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Ok, long story short. I have a new TV that occasionally switches on all by itself. In case anyone is wondering it's a Sony XH90 and this problem is documented on www. Not trying to go into details about this, makes no sense.

The thing is, someone suggested to me, try reversing the power plug. For the heck of it, I tried without any hope. But, to my surprise, it actually solved the problem!

I am an EE and know a few things about power line, electronic design, SMPS design. This TV is not grounded (double isolation design), same as all other equipment attached to it (exclusively through HDMI).

I know power line has line and neutral, but even then, how could reversing the power plug possibly make any difference. Any ideas?

Final conclusion

This issue was in fact a non-issue. What i described did happen, but not again after the TV had been rebooted by unplugging it, nor after re-reversing the power plug. It must have been a software bug that disappeared after a reboot.

Nevertheless, someone trustworthy did suggest the solution (where there's smoke, there's fire), and Andy aka did give a very likely solution in case the issue would have been real. Kudos to Andy for what he wrote, i will certainly keep his thoughts in mind whenever applicable.

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    \$\begingroup\$ How do you know reversing the plug helped? What if simply the unplug and replug helped? Try again with the original orientation to make sure. Make also sure that firmware did not update automatically after replug. But simply put, switch mode power supplies may not be exactly symmetric, at least not when connected to other equipment. And some other device may be waking it up via CEC. \$\endgroup\$
    – Justme
    Nov 24 '20 at 12:32
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    \$\begingroup\$ CEC can be ruled out. I'm 100% sure firmware did not update in between. But fair enough, in order to be sure, I re-reversed the power plug. Let's wait and see what happens. \$\endgroup\$
    – gommer
    Nov 24 '20 at 12:38
  • \$\begingroup\$ Do you have other power-line devices that might be transient noise sources? Like a welder, or some other big power-sucker that switches on/off? Transients can make some powerline-synced clocks run fast too. \$\endgroup\$
    – glen_geek
    Nov 24 '20 at 14:43
  • \$\begingroup\$ I'll keep an eye on the microwave in combination with the TV, nothing significant other than that. \$\endgroup\$
    – gommer
    Nov 24 '20 at 16:00
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Background (with a little speculation)

Inside the TV's switch mode power supply will be one or several Y capacitors that connect the internally produced DC voltages to either live or neutral. They are there to reduce the common mode noise produced by the high frequency switching transformer from affecting the DC outputs. Without the Y capacitors, all the internal DC rails will be wobbling up and down at 100 ish kHz due to inter-winding capacitive coupling between the flyback primary transformer and its various secondaries. The wobble will not be massive but could be around 1 volt p-p.

If this were not a TV, then there might be no real use for the flyback noise suppression capacitors but, consider this; the TV has to connect to an antenna of some sort and, you don't want the noisy internal 0 volts of the TV to be superimposing several volts peak-to-peak of 100 kHz onto your antenna or satellite input signal. You might be sharing the antenna system with other homes (say in an apartment building) so, the internal design of the TV has to accommodate this scenario. If using a satellite dish then it will be connected via long wires with high capacitance to ground and 100 kHz noise is going to cause some detriment somewhere to the received signal.

Infrared remote

The infrared remote receiver in the TV will be active all the time whenever the TV is plugged into a wall socket so it will be constantly looking for an infrared signal that is encoded as "ON". The infrared detection circuit will use high gain circuits that feed into a form of data slicer that gives a digital output so, how many bits of encoding are used to represent "ON" and how long will it take random noise (via a high gain infrared detection circuit) to erroneously reproduce the bit stream that represents "ON"?

I have had some experience here to draw on. A high-speed data link I designed (650 Mbits per second) when not connected to a valid signal would trigger approximately every millisecond or so to indicate it had received a correct frame ID and, about every minute or so it would find exactly the same frame header in exactly the right place hundreds of bits later on. It would then indicate that it had received a valid frame of data. Of course it hadn't (and we knew that) but, just like false alien transmissions that people rave about, the hardware told us differently. Just random numbers coinciding.

Tossing a coin

How many times would you have to toss a coin to get 16 heads in a row? The data stream was 650 Mbps and in 1 ms the data receiver would get 650 kbits (with one false positive) - so "tossing a coin" 650,000 times resulted in a good chance of seeing 16 consecutive heads. OK I can't remember whether it was 1 millisecond or 5 milliseconds but, the point is this; if you do the experiment enough times (and very quickly) the number of false positives will be huge!

What has this got to do with the question?

If the Y capacitors were connected to the neutral incoming AC lead, it would offer better noise reduction than if it were connected to live. Now clearly, the live and neutral wires can be interchanged so you could ideally choose to have Y capacitor noise reduction capacitors connected to the "earthier" of the incoming AC wires and, if this gives slightly better noise reduction on the infrared detection circuits then it might make a big difference in receiving a false "ON" demand every hour or so and detecting a false "ON" every month or year.

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    \$\begingroup\$ "In our business, one in a million is next millisecond" I suppose \$\endgroup\$
    – user253751
    Nov 24 '20 at 17:51
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    \$\begingroup\$ There's a famous quote that [in software] "one in a million is next tuesday". In hardware, I suppose it's next millisecond. \$\endgroup\$
    – user253751
    Nov 24 '20 at 18:26
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    \$\begingroup\$ @Andyaka Colloquially, a problem being a "1 in a million" means: "Don't have to fix, won't ever happen." Unless the event rate is in the MHz range, in which case "1 in a million" means "happens every second". \$\endgroup\$ Nov 24 '20 at 21:35
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    \$\begingroup\$ I think they did? The original quote by user253751 "In our business, one in a million is next millisecond" was a direct commentary to the "Tossing a coin" section of your answer. \$\endgroup\$ Nov 24 '20 at 22:07
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    \$\begingroup\$ Oh, so I should take it as a complement then? \$\endgroup\$
    – Andy aka
    Nov 24 '20 at 23:17
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You talk about reversing the plug, so it seems it is an ungrounded appliance. In that case reversing the plug should not have any influence, as there is no reference for live and neutral.

However, it is a tv, so it has a third connection being the antenna or cable. I'm going to assume a cable connection, as there are typically grounded. Now this provides a reference, where neutral-ground is about 0v, and live-ground is the line voltage.

Now when the filter between the three is symmetrical, all is well. But in that case one of the filters will do nothing, as there is o voltage difference, so it is safe to remove it and save a few cents per tv.

Now in may places in the world it is possible to know which prong is live and which is neutral, and this would be the best solution. But as you live somewhere where you can reverse the plug, a cheaper, unbalanced filter will cause problems with half the installed sets, as they have live and neutral reversed in regard to the design.

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  • \$\begingroup\$ In EU here. As most small appliances, the TV has a type C power plug which can be reversed. Similar ungrounded US plugs exist also. Nothing exotic really. And no antennae connected. Although it's a TV, it's only used as a screen, really (aside from the fact it's running Android. \$\endgroup\$
    – gommer
    Nov 25 '20 at 16:41
  • \$\begingroup\$ The point is that it does nothing to reverse the plug, unless there is a 3rd connection to the device. If it isn't a antenna cable, it is another cable, such as hdmi or the likes. \$\endgroup\$
    – Pelle
    Nov 25 '20 at 19:53

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