Hey Reddit, help me figure out why old tube TV’s when they shut of have a shrinking dot in the centre that slowly fades out.
Thanks
In: Technology
Tube TVs work by shooting a beam of electrons forward, and when the hit the from of the screen they cause it to glow.
Normally the electrons are going to go straight, but by using electromagnets the beam can get steered across the screen drawing each pixel.
When you kill power the electronics still sort of work for a little bit as the power stored in capacitors inside drain, but as that happens the voltage it has to work with drops. With les max voltage it can’t generate enough magnetic field to steer the beam all the way left and right, or up and down.
As max voltage gets lower and lower eventually the whole thing shrinks into a dot before cutting out.
A CRT has an electron beam that is scanned across the screen in a raster pattern left-to-right, then down a row, left-to-right, etc. It covers the whole screen 60 times per second. The scanning is controlled by magnetic fields.
When you turn off the TV, the magnetic fields go away slightly before the electron beam. So, for a fraction of a second the electron beam is concentrated in one place (or a line) leading to a bright point or line.
Here’s a demonstration of how magnetic fields effect a crt picture http://i.imgur.com/Nz2HqmX.gif
Old tube TVs worked by shooting a beam of electrons, then using magnets to bend the beam so it lights up different parts of the stream.
If the magnets are off, the beam goes straight to the middle.
When you turn off the TV, the magnets get turned off, but there might be enough electricity left in the beam to fire off a few.
That’s the bright spot.
A CRT TV uses an electron beam to excite phosphor coating on the screen to glow. While the TV is turned on, there is circuitry which causes the electron beam to rapidly move back and forth and up and down across the screen, drawing an image. When you turned off the TV, the circuitry moving the electron beam would lose power before the electron beam itself. So the electron beam would stop moving, and just shoot at the center of the screen, creating a bright dot that was the last thing to fade away.
As the circuitry to move the electron beam lost power, it would be able to move the beam less and less, so the image would kind of shrink down towards the center of the screen.
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