Author Topic: Question about cable internet interface  (Read 518 times)

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Offline ExcavatoreeTopic starter

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Question about cable internet interface
« on: February 11, 2020, 01:05:14 am »
This is a long shot, I know.  I'm in the US, but I suspect that similar devices may be used elsewhere.

I have what's known as "cable TV, with internet."    There is an appliance that connects to the RG6 cable, and connects to my network router, as well as my POTS.  (telephone)

I had a problem, so I was checking the connections, and I got a shock from the shield of the cable.  I thought it was a problem with my "drop," or maybe one of my Televisions, but it turned out to be the first thing I checked - the internet appliance.

When it's sitting there, with nothing connected except the power, there is 30 volts AC on the shield connection of the RG6 input.  It's high impedance, as with my body touching the connection, it drops to 10 volts.   Of course, when my grounded cable is connected to it, it's at zero, but with my cable cut (the reason for the problem) I had that 30 volts on all the cable in the house.

Is that sort of thing normal. or is my device defective.  The technician that diagnosed the cable break in my yard had no idea, and was in the "well, it works, so don't worry about it." camp.   "We really don't measure any voltages" he said. 

For those not in the US, US cable companies are know for their HORRIBLE customer service, and the inability to speak with anyone who knows anything.  All I can get is "check your connections" and "  We'll send someone out."  Well, they did that, and he didn't know either. 
 

Online ejeffrey

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Re: Question about cable internet interface
« Reply #1 on: February 11, 2020, 01:14:45 am »
That's pretty much true for any ungrounded appliance with a switch mode power supply.  The culprit is the class-Y capacitors required for EMI shielding.  There will be one from each power line (live and neutral) to the enclosure ground of the unit.  They form a voltage divider that gives you ideally half the supply voltage (i.e., 60 volts)when open circuit, but can be less if there is any leakage to ground.

In a grounded power supply that leakage current would be conducted to ground, but without that it just floats at half the supply.  Its normal and not really anything to worry about from a safety perspective although it can be a cause of equipment damage when you hotplug connections to a grounded instrument.
 
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Offline Siwastaja

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Re: Question about cable internet interface
« Reply #2 on: February 12, 2020, 09:33:28 am »
 
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