2
\$\begingroup\$

After attempting to use a power inserted for the OTA antenna I discovered an unexpected problem. The inserter is powered by an UnGrounded Ac/DC adapter (Wal-wart as they are often called). All worked fine until I connected the coax and discovered that the device will stay powered on even when the negative is disconnected from the AC/DC adapter.

After much trial and error I discovered that the mysterious ground / negative was going through the coax cable to the PVR and TV, then through RCA, HDMI, and coax between the PVR, TV, and DirecTv receiver. Only once was each device fully unplugged from one another would the inserter power off without any/ issues.

After more headache I figured out this was occurring because the DirecTv dish and OTA antenna are sharing the same grounding Stake outside. After disconnecting one of the grounds from the stake the problem disappeared.

Attached are diagrams of the configurations for clarity. I am extremely confused on how this is working. My knowledge of DC power is that both the positive and negative must be directly connected in order for the circuit to be completed. In my scenario only the positive needs to be connected and the circuit works. At first I believed the earth ground on each power supply was causing the issue, but nothing has a ground plug besides the PVR. All devices with TV 2 have no ground and have been eliminated as a potential issue.

Any insight on what could be happening and how is appreciated! This problem has truly piqued my curiosity!

enter image description here

enter image description here

enter image description here

\$\endgroup\$
8
  • 1
    \$\begingroup\$ it appears that the coax shield is providing ground path \$\endgroup\$ – jsotola Jun 15 '20 at 21:14
  • \$\begingroup\$ Questions on the usage of undocumented consumer products are not on topic here. Clearly you have non-understood paths of galvanic contact between sub-systems, likely as a result of signal connections. But while this may be an intellectual and practical curiosity, it is not an engineering design question, as an engineering design would start from systems of documented behavior. \$\endgroup\$ – Chris Stratton Jun 16 '20 at 3:51
  • \$\begingroup\$ What happens if you disconnect the positive wire from the AC/DC adapter? \$\endgroup\$ – Bruce Abbott Jun 16 '20 at 4:32
  • \$\begingroup\$ @BruceAbbott Power Inserter Goes Off if Positive is disconnected \$\endgroup\$ – user2860253 Jun 16 '20 at 15:57
  • \$\begingroup\$ @jsotola I Would have to agree with that statement just do not understand how the coax shielding could become the ground path when it is not connected to the power source \$\endgroup\$ – user2860253 Jun 16 '20 at 15:58
2
\$\begingroup\$

It's common design principle to have a single common ground point in circuits for signals and DC supply voltages. It's common that minus pole of a single supply voltage is connected to that in-circuit ground point. The signal cable shields are also connected to the same ground point.

Then something not so expected: Your wall-wart has internal connection between the mains AC input and minus side output. You have been lucky, the connection from the DC output minus happens to be to the neutral mains AC wire. In addition where you live there's a grounded neutral, both wires are not hot. Hopefully you have not in some way already reversed the mains AC input wires because that could be lethal.

The wall wart power supply isn't acceptable at least where I live because we have mechanically symmetric mains AC outlets. I guess it's faulty, for ex a wrongly assembled or broken part makes an unwanted contact.

Unplug, stop using it now and get local pro help to check the case. There can be more than you have shown.

\$\endgroup\$
3
  • \$\begingroup\$ Yes a single common ground between the DC electronics, AC, and Signals does appear to exist. Yes, Neutral and Ground are bonded together in breaker panel. Outlet shows wired correctly with one of those plug in testers. This circuit the equipment is plugged into is relatively new( within 10 yrs) in which I was part of adding so I see low chance of reversed wiring(human error does occur though). I don't have a second 12v power supply to swap or I would already have. I should purchase one. \$\endgroup\$ – user2860253 Jun 16 '20 at 16:05
  • \$\begingroup\$ What prevents others who in some still unknown way get in to reverse the AC input polarity of the wall wart adapter? And what happens if in the AC input the neutral side gets disconnected? Answer: The line side is connected through adapters primary to your signal GND. \$\endgroup\$ – user287001 Jun 16 '20 at 16:13
  • \$\begingroup\$ That is very true. I did ohms tests between each of plugs prongs and the DC barrel jack and found not traces of any shorts. However, I will leave it disconnected and look for a new adapter. \$\endgroup\$ – user2860253 Jun 18 '20 at 23:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.