Discussion:
Video Ground Loop Isolation
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David Kaye
2014-09-28 19:14:19 UTC
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If there were a ba.electronics I wouldn't feel so guilty about posting here,
but this topic is Bay Area and Internet-related.

Anyhow, I've replaced a video DVR unit and some cameras, and left others
intact. The previous DVR showed rolling hum bars on a couple screens and
occasional tearing on another. I mistakenly figured it was hum induced into
the co-ax from refrigerator motors and the like, but after turning off
various equipment throughout the restaurant I didn't get rid of the hum.

So, it's gotta be a ground loop issue. Most off-the-shelf CCTV systems I've
seen bring the camera power back to the DVR unit and supply from there, but
for some reason, the installer who put in the system years ago put up
individual 12v transformers at each camera. I'm not sure why, but that's
how things stand.

Now I need a bunch of ground loop isolation transformers to put at the DVR
end. Doesn't anybody keep these in stock in the Bay Area? I've looked them
up every which way and phoned everyone from Radio Shack to Fry's and Samy's,
and most folks either don't know what I'm talking about or they tell me I
have to mail order.

WAAAAAA!!! I miss Zack's! I want to be able to walk into a store and buy
some video isolation transformers (BNC to BNC) to get rid of my hum bars.
Is that too much to ask?




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b***@MIX.COM
2014-09-28 19:41:14 UTC
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Post by David Kaye
If there were a ba.electronics I wouldn't feel so guilty about posting here
You should feel guilty about doing nothing to create it.
Post by David Kaye
WAAAAAA!!! I miss Zack's! I want to be able to walk into a store and buy
some video isolation transformers (BNC to BNC) to get rid of my hum bars.
http://jensentransformers.com/datashts/vb1bb.pdf

Your other choice is to deal with the grounding problems at the cameras.

Billy Y..
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David Kaye
2014-09-28 20:19:50 UTC
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Post by b***@MIX.COM
http://jensentransformers.com/datashts/vb1bb.pdf
Thanks, but the loop problem is minor enough that I'm sure those little
in-line transformers would do just fine.
Post by b***@MIX.COM
Your other choice is to deal with the grounding problems at the cameras.
The cameras are in several locations on the ceilings, making it really
impractical to set up common grounds at the cameras. We'd have to tear out
walls and run over 500 feet of wire. Each camera is plugged into its own
"wallwart" as people seem to call them. Each is in a separate outlet, and
it turns out that some are on different circuits as well. I'd think that
using those little transformers there would be no actual ground at the
transformer side in that both sides of the secondary would float. But maybe
there's just enough induction to create a pseudo ground at the camera end.
That, coupled with the grounding at the DVR end could create a ground loop.
While it's not objectionable to look at onscreen, it interferes with the
motion detection feature of the cameras, keeping the DVR unit recording
constantly on those channels.




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Thad Floryan
2014-09-28 20:19:03 UTC
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Post by David Kaye
If there were a ba.electronics I wouldn't feel so guilty about posting here,
but this topic is Bay Area and Internet-related.
Anyhow, I've replaced a video DVR unit and some cameras, and left others
intact. The previous DVR showed rolling hum bars on a couple screens and
occasional tearing on another. I mistakenly figured it was hum induced into
the co-ax from refrigerator motors and the like, but after turning off
various equipment throughout the restaurant I didn't get rid of the hum.
So, it's gotta be a ground loop issue. Most off-the-shelf CCTV systems I've
seen bring the camera power back to the DVR unit and supply from there, but
for some reason, the installer who put in the system years ago put up
individual 12v transformers at each camera. I'm not sure why, but that's
how things stand.
Now I need a bunch of ground loop isolation transformers to put at the DVR
end. Doesn't anybody keep these in stock in the Bay Area? I've looked them
up every which way and phoned everyone from Radio Shack to Fry's and Samy's,
and most folks either don't know what I'm talking about or they tell me I
have to mail order.
WAAAAAA!!! I miss Zack's! I want to be able to walk into a store and buy
some video isolation transformers (BNC to BNC) to get rid of my hum bars.
Is that too much to ask?
Googling "bnc video isolation transformer" finds, among others:

http://www.bhphotovideo.com/c/product/329523-REG/Kramer_TR_1_TR_1_Video_Isolation_Transformer.html

and

SUB Only Ground Loop Isolation Transformer BNC M To F
http://www.mcmelectronics.com/product/SPECO-/82-6132

If you want to attempt a local source, best one is San Mateo Electronics
where I get some really unusual things. They were the only source of
Japanese to USA voltage transformers (250W, 500W, 1kW, 2kW) that I needed
to operate stuff I imported from the Chinese Club Discount Centre in Hong Kong.

They have a lot of CCTV and related gear, too, so call them:

http://www.smelectronics.com/

Their website shows only a small percentage of the things they have. They're
at 42nd Avenue and El Camino which is relatively easy to get to using Hwy 92
from I-280; I still visit them every time I have to special order Stella D'Oro
sesame breadsticks from the Molly Stone store across the parking lot from San
Mateo Electronics.

Thad
David Kaye
2014-09-28 20:33:17 UTC
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Post by Thad Floryan
If you want to attempt a local source, best one is San Mateo Electronics
where I get some really unusual things.
Thanks. I'd heard of them but totally forgot about them and have never
visited. I'll phone them first chance I get.




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Jeff Liebermann
2014-09-28 21:02:21 UTC
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On Sun, 28 Sep 2014 12:14:19 -0700, "David Kaye"
Post by David Kaye
So, it's gotta be a ground loop issue. Most off-the-shelf CCTV systems I've
seen bring the camera power back to the DVR unit and supply from there, but
for some reason, the installer who put in the system years ago put up
individual 12v transformers at each camera. I'm not sure why, but that's
how things stand.
Google for "Coaxial Video Ground Loop Isolator". About $13 ea.
However, I consider these to be a band-aid. Better to find the source
of the problem.

I always worry about AC wiring problems when I see hum bars. The hum
bars are usually caused by the grounding of both the DVR and camera
BNC connectors to the AC power ground. The loop is between the coax
grounds, and the two different AC grounds, which are probably at
different potentials. Before you electrocute yourself, carefully
disconnect each BNC connector from the DVR, and measure the AC voltage
between the BNC shield ground, and both the DVR case and DVR BNC
connector. If you see anything over maybe a few hundred millivolts,
you've found the source of the hum. However, if you find more than a
few volts of AC power, or see sparks, you've found a probable
electrical fault and electrocution hazard.

Please remember that you have but one life to give on behalf of your
customer.
--
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150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
David Kaye
2014-09-29 06:13:28 UTC
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Post by Jeff Liebermann
Google for "Coaxial Video Ground Loop Isolator". About $13 ea.
Well, yeah, I have seen several at $9 to $13 each. That's not the point.
The point is that I'd like to try one tomorrow and see if it works, not wait
until next week for delivery. There's about half a volt potential between
the co-ax shell and the outer connector on the DVR. It may be low enough to
be solved by a loop isolator.
Post by Jeff Liebermann
Please remember that you have but one life to give on behalf of your
customer.
Uh, yes. My original training is in electronics, and I've worked around
high power and high frequency radio and TV transmitters. I realize the
risks. I'd prefer to have everything routed back to one circuit with all
power supplies at the DVR end, probably using just one 12v supply, but this
also means having someone climb around and connect the cameras to the
original 12v twisted pair that was abandoned in the original installtion,
and then connect up to the DVR end where the 12v leads were simply cut off.

Today I asked the owner why on earth the original tech did it that way. He
said it was probably to save time. I don't comprehend that as an answer.




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Kevin McMurtrie
2014-10-01 19:54:19 UTC
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Post by David Kaye
If there were a ba.electronics I wouldn't feel so guilty about posting here,
but this topic is Bay Area and Internet-related.
Anyhow, I've replaced a video DVR unit and some cameras, and left others
intact. The previous DVR showed rolling hum bars on a couple screens and
occasional tearing on another. I mistakenly figured it was hum induced into
the co-ax from refrigerator motors and the like, but after turning off
various equipment throughout the restaurant I didn't get rid of the hum.
So, it's gotta be a ground loop issue. Most off-the-shelf CCTV systems I've
seen bring the camera power back to the DVR unit and supply from there, but
for some reason, the installer who put in the system years ago put up
individual 12v transformers at each camera. I'm not sure why, but that's
how things stand.
Now I need a bunch of ground loop isolation transformers to put at the DVR
end. Doesn't anybody keep these in stock in the Bay Area? I've looked them
up every which way and phoned everyone from Radio Shack to Fry's and Samy's,
and most folks either don't know what I'm talking about or they tell me I
have to mail order.
WAAAAAA!!! I miss Zack's! I want to be able to walk into a store and buy
some video isolation transformers (BNC to BNC) to get rid of my hum bars.
Is that too much to ask?
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I'd suspect the 12V transformers first.

Second, I'd check to see if a wire mesh in the walls is electrified.
The wire mesh in stucco can carry current from the PG&E drop to the
entire exterior of the building. It's less than a volt but at nearly
infinite current. Nylon screws for the camera mount would fix this.
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