FAILED POE ON HIK NXI’s

C

Camera Guy999

n3wb
Sep 18, 2020
21
8
uk
Ok so long time engineer recently started using NXI’s over the 7608’s but have been hitting an issue.

on 3 units now I have had the built in POE Hub fail after working for a few weeks, if I power the camera via an Independent poe switch plugged into the nvr or even plug the cameras into the hub and add 12v to the jack the cameras are fine but the poe on the NVR will not kick in and the poe power lights flash once when you turn the nvr on as you would expect and then stay off .

to clarify and avoid the generic comments:

*yea cameras all work on other machines.

*yes until the failure all cameras worked on the NXI’s.

*no there is no moisture damage.

*yes I have checked the poe settings on the NVR.

*yea I have checked the red/white cables with a meter for voltage and drawtest both are fine.

*yes I have tried power supply from another working nxi to rule that out too.

these units worked fine but the. Suddenly died.

I have read a lot of posts about overload and power spikes killing the poe hubs but none on how to fix or prevent the issue especially since I have surge protection in place…

My question is has anyone else experienced this poe failure and have you found a fix other than buying an independent poe hub and plugging it into the nvr or individually powering each camera?
 
I've fixed up several Hikvision NVRs where the PoE ports no longer supply power but work ok when the camera is powered with 12v.
The common cause has been the chunky TVS (transient voltage suppression) diode that's connected between the -ve of the 52v power and chassis being partially or fully shorted.
Because each PoE port also has a small TVS diode to chassis this has the effect of stopping the PoE classification process recognising that a PD (powered device) is connected as it clamps the port voltage when the classification current is applied by the PoE controller.
Disconnecting the blown TVS diode brings the ports back to life, and when the diode is replaced the NVR is functional again.
 
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I saw a video on this but when I tested the diode with a meter there was no short, I removed it regardless as per the video but it made no difference? Do I need to bridge the gap where the diode was or simply remove it?
 
Camera Guy999 said:
when I tested the diode with a meter there was no short,
Click to expand...
If the diode simply has developed a low breakdown voltage, the meter will not spot that, but the PoE operation can still be affected.
One way to check if a partially blown diode is causing the problem is to measure the voltage of the +ve side of the 52v power supply against the chassis of the NVR (not to the -ve side of the power supply) when at least one PoE camera is connected.

Camera Guy999 said:
Do I need to bridge the gap where the diode was or simply remove it?
Click to expand...
Given that you have identified the correct diode - (the ones I've seen have all been on the underside of the PCB near the edge where the 52v connector is) - removing it is a way to determine it's faulty if the PoE ports start working again.
 
Hmm, as I’ve removed the diode and it’s still off so we have established that the diode isn’t the issue, any idea what else it could be?
 
Further analysis would likely involve the use of an oscilloscope to observe the working of the PoE controller in determining if there is a PD connected, by it applying a stepped limited current and it observing the resultant voltages in order to confirm if the needed 25k resistance is present.
 
Camera Guy999 said:
This is the 3rd nxi this has happened to of mine it must be a known fault
Click to expand...
It's quite intriguing as to what the common cause might be.
It could be a design weakness - but I don't think we've seen other posts about similar failures.
But it's a fairly new model, so maybe not many users yet.

I take it that all PoE ports are non-functional?
Do you know if they all failed at once? eg from seeing the playback timeline.
Have you measured each of the 52v power rails relative to the chassis of the NVR, and what result did you see?
 
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Hi mate, so I did a measure and a draw test and the power supply is fine, I also tried it in a few other chassis with no luck,


the ports work for data but they aren’t supplying power for the poe despite receiving the power, no I haven’t gone as far as checking the rails yet I was hoping someone had come across this issue and had a fix before I got to deep into diagnosing this.

to confirm yes despite data still going through the ports all cameras dropped off at once
 
Camera Guy999 said:
Hi mate, so I did a measure and a draw test and the power supply is fine, I also tried it in a few other chassis with no luck,
Click to expand...
Not a draw test - but measuring the voltage of both the positive and negative of the 52v power rails relative to the chassis of the NVR.

This is because there are surge-protection diodes that are connected between the individual PoE port and the NVR chassis.
If the normally mostly floating 52v power becomes solidy or partially tied to the NVR chassis, these diodes can clamp the voltage that the PoE ports reach when the classification current is applied by the PoE controller, such that it does not recognise that a PD device is connected.
 
Hi Andy, so yes I checked the diodes and even removed the diode that supposedly causes this problem.

to save me having to research which other diodes to test do you have a photo you can circle?
 
Camera Guy999 said:
do you have a photo you can circle?
Click to expand...
This is from a DS-7608NI-K2/8P which had no working PoE ports, before removal of the failed TVS diode.
You can see the component number TVS55 and the big solder pads on the right hand side where the diode was before it was removed, under the 2 large ceramic capacitors.
The + of the diode was connected to the PCB chassis track.

1666545653624.png
 
Hmm so interesting one Alastair, I went into the recorder it’s self this morning of the last one to die and it shows “resistance is to high on the ports connected…

and this is where it gets interesting….

it’s an 8ch NVR but only 4 are in use… so on a whim I plugged the cameras into the 4 un used ports which all work fine, the original 4 on the nvr show unused as expected…

more interesting still I broke out a new camera and plugged it into the old port with a pre made cat 5 to be safe and it went back to resistance is to high?

this is starting to boggle me but looks like only the ports that had cameras on blew when it went down …
 
alastairstevenson said:
Ok, so that's a bit different from the ports just not working.
Presumably the ports that had the cameras on are still not powering the cameras?
Click to expand...

Hi,

no, they don’t seem to work and show one of 3 things:

connected, if I independently power the camera

resistance to high if I try to power them via the NVR’s poe

disconnected if no cameras are connected.
 
If you have an oscilloscope and access to a few electronic components, this might be an interesting experiment :

Take a short ethernet cable, cut off one of the RJ45 connectors and strip back the wires so there is maybe 1cm of bare on each wire.
Find or make up a 25k resistor, 1/4 watt will do.
join wires 4,5 and one end of the 25k resistor.
Join wires 7,8 and the other end of the resistor.

Connect the scope probe to the 7,8 wires and connect the RJ45 end to one of the working NVR PoE ports and power on the NVR.
You should see the voltage drop down a couple of times as the PoE controller goes through the detection / classification stages of the PoE handshake.
Then the voltage should drop down to match what's on the -ve side of the 52v power of the NVR. If it reaches this point, power is being supplied.

At that point, the detection and classification has decided that a PD is connected, and is applying the full 48v, then is doing an ongoing current check to handle disconnections.
After about half a second (I think it's actually 400mS) the 'PoE-check-if-still-connected' will decide there is no valid device and the cycle repeats about once per second.

Then do the same for one of the faulty ports and see what's different.

As an extension to this experiment, you could take a 2k or 3k 1W resistor, and quickly connect it across the 25k resistor just when the classification phase has completed, and power is applied.
If the timing is right - this will maintain the power, as the controller will now treat what's connected as a valid PD based on the increased current draw.

As a further extension - up the 25k resistor to around 30k or so, and see if the 'resistance too high' message is triggered.

If I had my hands on the NVR, this is the sort of diagnostic test I'd do when the fairly common problem of the shorted TVS diode is not found to be the cause of the fault.
 
Camera Guy999 said:
Ok so long time engineer recently started using NXI’s over the 7608’s but have been hitting an issue.

on 3 units now I have had the built in POE Hub fail after working for a few weeks, if I power the camera via an Independent poe switch plugged into the nvr or even plug the cameras into the hub and add 12v to the jack the cameras are fine but the poe on the NVR will not kick in and the poe power lights flash once when you turn the nvr on as you would expect and then stay off .

to clarify and avoid the generic comments:

*yea cameras all work on other machines.

*yes until the failure all cameras worked on the NXI’s.

*no there is no moisture damage.

*yes I have checked the poe settings on the NVR.

*yea I have checked the red/white cables with a meter for voltage and drawtest both are fine.

*yes I have tried power supply from another working nxi to rule that out too.

these units worked fine but the. Suddenly died.

I have read a lot of posts about overload and power spikes killing the poe hubs but none on how to fix or prevent the issue especially since I have surge protection in place…

My question is has anyone else experienced this poe failure and have you found a fix other than buying an independent poe hub and plugging it into the nvr or individually powering each camera?
Click to expand...

I haven't had an issue with them normally.
The only one that I had a POE died on it someone changed the connector end on a camera w/o unplugging it causing it to short out. But you suppose to plug in cameras in with NVRs not powered on either. Which I doubt that is documented.
btw, I install an online UPS with it.
 
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