Trying to tap into PoE power, but PoE handshake fails

[Signal32]

Hello,

I have a PoE camera and want to run some 5V accessories near the camera with using only one network cable to the location.
So I was wondering if I could "piggy back" off of the camera's PoE power. I can't really open up the camera - so I would have to hook into the ethernet connection and get the power from there.

The idea is to have the camera connect normally to PoE power and after full power is supplied, have a BUCK converter that draws from the 48V on the ethernet cable and steps it down to 5v.

The issue is that the PoE handshake is failing and I'm not sure what the reason could be.

My circuit is attached below (i.imgur.com/mZqmI1n.png), here is the idea:
- When the PoE camera is connected to the PoE switch the the switch will initiate the PoE handshake by sending voltages <25v.
- At this point, since the voltage is <25v, my circuit should have no effect due to the D3 28v Zener Diode which will not conduct - having Q1 isolate the BUCK converter from the input.
- After the handshake completes and the voltage goes up to 48v, D3 ill conduct, enabling Q1(2n7002) and providing power to the Buck converter.

However, when I connect the circuit, the hndshake never completes and the switch never ends up delivering any power.
If I isolate Q1's drain (Q1 drain will be left floating), then the PoE handshake is complete and everything is working.
Also, if I tap the PoE connection with my circuit , but after the PoE handshake has completed, then everything works.

Any idea what the issue could be here ?
- I tested if my circuit draws any power @25v - but the amperage drawn was < 0.1mA
- I tested the capacity of my circuit and it was 33pF - which shouldn't affect too much.
- I've tried hooking up a scope to observe how the signature is failing but I didn't see anything special there.

Compoenets:
BR1, BR2 = UM1B Diode Bridges 25A 1.1V@800mA [Datasheet]
Q1: 2N7002 N-MosFET [Datasheet]
D3: MM1Z27W 27V Zener [Datasheet]
D4: BZT52C15 15V Zener [Datasheet]

Any Suggestions appreciated, thanks!

[fourtytwo42]

It would be helpful if you added the active component values.

[Terry Bites]

Read-up on the POE detection standards.
Are you compliant?

[Signal32]

@fourtytwo42 - Good point - I've updated the schematic with values and added links in the OP.

@Terry Bites - I thought I was compliant - that's why I designed it as such. I can't figure out what's wrong with the design . Any suggestions on how to test to check I'm in compliance ? I don't have access to the PoE camera circuit to have a full picture of what might be happening - I just designed the circuit to activate after the classification signal in your .pdf, when the voltage is >27v. I am not sure why it is interfering with detection or classification.

[fourtytwo42]

That fet is a bit tight on voltage rating especially on potentially long cables however.
I think your problem might have to do with the ethernet pins you are connecting to, you seem to be coupling to rx/tx pairs in an unbalanced way and that certainly won't work. You don't say if this is a 10/100 interface or a 10/100/1000 but in either case you cannot connect to data lines except with a center tapped transformer. Perhaps you should limit yourself to a simple 10/100 poe that only uses the unused pairs namely 4/5 & 7/8.
Also don't know what the bridge rectifiers are for, polarity is defined in the standard.

[Signal32]

@fourtytwo42 - The PoE switch is 10/100/1000 and the device is 10/100. I guess this is the problem ? I can't use spare pairs in such a configuration - the switch will only send power over the data lines ?

If this is indeed the issue - is there any way to 'tap' into a  10/100/1000 switch powering a 10/100 line ? I guess adding a another mid-tap transformer effectively in parallel wouldn't work.

[kripton2035]

these are so cheap. why bother ?
https://www.aliexpress.com/item/32995117505.html

[fourtytwo42]

@fourtytwo42 - The PoE switch is 10/100/1000

That is indeed your problem (the 1000 bit)!
If you were into pro audio you would know what phantom power for microphones is, well poe for 1000 is the same principle. Centre tapped transformers for each of the data pairs, the phantom power being provided and received on the centre taps relative to each other (not ground).
That ali-express device looks strongly like 10/100 to me.
I would expect all the phantoms for a port on your switch to be tied together so if you are very lucky the phantom will be present on the 10/100 spare pairs.
As they are not carrying data in this instance you should be able to couple to them without a transformer BUT the switch may object, up to you to experiment! Probably best to tie each of the spare pairs together to prevent switch transformer saturation (as will likely occur if you only extract phantom power from one half of the winding).
Not quite so easy as you had hoped to be a parasite 

[m k]

the switch will only send power over the data lines ?

No, 100M is 100M and about a decade ago 4 pair power was still bad behavior.

So test it by disconnecting the data pair bridge.

[fourtytwo42]

the switch will only send power over the data lines ?

No, 100M is 100M and about a decade ago 4 pair power was still bad behavior.

So test it by disconnecting the data pair bridge.

You may have missed it's a 10/100/1000 switch so all pairs are active.

[m k]

I have not.

Either it is 100M compliance or not.
If it is then it has a mode where 2 pairs are spares and for old style PoE.
It can also be 100M compliance without PoE but that's not rational.
10M without PoE is more rational but not much.

[Signal32]

these are so cheap. why bother ?

Indeed something of the shelf is the fallback option if my DIY doesn't work - I was going for a more compact format. Though, something like this because it delivers 2A.

You may have missed it's a 10/100/1000 switch so all pairs are active.

I think this may be the problem. Here is what I think is happening:
From here:

POE treats each pair as a single conductor, and can use either the two data pairs or the two spare pairs to carry electrical current.

I've tested all pairs when connecting 10/100/1000 switch to 10/100 camera (without anything extra or in between) and indeed power is only provided on the data pairs. The spare pair are not powered.
So what it looks like is happening is that when the PoE Switch "sees" that this is a  10/100 device, it only sends power down the data lines.
I've tested this with multiple 10/100/1000 PoE switches and the behavior is the same - the spare pairs are never used.
I suspect that 10/100 native PoE switches are more likely to use the spare pair but for 10/100/1000 switches it's easier to just implement the data-pair power only behavior as there is overlap between 10/100/1000 and 10/100 mode.

Does that sound like what is going on ?

Is there any way I could "tap" into the power going down the data pairs ?

[fourtytwo42]

Ahh well done for finding what pairs your switch is using when attached to your camera because that's what matters!
As I pointed out before it's whats called phantom power so the only way you can get it from the data lines is a center tapped transformer on each pair AND you may have signal quality problems as a result of adding such a tap part way between the switch and the camera (you didn't mention the distances involved).
I think you mentioned 12V@2A somewhere, that's a fairly hefty 24W on top of whatever your camera uses ? Are you sure the switch will support the total and are you sure the switch will be happy with a negotiation for a lower power and then get another 24W slapped on top ??
Just a few more things to think about 

PS that commercial unit you found is so cheap you may as well buy it and try it, much better than pouring over specs forever!!
(but if it relies on the spare pairs only it aint gonna work, all depends if it contains transformers).

[m k]

Does that sound like what is going on ?

Yes.
And after a 2nd thought my not rational is also fading so bean counters must be smiling.
When today's connection is mechanically splitted there are only 2 pairs left and zero old style power.

Is there any way I could "tap" into the power going down the data pairs ?

It's in the LTC picture but what then.
https://www.intel.com/content/dam/doc/application-note/8255x-fast-ethernet-controllers-without-magnetics-appl-note.pdf

Can you find any labels around the switch where PoE is companied with some extra characters?
A is pairs 1 and 2 power, B is 3 and 4 and + is all of them.
There should be a standard for voltages also but it seems to be less accurate.
Seems that your switches are 10/100/1000 speed compatible but PoE type A only.
For 24 W you need PoE+ so pretty difficult is the route.

[Signal32]

you didn't mention the distances involved

Switch -> Camera is 100ft and I am trying to tap very close to the camera, 1ft away.

get another 24W slapped on top ??

My power requirements are only 3W on top of what the camera uses, it should be 10w total, so should be within normal PoE budget.

PS that commercial unit you found is so cheap you may as well buy it and try it, much better than pouring over specs forever!!
(but if it relies on the spare pairs only it aint gonna work, all depends if it contains transformers).

So the commercial unit is different than what I am trying to do. I do have it and it works but is not ideal for my use case. That unit takes in a PoE connection to the switch and outputs a non-PoE 10/100 RJ45 data connection and a 12V output. This means that you will have to connect PoE to the commercial unit and then plug both the non-PoE 10/100 output AND the 12v into the camera.
I've opened the unit and indeed it has a center-tapped transformer and doesn't use the spare pairs at all.
Note that the commercial unit is the one that does the PoE negotiation - the camera will never "know" that a PoE switch is involved, it will get the regular non-PoE interface and separate 12v power. 
What I'm trying to do is a bit different. I am trying to have the camera do the PoE negotiation so it would only need one wire to it - the PoE connection from the switch. With my device I don't want to connect the 12v power to it also.

There should be a standard for voltages also but it seems to be less accurate.

I want to have 48v and 53v working - I have switches that output these 2 voltages.


I guess I'll give another shot with putting a center-tapped transformer in parallel and see if the camera can still work in such conditions.

[fourtytwo42]

So if you had started this thread with a little more information like the camera you were using it would have saved everybody a lot of guessing!