CRTube mumbling continue - Page 1
Forum section: electronics / beginners

author: m k
author_rank: Super Contributor
timestamp: November 23, 2024, 07:29:13 pm
content: Quote from: TimFox on November 19, 2024, 02:57:40 pmI think I get it now.My problem was that the thought process partially excluded the second loop.Lesson here is to not think particles first, particles have positions, thinking can be distracted.When control grid voltage is negative it repels electrons from cathode, some electrons are emitted from outer surface of the grid.That continues until voltage turns positive, but nothing really changes at that moment.Electron density on cathode is a combination of both loops/circuits, cathode-anode and grid-cathode.For negative control situation it can't be so that one set of electrons go from cathode to anode and another from control grid to cathode.Electrons are real particles, they occupy space, so their density is literally how far away other electrons are.Surface of material is no different, but atoms behind it create a different environment, less attractive when heated, more when charge space in front of it has more charge.So charge space is only one, cathode and grid surface electrons included, the force there is a photon cloud, initiated by electrons and forcing them as it does.Voltage is not a fundamental, it's just a potential difference.So when control grid voltage turns positive electron density just go up a tick.So electron flow to anode and control grid increases, nothing unusual, this continue until electron density is so high that electric arc happens, at that point electron flow to anode decreases.When grid voltage is negative enough released electron density is so low that picture is missing, system is in cut-off state.All spaces between different parts after control grid are electrostatic lenses.Scattering or collecting depending how before and after voltage polarities are, mainly on purpose, but some as a side effect.Internal shape of grid can also alter electron track.Since lenses are electrostatic nothing is diffracting electron track from perpendicular lines of tangents of arc shape of lens.So electrical adjustment of electron track can be made very accurately.https://en.wikipedia.org/wiki/Electron_gunhttps://www.kimballphysics.com/learning_center/electron-gun-beam-systems/

author: TimFox
author_rank: Super Contributor
timestamp: November 23, 2024, 07:59:55 pm
content: Not quite.Remember that the electrostatic field (E) is a description of the force on a charge carrier:F=QE, whereboldface means a vector quantity (with magnitude and direction).The E-field is thevoltage gradientin the space between electrodes."Voltage" and "potential difference" are synonymous.The "grid" in a vacuum tube affects the electrostatic field at the cathode, and changes the cathode current: see the Child-Langmuir law in any appropriate reference for the effect of anode, grid, and space charge.In a CRT, the "grid" is really a hole in an electrode, not a grid of wires as in a triode tube; it is sometimes called a "Wehnelt", using a German word.In a CRO, the grid voltage is determined by the external circuit, should be negative with respect to the cathode, and does not emit electrons: electrons pass through the hole, with a current depending on the E-field between the cathode and grid.In practice, the total field at the cathode depends on the "space charge" (density of electrons emitted by the cathode forming a "cloud" near the cathode surface), the grid-cathode voltage difference, and the cathode-anode voltage difference. The grid is closer to the cathode and has a stronger effect on the field at the cathode.If the grid is sufficiently negative, then no electrons get past it and the current is "cut off".In CRO use, one avoids having the grid positive with respect to the cathode, since some of the cathode current will then flow in the grid circuit.Useful design of the electrodes gives an E-field that acts as a lens to accelerate and focus the electron current in a useful manner: the electrodes should not scatter or collect electrons before the phosphor screen.These electrodes are normally rotationally-symmetric (cylindrical) about the beam axis."Since lenses are electrostatic nothing is diffracting electron track from perpendicular lines of tangents of arc shape of lens." is not a correct description of electron optics.Another set of electrodes, sometimes last and other times followed by further acceleration, are used to deflect the beam away from the axis, one set for horizontal and the other set for vertical deflection, in order to change the position on the screen where the electrons hit.

author: janoc
author_rank: Super Contributor
timestamp: November 23, 2024, 08:48:04 pm
content: Quote from: TimFox on November 23, 2024, 07:59:55 pmIt is not quite a German word, more like a Germannameof the guy who has invented it:https://en.wikipedia.org/wiki/Wehnelt_cylinder

author: TimFox
author_rank: Super Contributor
timestamp: November 23, 2024, 09:46:08 pm
content: From your source:"A Wehnelt cylinder (also known as Wehnelt cap, grid cap or simply Wehnelt)" ...In this context, a Wehnelt acts as a control grid, but is actually a round hole in an electrode rather than a wire grid.It is part of the electron-optical "lens" in the electron gun.

author: m k
author_rank: Super Contributor
timestamp: November 24, 2024, 05:13:27 pm
content: I tried to avoid word "field."Second picture, space between G1 and G2, there are electrons, not active, but still, same between deflection and secondary anode.I tried to include that the whole space inside a tube is electrically active, more or less.One description problem is standard words and their practical representation.Context must be exact or mental image is probably wrong.Second and third pictures try to explain real shapes of grids in this context.Photons are the vector, or as many vectors as is required.In my opinion we should have words describing happenings.It's sort of elitist or lazy to through a plain formula.Good thing around here is that a formula is usually like an addendum.Scattering and collecting from me was not accurate enough.It's electron beam related first, not grid related.I'd say that after hundred years all kind of descriptions should plenty.But not quite it seems, quantity of descriptions of problems seem to be limited.Maybe I just can't find them, but isn't that a practically same thing.For now I would like to know what kind of problems can be spotted easily.Wrong voltages are clear, lower voltage currents also, if known, but how is HV current?I've never measured HV current directly, there shouldn't be "any" problems with floating meter, but I wouldn't like it, even after all kind of distance precautions.From picture all asymmetrics are easy to spot, but are there rules of thumb what kind of problem picture is a game changer, one way or another?

author: TimFox
author_rank: Super Contributor
timestamp: November 24, 2024, 06:11:30 pm
content: In general language, there are several meanings of "vector", e.g., a mosquito that transmits disease.However, in physics and mathematics, there is a universal meaning of vector:"A vector is a quantity or phenomenon that has two independent properties: magnitude and direction. The term also denotes the mathematical or geometrical representation of such a quantity. Examples of vectors in nature are velocity, momentum, force, electromagnetic fields, and weight."(In mathematics, this can be further generalized in linear algebra, but that is not relevant here.)To understand electrical forces on charge carriers, one needs to understand vector fields (electrostatic for these CROs, but also magnetic fields for other applications).It is not useful to invoke "photons" as vectors in this context.When designing electron guns, the shape, dimensions, and spacing of the electrodes give the details of theEfield that accelerates the electrons along their path.

author: m k
author_rank: Super Contributor
timestamp: November 24, 2024, 08:18:12 pm
content: Quote from: TimFox on November 24, 2024, 06:11:30 pmProbably not.Idea was to bring up that as far as we currently know there are no magic in fields.Calculations may have some counter intuitive parts, but all are results of experiments.

author: TimFox
author_rank: Super Contributor
timestamp: November 24, 2024, 08:32:55 pm
content: Fields are not magical, and have been carefully studied since the 19th century.They quantitatively explain physical situations and processes. The math can be complicated for complicated situations, but there is plenty of engineering software that can deal with that, using classical electrodynamics.You may think that an electric field in vacuum is not real. Normal matter, including your body, is very close to electrically neutral (zero net charge), so the effects of anEfield in air may not be felt (except in ones hair).However, magnetic fields in vacuum or air are obviously real. I once needed to take a normal wrench into the bore of a 0.5 T MRI scanner: the force and torque on that 250 g of steel required a stiff wrist while repairing the problem.

author: IanB
author_rank: Super Contributor
timestamp: November 24, 2024, 08:42:04 pm
content: Quote from: TimFox on November 24, 2024, 08:32:55 pmI'm maybe surprised that the manufacturer of the scanner did not also provide a kit of nonmagnetic tools to do maintenance on it? For example, I think some stainless steel grades are not magnetic and so might be a good material for such tools.

author: TimFox
author_rank: Super Contributor
timestamp: November 24, 2024, 10:07:53 pm
content: Quote from: IanB on November 24, 2024, 08:42:04 pmActually, I mis-remembered the event. I had to use a heavy high-power soldering iron to repair a shield connection. Since the joint was on the interior of the gradient-coil cylinder, near the mid-plane of the device, the problem was actually the torque that tried to align the iron to be parallel to the axis. Since the magnetic field inside the superconducting solenoid is quite homogeneous and the vectorBis close to the axial direction, there should be zero net force on an iron object near the mid-plane, just a torque (similar to that on a compass needle).However, tools left on the patient couch near the end of the solenoid's bore quickly flew into the interior. I was younger then.We had non-magnetic small tools, usually made from copper alloys (bronze or Be-Cu). Although 300-series stainless steels are often considered non-magnetic, they are in fact weakly magnetic (especially if work-hardened).