I would like to power my Omnitik AC from a 12v battery.
While the specs on the Omnitik say 10v - 57v is supported, the included PoE injector says 18v-57v and I suspect is not working correctly with a 12v supply.
I may have mixed up these versions when the SXT’s and the Omni were installed, or if not perhaps I can just buy one of the 10v versions and use that instead.
Edit: This is all very confusing, this statement seems to indicate even on the 10v one it still needs 18v input:
“input needs to be at least 18V to accomodate any losses in cables”
Both injectors are fully passive, so the lower limit of their voltage range has nothing to do with the injectors themselves. The Ethernet cables are thin, so with ones several tens of meters long, which are typically used when the radio units are on a tower and the power source is at ground level, a significant voltage drop happens between the injector and the device. It’s even worse - the lower the voltage, the more current the device drains to get the same amount of power (voltage times current), whilst the voltage drop on the cable is directly proportional to the current.
So unless you have just a few meters of cable between the injector and the Omnitik, 12 V is not enough even for a device, for which 10 V at input is otherwise sufficient, and you either need a second battery or a DC-DC upconverter. And if you use an upconverter, one creating 48 V is better than one creating just 24 V. Because the current drained by the device when you feed the line with 48 V will be less than half of the one drained at 20 V, so you’ll spend less power to just heat the cable. It’s the same reason why power grid companies use high voltage lines to transport electricity.
Other than that, the AC in the Omnitik name suggests that you could make use of gigabit speed on the Ethernet interface; if it is the case, be aware that only the RBGPOE supports gigabit speed, whilst RBPOE supports ony 100 Mbit/s (the two pairs used to deliver power are not available for data transmission).
Thank you sindy that’s really great info, I appreciate it.
The cable from Omni to PoE injector in our case is a 3 metre Cat 6. It’s a small domestic tower install on a farm so a short run.
We’ve ordered a DC-DC step up which is capable of 35v (https://core-electronics.com.au/150w-dc-dc-boost-converter-10-32v-to-12-35v-6a.html) I would have set this to 18v so your info about the higher voltage producing less current is very useful. As we’re running off a battery the lower the draw the better, I’ll set the Boost to max at 35v.
The Omnitik has been running for a few weeks on 12v with no (apparent) problems, however there was nobody at the site. Once they arrived and started streaming a movie through it that’s when it shut down (halfway through the movie), so I’m guessing additional load increases the power requirement, which would make sense.
It does. It is “only a few watts”, but that can mean that voltage drop on the UTP cable becomes just a tad too big.
As you’re powering the gear off a battery, you may want to do some measurements of efficiency of the DC upconverter. The efficiency might vary with different Vout-Vin difference and you want to find a sweet spot where overall efficiency (including the DC conversion inside omnitik) is best but offers stability of operations (you you probably don’t want to go lower than say 18V on DC upconverter’s output). You should measure the efficiency at omnitik’s most common state (which might be state without any wireless clients).
Today i visited the site and inspected the cabling, I found a loose positive power cable connection to the PoE that powers the Omnitik. After tightening this it’s been running for around 8 hours with no issues on 12v and we’ve streamed 4k for a few hours no problems.
We did not deploy the DC booster for the time being, will see how this setup performs and add the Boost if necessary down the line. Heat is a concern (we’re in Australia) and the heat sinks on this booster show that it obviously generates plenty. Fingers crossed it’s all down to a loose power cable…
With just 3 meters cabling distance between the battery and the device, you can basically place the injector next to the device and bring the power to it using a thicker cable running in parallel to the Ethernet one if you experience some intermittent behaviour again. But most likely it will work as-is forever, the drop on 3 meters of Ethernet cable is negligible.