Subscriber Discussion

Could A PoE Injector Be Used To SUPPLEMENT An Existing 802.3af (15W) Switch Port For A More Demanding PTZ Camera

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Alan Grant
Jun 13, 2018
IPVMU Certified

Late question but want to know nevertheless: Could a PoE Injector be used to SUPPLEMENT an existing 802.3af (15W) Switch port for a more demanding PTZ camera, for example, or must the required power all come from either the PoE Switch port or the Injector?

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Orlando Ayala
Jun 13, 2018

I don't know of any PoE injectors that pass through additional power. I would just turn off the power to the port you are currently using for the PTZ and use it as the LAN input for a sufficient injector.

U
Undisclosed #1
Jun 13, 2018
IPVMU Certified

That’s a fascinating question!

So, just so I understand, you want to plug the passive POE injector into an active POE capable switch?

Without trying it, I’m going to say it won’t work the way you think it will, and will only provide the passive POE power.

The passive POE power is always on, and typically uses the non-data pairs 4,5,7,8.  So that’s there.

The 802.3.af typically (though not always) uses the data pairs 1,2,3,6 for its power.

The device side 100M POE modules that I am famailar with at least, the center taps from the data pairs connect in parallel to the non-data pairs.  

So it seems it might work, except that the switch POE isn’t always on, it requires a “signature” resistance of 10K be the load.  This 10k resistance is normally provided by the camera, even when off (Duh!).

Here’s the rub, the fact that there will be a complete circuit on the non-data pairs will likely screw up the signature resistance detection on the data pairs, meaning that the POE switch will never decide to give it power.

Also, I’m not as familiar with POE modules on the switch, so it’s conceivable, though not likely IMHO, that a current will go from the passive POE into the camera and thru the switch, damaging it.

Still, I’ll give it a shot and let you know...

 

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Brian Rhodes
Jun 13, 2018
IPVMU Certified

Hello Alan:

PoE Power-source equipment does not itself pass-thru existing power.  So this means you cannot multiply PoE (802.3af) to a max of ~30W.   Power would be sourced from one device only, whichever is closest to the PD requesting it.

If you needed more power than 15W (802.af), then you would need to add a 802.3at (or greater) midspan in line.  

We tested this scenario in PoE Midspan With Switch Tested.

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U
Undisclosed #1
Jun 13, 2018
IPVMU Certified

Thanks for refreshing my memory about that discussion.

@Alan, so I did try something a little similar, here’s the picture:

Though this does supply power on both data and non-data pairs, they are both passive.  I was trying to show that the camera would not get fried even if power was supplied on both.

I didn’t test to see if a camera that required more than the wattage available from just one injector would actually work, though.

Again though, even if it would, you have the problem of the switch not supplying the power because of the passive injector’s interference in the circuit.

One though that no sane person would entertain would be, what would happen if you didn’t plug in the power to the injector until after the switch had started supplying power.

I’ll entertain that when I have the chance...

UM
Undisclosed Manufacturer #2
Jun 13, 2018

If you're using 802.3af compliant devices it won't work as the device and poe injector negotiate poe requirement before power is applied. 802.3af does not support use of 2 power injectors for one device.  Using passive PoE you could apply 15W on the data pairs and 15W on the spare pairs, but you risk frying the camera, or at the very least reliability issues.

U
Undisclosed #1
Jun 13, 2018
IPVMU Certified

Using passive PoE you could apply 15W on the data pairs and 15W on the spare pairs, but you risk frying the camera, or at the very least reliability issues.

Why fry?

The voltage should be the same.

Only the watts could increase, but then only to the degree that the camera requires it.

See my test above, separate passive on both spare and data pairs.

UM
Undisclosed Manufacturer #2
Jun 13, 2018

It depends on several things -
1) the stability of the power supplies (no risk of over voltage).

2) the camera input circuit must be designed to cope with this. (An 802.3af device is not designed to be used like this)

3) it is a totally non standard set up so if some one swaps the camera or connect another camera to the cable, (not knowing the installation) , it may damage the new camera.

 

U
Undisclosed #1
Jun 13, 2018
IPVMU Certified

1) the stability of the power supplies (no risk of over voltage).

Overvoltage is a risk, though an unlikely one, in any scenario.

2) the camera input circuit must be designed to cope with this. (An 802.3af device is not designed to be used like this)

See my schematic above in my first post.  It’s a typical 802.3.af module.  There is no relay or other mechanism that prevents inrush on one pair or the other.  

I agree the spec says that the device does not have to account for this possibility to be compliant.  

3) it is a totally non standard set up so if some one swaps the camera or connect another camera to the cable, (not knowing the installation) , it may damage the new camera.

Again, I see no risk to the camera since voltages are in parallel.

However, as I mentioned in my original post, there could be a risk to the POE switch, if the voltage from the passive injector exceeds the voltage from the switch, say 50v vs 49v.  This one volt differential might produce an undesired ground loop like current in the switch with a voltage of 1v. But I don’t know enough about the switch electronics to know.

In any event, I’m not advocating the use of dual POE, just trying to provide information about what happens from an academic perspective.

UM
Undisclosed Manufacturer #2
Jun 13, 2018

I'm glad you make it clear this is purely academic discussion, as it is not something try on a customer site. 
The schematic in your post is typical of part of an 802.3.af module but is far from complete. Its missing some key elements such as the 802.3af controller IC and related circuits, designed to protect the injector and powered device.

I think the discussion is misleading as a camera requiring >15W is not an 802.3af device. It may be an 802.3at device, but again I would recommend using 802.3at compliant devices rather than cobbling something together that may or may not work.

The answer to the original question is a simple no, assuming the PTZ is 802.3af or 802.3at compliant.

U
Undisclosed #1
Jun 13, 2018
IPVMU Certified

The answer to the original question is a simple no...

Then we agree.

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