Strange Voltages

G

GC

Guest
Here I am again with more camera questions....
We have a PTZ camera on a pole, running off 24volt AC power supply....
We use the same type of power supply and cabling for all of the same PTZ
cameras....

At the power supply end, I have 25volts unloaded, then 24 volts when loaded
(Camera and housing attached)

On the camera end, I have almost 25volts..... but when we put the cameras
on, the voltage drops to 17 volts.....
We measured the voltage at the bottom of the pole... 17 volts..... but back
at the power supply still 24 volts....
Somehow we get a drop of about 7 volts on the cable run, which appears to be
quite a lot when its only about a 20 metre run. We don't get this happening
for longer runs than this.....
So it looks like the cable.....
What could cause a drop this large?? The cabling in run in conduit under
ground.
If the conduit was full of water could that cause this???

Thanks
 
On Fri, 8 Aug 2003 07:19:10 +1000, "GC" <mobaccREMOVE@tig.com.au>
wrote:

Here I am again with more camera questions....
We have a PTZ camera on a pole, running off 24volt AC power supply....
We use the same type of power supply and cabling for all of the same PTZ
cameras....

At the power supply end, I have 25volts unloaded, then 24 volts when loaded
(Camera and housing attached)

On the camera end, I have almost 25volts..... but when we put the cameras
on, the voltage drops to 17 volts.....
We measured the voltage at the bottom of the pole... 17 volts..... but back
at the power supply still 24 volts....
Somehow we get a drop of about 7 volts on the cable run, which appears to be
quite a lot when its only about a 20 metre run. We don't get this happening
for longer runs than this.....
So it looks like the cable.....
What could cause a drop this large?? The cabling in run in conduit under
ground.
If the conduit was full of water could that cause this???
Click to expand...
The little electrons would have to swim!

Have you looked at the power at the camera end with an
oscilloscope, to ensure it's still clean DC and (for some
odd reason) hasn't picked up lots of AC. When I get
unexpected readings on a meter I always like to check
them out with a scope.

Mike Harding
 
"GC" <mobaccREMOVE@tig.com.au> wrote in message news:bgues4$go7$1@lust.ihug.co.nz...
Here I am again with more camera questions....
We have a PTZ camera on a pole, running off 24volt AC power supply....
We use the same type of power supply and cabling for all of the same PTZ
cameras....

At the power supply end, I have 25volts unloaded, then 24 volts when loaded
(Camera and housing attached)

On the camera end, I have almost 25volts..... but when we put the cameras
on, the voltage drops to 17 volts.....
We measured the voltage at the bottom of the pole... 17 volts..... but back
at the power supply still 24 volts....
Somehow we get a drop of about 7 volts on the cable run, which appears to be
quite a lot when its only about a 20 metre run. We don't get this happening
for longer runs than this.....
So it looks like the cable.....
What could cause a drop this large??
Click to expand...
Bad joint most likely.

The cabling in run in conduit under ground.
If the conduit was full of water could that cause this???
Click to expand...
Nope. That should drop the unloaded voltage.

The only thing that changes with the camera connected
and not connected is the current thru the cable. That will
certainly drop significant volts in a bad corroded joint.
 
On Fri, 8 Aug 2003 07:19:10 +1000, "GC" <mobaccREMOVE@tig.com.au>
wrote:

Here I am again with more camera questions....
We have a PTZ camera on a pole, running off 24volt AC power supply....
We use the same type of power supply and cabling for all of the same PTZ
cameras....

At the power supply end, I have 25volts unloaded, then 24 volts when loaded
(Camera and housing attached)

On the camera end, I have almost 25volts..... but when we put the cameras
on, the voltage drops to 17 volts.....
We measured the voltage at the bottom of the pole... 17 volts..... but back
at the power supply still 24 volts....
Somehow we get a drop of about 7 volts on the cable run, which appears to be
quite a lot when its only about a 20 metre run. We don't get this happening
for longer runs than this.....
So it looks like the cable.....
What could cause a drop this large?? The cabling in run in conduit under
ground.
Click to expand...
Subject to Mike's point about AC, you have 17 at the camera and at the
base of the pole. You have 24 at the PSU. Clearly the drop - being
*all* in the underground bit - indicates a fault there. Put a known
load on the cable at the base of the pole and do some ohms law calcs
and work out the impedance. You could even work out which leg of teh
supply has the high impedance.

If the conduit was full of water could that cause this???
Click to expand...
It can contribute at an exposed pair joint. Usually water shunts
present quite time-variable impedance. Plays merry hell on phone
pairs.
 
We did a bit more playing today.....
Tried a few more things....
I parked my car next to the camera where I had an invertor..... We plugged
the power supply into that.....
Used about 2 metres of figure 8 cable....voltage from the power supply to
the bottom of the pole with a load was only about 0.5volts...
So we had about 23.5 volts on the camera.... thats much better....
This ruled out the camera being faulty. It all worked fine(I already ruled
this out before when I replace the camera and housing etc)
So then we tried running a figure 8 cable along the ground from the bottom
of the pole to the existing power supply......
It would have been probably about 40-50 metres unrolled.... and another
50-60metres still on the reel....
Voltage dropped down to 16 volts with this cable....
We meaured the resistance in this cable, it was about 5 ohms....
The existing cable underground was about 4 ohms.....

So what its looking like is, its the length of the cable that is a
problem.... We didn't measure the current but someone told us the cameras
draw 2 amps..... It think in the manual it says it can draw up to 50 VA.
So perhaps using a thicker cable would boost it a little.... The cable
didn't feel hot :)
Or what we are going to try to do is get a GPO installed in a box on the
camera pole.... I noticed most of the other cameras simliar to this one
are done that way... as opposed to running the cable to another location...

Anyway thanks for peoples help!




"GC" <mobaccREMOVE@tig.com.au> wrote in message
news:bgues4$go7$1@lust.ihug.co.nz...
Here I am again with more camera questions....
We have a PTZ camera on a pole, running off 24volt AC power supply....
We use the same type of power supply and cabling for all of the same PTZ
cameras....

At the power supply end, I have 25volts unloaded, then 24 volts when
loaded
(Camera and housing attached)

On the camera end, I have almost 25volts..... but when we put the cameras
on, the voltage drops to 17 volts.....
We measured the voltage at the bottom of the pole... 17 volts..... but
back
at the power supply still 24 volts....
Somehow we get a drop of about 7 volts on the cable run, which appears to
be
quite a lot when its only about a 20 metre run. We don't get this
happening
for longer runs than this.....
So it looks like the cable.....
What could cause a drop this large?? The cabling in run in conduit under
ground.
If the conduit was full of water could that cause this???

Thanks
Click to expand...
 
On Fri, 8 Aug 2003 16:41:10 +1000, "GC" <mobaccREMOVE@tig.com.au>
wrote:

We meaured the resistance in this cable, it was about 5 ohms....
The existing cable underground was about 4 ohms.....

So what its looking like is, its the length of the cable that is a
problem.... We didn't measure the current but someone told us the cameras
draw 2 amps..... It think in the manual it says it can draw up to 50 VA.
So perhaps using a thicker cable would boost it a little.... The cable
didn't feel hot :)
Click to expand...
Try using a 30VAC transformer just for that camera which will allow
for the 6V drop would be the cheapest/easiest option.
 
Just from memory (I have not spent the time to research the real details so
no flames without facts)
The quoted currents and voltage drops all seem about right.
The missing link is ???
What size cable are you using??
It looks like you are using telephone wire or speaker wire because that is
about the size that will get the voltage drops you are seeing.
Try automotive cable or even ordinary mains figure 8 and see what sort of
resistance you get.

Oh! And since this would appear to be a commercial operation, where is the
licensed electrician who should be able to understand the very simple OHMS
law problems here?
--

John G

Wot's Your Real Problem?


"GC" <mobaccREMOVE@tig.com.au> wrote in message
news:bgvfpp$nik$1@lust.ihug.co.nz...
We did a bit more playing today.....
Tried a few more things....
I parked my car next to the camera where I had an invertor..... We
plugged
the power supply into that.....
Used about 2 metres of figure 8 cable....voltage from the power supply to
the bottom of the pole with a load was only about 0.5volts...
So we had about 23.5 volts on the camera.... thats much better....
This ruled out the camera being faulty. It all worked fine(I already ruled
this out before when I replace the camera and housing etc)
So then we tried running a figure 8 cable along the ground from the bottom
of the pole to the existing power supply......
It would have been probably about 40-50 metres unrolled.... and another
50-60metres still on the reel....
Voltage dropped down to 16 volts with this cable....
We meaured the resistance in this cable, it was about 5 ohms....
The existing cable underground was about 4 ohms.....

So what its looking like is, its the length of the cable that is a
problem.... We didn't measure the current but someone told us the cameras
draw 2 amps..... It think in the manual it says it can draw up to 50 VA.
So perhaps using a thicker cable would boost it a little.... The cable
didn't feel hot :)
Or what we are going to try to do is get a GPO installed in a box on the
camera pole.... I noticed most of the other cameras simliar to this one
are done that way... as opposed to running the cable to another
location...

Anyway thanks for peoples help!




"GC" <mobaccREMOVE@tig.com.au> wrote in message
news:bgues4$go7$1@lust.ihug.co.nz...
Here I am again with more camera questions....
We have a PTZ camera on a pole, running off 24volt AC power supply....
We use the same type of power supply and cabling for all of the same PTZ
cameras....

At the power supply end, I have 25volts unloaded, then 24 volts when
loaded
(Camera and housing attached)

On the camera end, I have almost 25volts..... but when we put the
cameras
on, the voltage drops to 17 volts.....
We measured the voltage at the bottom of the pole... 17 volts..... but
back
at the power supply still 24 volts....
Somehow we get a drop of about 7 volts on the cable run, which appears
to
be
quite a lot when its only about a 20 metre run. We don't get this
happening
for longer runs than this.....
So it looks like the cable.....
What could cause a drop this large?? The cabling in run in conduit under
ground.
If the conduit was full of water could that cause this???

Thanks
Click to expand...
 
On Fri, 8 Aug 2003 16:41:10 +1000, "GC" <mobaccREMOVE@tig.com.au>
wrote:

We did a bit more playing today.....
Tried a few more things....
I parked my car next to the camera where I had an invertor..... We plugged
the power supply into that.....
Used about 2 metres of figure 8 cable....voltage from the power supply to
the bottom of the pole with a load was only about 0.5volts...
So we had about 23.5 volts on the camera.... thats much better....
This ruled out the camera being faulty. It all worked fine(I already ruled
this out before when I replace the camera and housing etc)
So then we tried running a figure 8 cable along the ground from the bottom
of the pole to the existing power supply......
It would have been probably about 40-50 metres unrolled.... and another
50-60metres still on the reel....
Voltage dropped down to 16 volts with this cable....
We meaured the resistance in this cable, it was about 5 ohms....
The existing cable underground was about 4 ohms.....

So what its looking like is, its the length of the cable that is a
problem.... We didn't measure the current but someone told us the cameras
draw 2 amps..... It think in the manual it says it can draw up to 50 VA.
So perhaps using a thicker cable would boost it a little.... The cable
didn't feel hot :)
Or what we are going to try to do is get a GPO installed in a box on the
camera pole.... I noticed most of the other cameras simliar to this one
are done that way... as opposed to running the cable to another location...

Anyway thanks for peoples help!


Sounds to me like the cable has some sort of a "hot spot" in it - ie:
Click to expand...
a bad join or a cut to the cable that has gone partway through the
conductor so as 99.9% of the length is ok - but one small part has
high (er) resistance and causes voltage drops under load.

I would be considering drawing another cable through there (only
proper solution really) or it that isn't possible - you could try
using a higher voltage through it that would reduce the current (IE:
50-100-240v depending on the voltage rating of the wire - and what
transformers you may have in your junk box) and a transformer at the
camera end to drop it down to 24v again.

Also you could consider upping the voltage of the supply transformer
to compensate for the loss in the cable, (ie: take it up to say 30v to
compensate for the 6v loss) however this will end up producing more
heat at this bad join - and it may then fail completely or burn out in
time. A fire may occur too if its a small section that is high
resistance and the heat is concentrated. This might melt the PVC
conduit underground and make cable withdrawal and replacement
impossible
I don't think water would cause this unless there was a break in both
Click to expand...
wires and it was causing a loading on the circuit though the water.
Water would have to be impure to conduct too.
"GC" <mobaccREMOVE@tig.com.au> wrote in message
news:bgues4$go7$1@lust.ihug.co.nz...
Here I am again with more camera questions....
We have a PTZ camera on a pole, running off 24volt AC power supply....
We use the same type of power supply and cabling for all of the same PTZ
cameras....

At the power supply end, I have 25volts unloaded, then 24 volts when
loaded
(Camera and housing attached)

On the camera end, I have almost 25volts..... but when we put the cameras
on, the voltage drops to 17 volts.....
We measured the voltage at the bottom of the pole... 17 volts..... but
back
at the power supply still 24 volts....
Somehow we get a drop of about 7 volts on the cable run, which appears to
be
quite a lot when its only about a 20 metre run. We don't get this
happening
for longer runs than this.....
So it looks like the cable.....
What could cause a drop this large?? The cabling in run in conduit under
ground.
If the conduit was full of water could that cause this???

Thanks
Click to expand...
 
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