheet specifies 12-28V so I assume it works with 48V!"
Yes. Logic. None.
Well, doesn't that mean that it works at -16V? 
It surely works at -16.
there are jokes going on ground here
There is a great Croatian saying: “Pretpostavka je majka svih zajeba.”
Which roughly translates as: Assumption is the mother of all screw ups.
Im impressed you didn’t fry the sxt with 48V…
Easiest way to power your sxt would be to use poe injector and power adapter you got with the router. Just disable PoE on that port on RB5009.
Or as @pe1chl suggested, use converters.
Thanks to all, your are incredible!
Yes! I’m happy with the mikrotik team desicion to put power protection on SXT 
I think that POE was adaptatives to load necessity, and now I learn that POE is a by-pass power.
Vast majority of PoE switches are like that. The ones supporting standard 802.3 af/at/bt require powering with 48V+ power supply, the passive ones may tollerate (or require) other supply voltages. All of them simply pass supplied voltage to PoE-out ports. The only negotiation between PSE (PoE switch) and PD (powered device) is whether connected device is actually PD (supplying power to non-PoE device can destroy that device) and power class of PD (which allows PSE to not provide power to PD which requires more power than PSE can supply).
The negotiation is never about voltage. The rare PSEs that are able to supply different voltages require manual setting of PoE-out voltage and it's only possible to select between voltages supplied to the PSE (either via multiple power supplies of different voltages or via built-in multi-voltage power adapter; none of PSEs I know have internal DC-DC voltage coverters for PoE use).
And as with any manual settings one has to be extremely careful not to set things the way it can cause damage (e.g. forced PoE on). And that requires careful reading of specs rather than wishful assumptions.
I am still wondering if the CRS418-8P-8G-2S+RM can really supply +48V 802.3AF/AT or 24V passive PoE in a mix on the PoE outputs (and without power supply modifications). Newsletter #127 says yes, but in the specification on the product page itself this is not mentioned.
Product page (the propaganda part of it) says
PoE-out part of specs also talks about "low voltage" and "high voltage" limits.
But yes, nothing is explicit about concurrent availability of both low voltage and high voltage PoE-out. The "related" product TN19-0250-55 (Open Frame 250W replacement PSU for the CRS418 series) also only mentions 55V output. But that could be an omission as well, PSU has 6 pins on output header and could actually provide 3 different voltages (12V for the board, 55V for high-voltage PoE-out and 24V for low-voltage PoE-out). The only way of knowing the whole truth would be to do measurements on actual device.
Yes our group has bought them, I hope to get one soon to see how it works.
I can confirm the CRS418-8P does indeed support both high and low PoE-out voltages on different ports at the same time, similar to the older CRS328-24P.
TN19-0250-55 PSU datasheet https://www.tigerpower.com/wp-content/uploads/TN19-0250-XXe.pdf shows just a single output (3 pins each positive and negative), so step-down DC/DC converters on the main board generate lower voltages.
