Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!uakari.primate.wisc.edu!samsung!cs.utexas.edu!sun-barr!lll-winken!tekbspa!optilink!brad
From: brad@optilink.UUCP (Brad Yearwood)
Newsgroups: sci.electronics
Subject: Re: funny phosphorus illumination after power to tv has been turned off
Message-ID: <2893@optilink.UUCP>
Date: 3 Jan 90 22:48:14 GMT
References: <6805@lindy.Stanford.EDU>
Organization: Optilink Corporation, Petaluma, CA
Lines: 26

In article <6805@lindy.Stanford.EDU>, sorka@ucscb.UCSC.EDU (Alan Waterman) writes:
> 
> When I turn my TV off, areas around the center of the screen dance around
> brightly in multicolored flames...
> 
> The only thing that is possible is that the phosphorus is being bombarded
> by electrons, but how can this be if the power is turned off? Is the CRT
> drawing on capacitors or something?

You have capacitance in the picture tube itself and in the high voltage system
(diode/capacitor voltage multipliers are frequently used), so the screen
of the tube may still be charged to a high potential even with all power
turned off.  This is why repair people (the surviving population thereof)
remember to discharge the high voltage supply anode _and_ the picture tube
anode connection before sticking their hands into a TV set or monitor.

You have a cathode which, though no longer being actively heated, is still
hot.  It is reasonable to expect that the cathode will emit quite a few
electrons, and that many of these will find their way to the positively
charged phosphor screen, until the cathode cools or the charge dissipates
(which process is of course helped along by these same electrons).  Remember
also that the cathode is working within an excellent vacuum, so it will cool
only by radiation and by conduction through its connections and support
structures.

Brad Yearwood   {pyramid, tekbspa, pixar}!optilink!brad