Is this the right PoE device for me?

PSE, PD, 802.3af/at, passive PoE, mode A or mode B, there is a lot of jargon out there for PoE devices that makes choosing the correct device a bit daunting but once you get your head around these terms it becomes quite simple to find a compatible PoE device :)
  • PSE: power sourcing equipment(device outputting the PoE)
  • PD: powered device (the device that is getting powered from the PoE)
  • 802.11af/at: An industry standardised model of PoE, these are also known as active PoE as there is a handshake/negotiation before powering
  • passive PoE: the PoE out is like a tap its either on or off, there is no 'smarts' to this so no handshake/negotiation before powering
  • mode A or B: the mode talks about what pins/wires the power and data is being supplied on, mode A will be pins 1-2 for voltage and 3 and 6 for return where mode B is 4-5 for voltage and 7-8 for return.

Knowing this is important as you will need to check what is supported from the PSE and PD to make sure they are compatible, if you have a device that supplies power out of the Mode A pins only you will not be able to power on devices expecting this from the mode B pins.
Other incompatibilities can caused by the af/at support not being the same from the PSE to the PD, most Mikrotiks that support af/at will still be able to be powered from passive PoE and vice versa but not all vendors offer this support so if you are powering from or to a non Mikrotik device most of the time its best to get the same protocol on either side :)

Once you have figured out that the protocol and pin layout that should be compatible between the PSE and PD you will need to check the power requirements of the PD and what your PSE can output, the thing to note about Mikrotik PoE that unless you are using the crs328 or the crs354 the voltage output will be the same as what you put in, this means if you put in a 24v psu into the PSE the output power will be 24v but if you put in a 30v psu the output will be 30v.

Below is a screenshot of a Powerbox Pro that will be our PSE in this example and a cAP ac that will be our PD. The first thing I am looking at is the DC jack input range as well as the PoE out range. I want to make sure that it will support the required voltage from the PD. We can see that the cAP ac is going to be able to accept 17-57v so as long as we power the Powerbox Pro with PSU is within that range we should be ok as the Powerbox Pro can support from 12-57v.
The second thing we need to look at is the output per port as well as the max output. The output per port will be how much power it can supply from each port. On some devices you will have 2 ranges: 1 low (below 30v), and one high (above 30v). Below you can see that the Powerbox Pro has 1A of power output if supplying 18-30v where we have 0.45A when powering from 30-57.
To work out the power output (wattage), multiply the voltage by the amperage. So for example powering a device from the Powerbox using 24v will give us 24W of output power but 48v will only output 21.6W, this is important as we recommend that the power box be able to output the recorded max wattage needed for the cAP ac. Using our example above you can see that both the 24v and 48v can power this device without attachments as the maximum power it will use is 13W but with attachments you will either need to use 24v or something higher than 54v (54v * 0.45A = 24.3W)

The last thing to notice is the max total out, along with a per port limitation there is also a limitation of how many amps the device can output at a time, looking below we can see that this is 2 amps, if we were to try and power 4 cAP ac from this devices at the maximum power consumption with attachments we would not be able to do this - as just plugging 2 of them will get us to our 2A limit (24v *1A = 24W). We can fix this by plugging in a 48volt PSU instead as it will only need to output 0.45A on each port to get the required power as it will fall under the 2A total limit :)

Powerbox pro:

cAP ac: