I have an industrial instrument that requires “11 … 30 VDC, 500 mA IEEE 802.3af Power over Ethernet, class 2” and am trying to buy a PoE injector to suit it.
I’m looking at a “TP-LINK TL-PoE150S Gigabit PoE Injector Adapter, IEEE 802.3af compliant, Up to 100 meters (328 Feet)” on Amazon and can’t tell if it is compatible or will damage my instrument. I’ve downloaded and read its datasheet and its manual but both just say “Auto-detects the required power supply”. Apparently its voltage is 48 V, which might be too much for an instrument that says it can handle up to 30 V.
In the Customer Questions and Answers is the following group:
Q: What is the output voltage for the PoE ? can it support Unifi APat 24V 500ma
A: As long as your AP is 802.3af compliant, you shouldn’t have any problem.
A: There is a switch that changes the voltage and on mine it only goes up to 12V. Mine is the “kit” one that is not 802.3af compliant though.
A: No it wont. Ubiquiti Does not use POE standards. You would need to use their standard POE Brick that comes with AP’s or a Tough Switch that is made by them.
I looked at some other IEEE 802.3af PoE injectors too, but this detail actually doesn’t seem to be clear about any of them. I did find some information about a built in resistor in some PoE based devices that encodes the proper voltage, but the voltage ranges listed were all wrong for my device as well as for this injector.
Can anybody clear this up for me? It’d be greatly appreciated! Thanks!
POE works by the device and supply negotiating before any power starts to flow. the risk of overloading is really very small (i.e. something needs to malfunction)
The risk of it simply not working due to incompatibility is slightly higher.
802.3af POE provides 48V. If the powered device can’t withstand 48V, it’s not 802.3af compliant.
There is a multi-step detection and negotiation process during which the type of POE and power requirements are determined, but this is mainly for negotiating current limiting, not voltage.
In a recent install I was involved in, they had a Wi-Fi radio that used PoE 24V. The installer specifically warned that current (sorry) standard PoE was 48V and if the radio was hooked to this, it would burn out. In fact, we inserted a small 5-port non-PoE between the 24V PoE injector and the (48v) PoE switch to ensure nobody would accidentally feed the radio directly.
So my suggestion - don’t do it, unless it’s cheaper to buy a new device than a 24V PoE injector.
Just to wrap up, I finally heard from the maker of the instrument. The manual specifications are misleading. What they mean is that there are two different methods you can supply power to the instrument. One method is through a cable that has a red and a black wire, and the 11 to 30 volt specification applies to that method only. The other method is through another cable, the ethernet cable, and the IEEE 802.3af specification applies to that method only. There is no method of supplying power where both specifications are in force, and thus the quoted phrase I cited never applies as such. I think I understand that the two specifications are mutually exclusive, though I’m not actually sure about that one.
Anyway, I bought the TP-LINK supply, and it works fine for this.