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Bit of a silly question, I know, but one I couldn't find the answer to in a decent amount of searching... why exactly do CRTs use such a high accelerating voltage? Is it that the electron beam has to have a certain KE to light the phosphor? Or is there some other limitation that requires the electrons to have a higher energy?

Here I'm assuming that the designers would have preferred a lower voltage; lower risk of shock, less paranoia about x-rays, et al.

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    $\begingroup$ To light up the phosphor dots., AFAIK, takes a lot of energy. $\endgroup$ – user140606 Jan 23 '17 at 23:06
  • $\begingroup$ @Countto10 - Not sure if it necessarily takes a lot of energy to light up any type of phosphor since there are vacuum fluorescent digital displays which operate with only around 50 volts. $\endgroup$ – Samuel Weir Jan 24 '17 at 0:18
  • $\begingroup$ @SamuelWeir Very good point, forgot about VFDs. $\endgroup$ – 0xDBFB7 Jan 24 '17 at 0:40
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    $\begingroup$ VFDs have different tradeoffs. For a TV, you need for a single electron to make a good amount of light, since the beam is rapidly traversing the entire display. And, that light needs a certain persistance, i.e. needs to stay relatively steady over the retrace period. So, you need a lot of energy deposition, so you want a fairly high electron energy in the first place. $\endgroup$ – Jon Custer Jan 24 '17 at 0:48
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    $\begingroup$ @Lagerbaer - Yes, I was thinking that, too. Something like a 50 eV electron beam may possibly even be significantly affected by the orientation of the TV with respect to the Earth's magnetic field. $\endgroup$ – Samuel Weir Jan 24 '17 at 1:11
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The electron beam experiences two forces. There is a set of high voltage plates that accelerates the electrons to a high velocity. There is also a set of coils which produce a magnetic field that steers the electrons. Due to the dimensions of the CRT, including the length that the electrons travel in getting to the screen, and the maximum angle that they need to be steered through in order to reach the top and bottom of the screen, they need a given velocity if the magnetic field is to steer them through the appropriate angle, because the magnetic force on the electron is related to both the magnetic field strength and the electron velocity.

From a practical standpoint, it is more convenient to build a "shorter" CRT device to ensure that it doesn't take up too much space in a room. This constraint leads design engineers to go for a larger maximum angle required to hit the top and bottom of the screen, which requires higher voltages and magnetic fields.

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  • $\begingroup$ So electrostatic-based deflection systems do not require as high a voltage, then? $\endgroup$ – 0xDBFB7 Jan 24 '17 at 2:05
  • $\begingroup$ If you don't have a length constraint (e.g., you can afford to build a CRT tube that is 10 or 20 feet long), I am certain that you can use lower voltages and magnetic fields. Unfortunately, no one would buy the thing, which sort of begs the question of why anyone would build it. $\endgroup$ – David White Jan 24 '17 at 2:13
  • $\begingroup$ Looking it up now, it does indeed seem that non-magnetically steered tubes have much, much lower voltages- one here says it runs at <1kv, contrasting with large CRT's 30kv. This seems to confirm this as the cause $\endgroup$ – 0xDBFB7 Jan 24 '17 at 2:32
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    $\begingroup$ ..."because the magnetic force on the electron is related to both the magnetic field strength and the electron velocity." - That's true. However, the faster the electrons are moving, the less time the force due to the magnetic field has to alter the course of the electron beam. So offhand it's not clear to me that there is a net benefit to using a higher energy electron beam if one wants to to steer the electron beam over a wide range of angles in a CRT. $\endgroup$ – Samuel Weir Jan 24 '17 at 3:00
  • $\begingroup$ @DavidWhite: have a look at good old analogue oscilloscopes, they mostly use electrostatic deflection since its more precise and faster to control. Also a good portion of the reason to use magnetics is to lens the beam better, since electrostatics and wide angles have heavy problems in focussing. $\endgroup$ – PlasmaHH Jan 24 '17 at 8:49

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