I'm looking to setup a basic demo of backfeeding AC and how it effects some AV equipment. My idea was to wire up an extension cord with a three prong if on one end and a simple single gang outlet on the other. This is US equipment. Then wire an AC transformer to the circuit. Plugin the piece of equipment and slowly increase the voltage coming from the transformer to demonstrate how it effects the equipment. Max voltage would be about 70 volts. Is this even a proper demonstration of what backfed AC is? Would this just not do anything? Will I burn my house down? Or explode?


I work for a cable company. In this area we have a lot of old houses. Old people. And old TVs. When we have a customer who is experiencing tiling or other abnormal behavior and all RF issues have been addressed, we check each outlet (inhouse cable line) for AC or foreign vtage before it hits the splitter. To test we take the (-) lead from our meters, ground it out and test the center conductor of the coax. There isn't supposed to be any AC. Some trace energy is fine. But we find lines with anywhere from 2 - 70 VAC. We ground out the line in question and no more issues.

The sitters seem to act as a weak capacitor building up stray AC until it causes an issue.

If we have time we check all outlets with an outlet bug to make sure they're properly grounded and wired. Its usually either a broken ground wire, a two wire circuit with three prong outlets, or older TVs and VCRs that are backfeeding.

What I want to demonstrate is how it looks with various amounts of VAC. I have the test board with the coax all setup that's easy. Originally I was going to take a variable AC transformer and wire it to a coax line Hooke up to a splitter and thus the test RF network. Thinking about it again I still feel like that is my best option. But it'd be hard to get real-time readings of VAC POST splitter. So then I wondered about my above question. Why not just wire up a single circuit I can plug in, and send vtage to the outlet itself. I guess that wouldn't be the same though....

Basically I don't know enough about VAC to know if I plug something into an active outlet and somehow send energy back in the other direction if I'd cause a glitch in the Matrix or worse! :)

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    \$\begingroup\$ What is your definition of "backfed AC" and why do you want to demonstrate it? I can not picture what you are proposing to do. You will probably need to draw a diagram. \$\endgroup\$
    – user80875
    Nov 4, 2017 at 21:03
  • \$\begingroup\$ Charles thanks for your response. I've edited my question to add further detail \$\endgroup\$
    – RyGuy
    Nov 4, 2017 at 21:15
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    \$\begingroup\$ Still very unclear and made up terminology. Sound like you are seeing the Y-caps of ungrounded equipment dividing the AC line voltage to the chassis. \$\endgroup\$
    – winny
    Nov 4, 2017 at 21:21
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    \$\begingroup\$ I read your whole post and do not really understand what phenomenon you are seeing or what might be causing it. Maybe some pictures/diagrams would help, to show how you hook up your meter and what you see. Also, I think you should add a diagram of your proposed backfeed setup. The verbal description was unclear to me. \$\endgroup\$
    – user57037
    Nov 4, 2017 at 21:40
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    \$\begingroup\$ Please explain "backfed AC" in terms of current and phase. Do you mean the device shunts some current to ground? If your picking up AC on coax , it could be that the coax is inductively coupling with an AC electrical line. Just because you measure 70VAC on the coax doesn't mean there is any energy behind that 70V as meters are high impedance. If you measured a reasonable amount of current (more than mA's) then I'd start to worry. If you want to block the 60Hz on a coax then get a filter \$\endgroup\$
    – Voltage Spike
    Nov 4, 2017 at 21:40

1 Answer 1


It seems like you are trying to simulate 60 Hz voltage that is induced or otherwise appearing in troublesome levels in cable television coax wiring. You should definitely not use something that is connected directly to a wall outlet. If you have a small variable transformer available, you could feed that with a 1:1 isolation transformer. Put a resistor in series with the output of that so that you have a high impedance source. You probably shouldn't use anything less than 25K ohms. See if that give the effect that you want to simulate.


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