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Where do the electrons go in a CRT

  1. Jun 9, 2012 #1
    Do electrons flow in a "circuit" in a CRT? When they hit the phosphor coating on the screen, does the final anode complete the circuit and allow current to flow back to the circuitry or does the anode only serve to accelerate the electrons which then "accumulate" by combining with the phosphors and cause chemical changes by reducing atoms?
     
    Last edited: Jun 9, 2012
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  3. Jun 9, 2012 #2

    Simon Bridge

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    That's a good question and one few students feel the need to ask.
    You should be able to figure it out if you've ever run a CRT TV or monitor for a while and run your hand over the surface :) (something that is decreasingly likely)

    The electron ends up in the glass, which attracts dust. The excess charge eventually bleeds off into the atmosphere or into the device case... which is usually earthed.
     
  4. Jun 9, 2012 #3

    sophiecentaur

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    The circuit must have a 'formal' return path for the several milliAmps of the beam current. The screen has a brightness, equivalent to several tens of Watts so there must be significant Power in the electron beam. The front of the screen needs to be grounded to complete the circuit. If it were not, the screen would just charge up to the Eht voltage and repel the electrons as they arrived. The small charge that builds up on the tube face could be due to induction, I think, rather than leakage through the glass.
     
  5. Jun 9, 2012 #4
    I have forgotten most of my chemistry at this stage but is it possible that the electrons could combine with the phosphor and ionize it or add to the phosphor molecules and create a new compound. What happens for instance when the screen is "burnt" by the beam?
     
  6. Jun 9, 2012 #5

    sophiecentaur

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    Whatever you do with the electrons arriving at the screen, the negative charge would still (and very soon) build up. Whatever goes on in the phosphors they must start off with neutral charge and end up with neutral charge. Those electrons that arrive just have to be taken away to complete the circuit. The only (crazy) alternative would be to supply the front of the screen with positive ions at a fantastic rate, to mop up the electrons (and all of this would need to be done ina Vacuum!!). Needless to say, these positive ions would need to be manufactured by stripping off electrons and those electrons would have to be deposited to ground. So you still have your complete circuit - in just a much more complicated way.

    When the electron beam energy is too high in a given period of time (say the spot stopped being scanned) the spot will overheat and melt / break down the phosphors in that spot. The electrons will still not hang around there.
     
  7. Jun 9, 2012 #6
    As sophiecentaur indicates there is a formal return path for electrons. There is an aluminium overcoat on the back of the CRT screen, and a conductive material (aguadag) all along the sides of the crt bell. These are connected to the 2nd anode and collectively represent the anode of the circuit. If you look at a TV set, you will see a large wire connected through a large boot to the bell of the CRT. This is the return path from the CRT.
     
  8. Jun 10, 2012 #7

    Simon Bridge

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    Typical commercial CRTs for televisions or computer monitors have electron guns that work in the range 5−30 keV. GO read the label. Such electrons have a very short penetration depth in solids and will essentially all be stopped in the collimator or the glass of the tube.

    The result is that the screen will develop a net negative charge. This is the cause of the build-up of dust on the screen and of the low level discharge that you can feel if you run your hand over a CRT surface after it has been running for a while.

    Left to itself the charge bleeds off. Mostly into the surrounding atmosphere, but also into the bulk of the CRT's case.

    The inside of standard (non-storage) CRTs is coated with a thin layer of metal (usually aluminum) to increase conductivity. Tubes intended for television use normally have an aluminised interior which acts as a reflector on the back of the screen, and forms one plate of a capacitor. Aquadag is also painted on part of the outside of many tubes. The glass, tube, internal aluminium coating and the outer Aquadag then form a capacitor used to filter the high voltage EHT supply.

    Aluminising the inside of the phosphor is important so that charge does not build up--Any charge accumulation on the phosphor will raise its potential. That reduces the effective acceleration potential of the electrons, which causes the screen to dim.

    You can also get a secondary emmission - where the phosphor releases an electron.

    I didn't think the main purpose of all this aluminising was to provide a "return path" for the electrons in the beam - but it certainly helps keep the tube charge down. Idon't recall any of this feeding back into the cathode either ... iirc the tubes were all earthed one way of another.
     
  9. Jun 10, 2012 #8

    davenn

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    Actually the electron guns ( 3 for a colour TV) work at ~ 6 to 12 V. The EHT is supplied to a connection on tube on the bell of the tube ( as previously commented on) and brings the inside of the front end of the tube up to a high positive potential. This is the Anode of the tube. Its this EHT that "attracts" the electron beam(s) from the gun(s) to the front of the tube.
    On the way there they are guided and aimed by the deflection coils to produce the beam scanning across the tube's inside front surface


    Dave
     
  10. Jun 10, 2012 #9

    Simon Bridge

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    6-12V accelerating potential gives 6-12eV electrons doesn't it? That does seem very small... you sure that's not just the input voltage and it gets stepped up?

    It's been a while since I've dismantled a CRT TV so I won't argue the toss - I was trying to say that the electrons are not very energetic.
     
  11. Jun 10, 2012 #10

    sophiecentaur

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    After the gun(s) comes the tubular anode to get the high energy beam. The reason for starting off with slow beams would be to get the electon optics right. Convergence is very hard to get right.

    In order to have a safe voltage for all the rest of the electronics, the negative end of the tube needs to be earthy. All the really high volts need to be positive and at the screen end.
     
  12. Jun 10, 2012 #11

    davenn

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    I have been servicing TV's for more years than I care to remember :)

    Thats the voltage on the filament(s) there is NO step up, and is common for many tubes TV and others Some transmitting tubes can run as low as 3V but with currents of many many amps

    Dave
     
  13. Jun 10, 2012 #12

    Simon Bridge

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    I can understand why you wouldn't care to remember that ... I found TV servicing got boring when everything got modular. My old valve TV (B&W) had a kV warning on the CRT and lots of stray fields ... the picture quality depended on things like whether the back was on and where you sat. If I replaced a blown component it would throw the whole thing out because, of course, the old components had all drifted off their marked values. It was loads of fun.

    There's lots of different filaments at the gun end arn't there - isn't there a heating filament behind the cathode? Then there's a row of hat-shaped plates with holes in them in the collimator and a cylinder for the final acceleration just before the deflection plates.

    I vaguely remember that the electrons in very large screen TVs could be relativistic but never checked.

    I did once get my hands on an old radar crt ... those were little more than what you see in text books.

    The next set of CRTs I had a lot to do with were teaching rigs - totally exposed tubes 4" screens and about 10" long, clear for most of their length to expose the workings so they were only aluminised behind the phosphor and no aquadag at all. They could build up quite a charge even though I they had an earth pin.
     
  14. Jun 10, 2012 #13

    davenn

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    Yes, each gun has its own filament (heater) behind the cathode. But particularly in times gone bye going back to the older vacuum tubes, there were 2 methods of electron emission.
    Direct -- straight off the filament and out towards the anode, via any screen and grid plates that may be in that tube
    and
    Indirect -- where the filament heated a cathode which then emitted the electrons

    quite often in the indirect method, the filament wouldn't be shown on the circuit diag, just the cathode ( one would always assume the presence of the filament and there would be a note somewhere or at least a winding on the transformer designated for the filament voltage.

    In a lot of CRT colour TV's, the filament winding would be nothing more than a couple of turns around the ferrite core of the EHT (line output transformer)

    There were 2 styles of the 3 gun arrangement in colour CRT's inline and triangular

    searched hi and low for some decent drawings of a CRT system, seems to be severely lacking
    the best one was this one ....

    attachment.php?attachmentid=48222&stc=1&d=1339377854.jpg

    cheers
    Dave
     

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  15. Jun 10, 2012 #14

    Simon Bridge

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    Well you did better than me :) I also had a look for a schematic of the regular teaching setup ... which is simpler. Also, considering the topic, the diagram does not show an explicit return path for the electrons after interaction with the phosphor. That's something else that is lacking and makes the question quite intelligent.

    I also recall trying, without success, to find ion-gun tube setups that used positive ions ... ideally the + and - ion setups should be superficially identical. The reason was to annoy students when they go to do the magnetic deflection part of the demo. Giving students unexpected results is good for them: forces them to confront and overcome their own preconceptions. However, they have enough trouble figuring out why the x and y plates have different sensitivities (in the teaching setup they are one behind the other... I managed to reverse the X and Y labels on half the apparatus mwa hah hah haa).

    en ee way ... now we are just getting nostalgic. I think OPs question has been answered.
     
  16. Jun 11, 2012 #15

    sophiecentaur

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    I think the relative difference in masses / speeds would have a not-inconsiderable effect on the design, though. (Not to mention the problem of a return path for the ions?)

    I bet they lurved you for that.
     
  17. Jun 11, 2012 #16

    rcgldr

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    The accelerating anodes have high voltage.

    How stuff works article on CRTs, includes brief mention of the "return" path.

    http://electronics.howstuffworks.com/tv3.htm
     
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