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Q: why can i switch the screen of a led tv on & off via its built-in usb jack? Answered

hi, had a broken usb-cable causing a short in a usb-a 2.0 jack of a tv-set. thereafter the screen went black (tho the rest kept working, thus: audio, remote,...).did open up the beast and, after having given it for a goner, started playing around a bit. bridged the (metal)-mounting-frame of the inside with the vcc+ pin of the usb, and, look-look, the screen came back on... turns out: each time i apply this pulse the screen turns off, and on a 2nd pluse it turns back on, a.s.f... sort of works like a switch (i always wanted, incidentally).... i am just a lucky stupid bastard - but can any pro here explain what the hell/heaven is going on here? thanks, pit

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Orngrimm

16 days ago

As long as i dont have a schematic of the powertrain and USB-Supply, i cannot even start to answer this question.
Could be anything from an intended behaviour to luck in chained resets and such stuff...

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la xerraOrngrimm

Answer 16 days ago

danke, maybe that helps: the unit is an *engel*, model led 19. 30w

btw, did some more poking around, and thats the pattern: if the screen is on and i unplug the backlight cable it goes (obviously) dark, and if i plug it back in it stays dark - unless: i give a pulse as described in the post.
but: if the screen is on and i unplug the blacklight and thusly it goes dark AND then i turn off the tv via power-function and wait for the standby-light to go out and then unplug the unit AND then reconnect the backlight-cable and then switch the tv back on: the screen is lit...

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la xerraOrngrimm

Reply 14 days ago

well, looks like another riddle i can live/die with... sh it happ ns
cheers anyhow

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Jack A Lopez

17 days ago

Usually, shorting one of the power supply rails, of any kind of electronic equipment, is a condition to be avoided, even briefly, for reasons you might expect; i.e. because shorts allow big current to flow, and this can blow fuses and break things.

I have no idea why briefly shorting the USB's 5V pin, causes the screen to turn off, and also back on again.

If I had to guess, I would guess the TV is trying to protect itself. That is to say it detects too much current flowing to the USB port, then turns some things off, kind of the same way a circuit-breaker turns things off when it detects an overcurrent condition.

The part where it turns things back on again, in response to the same stimulus, is very mysterious.

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la xerraJack A Lopez

Answer 16 days ago

when i caused the short via the cable plugged into the usb the tv didnt turn off, kept on running - just the screen had gone black

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Jack A Lopezla xerra

Reply 16 days ago

Yeah. That just adds to the mystery for me. I mean, I understand that everything did not turn off, rather just the screen turns off. And this additional info does not make it clear to me, like, why it is happening.


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Downunder35m

17 days ago

It must be an older TV.
For a time it was quite fance to have multimedia feature.
Meant you can puld in a USB stick or your camera and the TV would go the right player for it.
The TV recognises this by the current draw of whatever is connecting.
From there the basic OS starts and starts if the attached device is accessible, a cam for example would fail.
If so then the corresponding drivers are loaded and the right app for the job shows you what is on the drive.

However, in your case you are creating a dead short and are lucky the TV has a protection for it on the 5V rail.
Most TV's have this protection for the USB ports eperately from the rest of the system.
That would mean if some short happens, a smart TV for example would give you some sort of error message on the screen.
It appears your TV shuts off the 5V through the power supply.
Anything attached to the same 5V rail rail and protection would then be without power.
Could be your display driver, graphis system or just the backlight (did you test with a flashlight against the screen?).
Unpluggin it all and leaving it for a few minutes should then reset the short detected and bring back life to the TV.
I do not recommend using this effect you have as a feature!!

If your TV has an audio only mode then it could be possible you are triggering the screen off by creating your short.
Still it means you produce a fault on the USB port and risk damaging the power supply for everything connected to it.
Take it as a lucky find to revive your TV and don't create more shorts ;)

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la xerraDownunder35m

Answer 16 days ago

_a, did make the backlight check, no flashlight tho... used pitch-black night instead... all dead
_b, no, this model (see reply above) has no audio-only feature