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2190 posts

Uber Geek
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Topic # 173768 5-Jun-2015 11:38
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I'm pretty sure this query is going to make me sound like an imbecile, but anyway.

We have some basic IP cameras (D-link DCS 930L). We didn't need anything fancy, and these play really nicely with our NAS, which is set up with D-Link's "My Surveillance" feature. Assuming everything works, we may invest in some better cameras later, but that's for another day.

I'd like to move a couple of the cameras to a place where there isn't a ready power socket, so have been reading about PoE but google sends me into a world of terminology that includes "active", "passive" and 802 standards, and I need a bit more of a 101.

So far as I can tell, the cameras don't natively support PoE (though I did get fooled very briefly after seeing PPPoE on the box - like I say, imbecile). So what I think I need is a PoE injector and splitter set - you move the power brick to a power socket near your switch, the injector combines the data and power feeds into a single cable, then the splitter splits out data and power feeds at the other end, and you connect the data feed to the ethernet port and the power feed to the power port using the appropriate connectors. The power travels over the spare pairs in the ethernet cable (the cameras only support 100Mb connections.

The cable runs aren't huge, probably 20 meters tops.

Is that about right? Or have I missed something critical that will end up frying either the cameras or me? Will any cheap injector/splitter set work, or is there something in particular I should be looking out for?

Separately, I also need to upgrade the switch, and see some that have inbuilt PoE. I am guessing that this will only support devices at the other end that natively support PoE? Or can you use these with a splitter too? Do PoE devices actually receive power through the ethernet socket?

Finally, can anyone enlighten me as to the difference between active and passive PoE? How do I tell whether a device that has PoE needs an active or passive supply?

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454 posts

Ultimate Geek
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  Reply # 1317897 5-Jun-2015 13:29
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You are pretty much right for a device that doesn't support PoE having and injector then a splitter is the only way to go.

General PoE info: 
There are 2 main types of PoE, what you refered to as Active, which has a PoE switch at one end that only supplies power when a PoE device at the other end requests it.   This is good because you can plug in anything to the cable and there is no risk of something going wrong.  If the device needs power it gets it.  But it's more expensive to manufacture.

The passive PoE is cheap, and if you have a switch or injector with passive PoE it *will* send power down the cable without doing any checks whatsoever.  So if a device that isn't expecting power gets plugged in at the other end you have a chance of blowing it.    Budget minded gear generally uses this, Ubquiti gear uses it a lot.

You are talking about a variant of the passive type where you use a pair of injectors/splitters to get something that was never designed for PoE to use it anyway, I can imagine it being 'fun' getting the right plugs at each end, but it should work.

Warning: reality may differ from above post

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Uber Geek
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  Reply # 1318202 5-Jun-2015 23:21
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Here you go, a simple pair of active injector and splitter
Since these units don't do any voltage conversions. You might have voltage drop issues depending on cable length and amount of current used by the cameras. Especially as the instructions say that your cameras use 5V. So you will probably need to inject 12V or 24V. And then convert it down to 5V at the camera end of the cable.


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  Reply # 1318456 6-Jun-2015 21:39
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A side not on PoE, all Ethernet devices are required to have transformers in the data lines for electrical isolation, so having power on the cable of a non-PoE device is not an issue (and if it is, then you are better off without the non-compliant device).

You can never have enough Volvos!

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