0

I want to know how the inside circuit of a CRT looks, from what I've understood it is that you heat the cathode, then some of the cathode electrons are released, then it gets focused and accelerated using the anode, and finally it hits the fluorescent screen.

My first question is: when you connect the anode and the cathode together with high voltage, that makes the electrons of the cathode attracted to the anode right? then how come they are in a circuit? shouldn't some of the released electrons go inside the anode to complete the circuit?, and if that happens I suppose the electron route is like this: cathode > vacuum > anode > wire > cathode and so on. Correct me if I'm wrong please.

My second question is why is the fluorescent screen connected to the anode? I got that mentioned in my text book and I don't understand why, shouldn't the screen just be connected to the ground? I feel like there are many types that you can connect a CRT circuit with, if that's the case please enlighten me.

And finally, if you don't quite understand me, then kindly just put a detailed explanation of how the electrons move in the CRT inside circuit.

Regards.

Abdo Ismail
  • 11
  • 1
  • 2
    There is a lot of individual questions which basically boil down to generic questions how a CRT works. So what research have you done so far? Have you read for example sites such as Wikipedia or Howstuffworks to know the basics? – Justme Apr 19 '22 at 14:34
  • @Justme I know most of the important details and formulas, I just don't understand the circuit, like how do the electrons cycle? do they even cycle? – Abdo Ismail Apr 19 '22 at 14:57
  • In a lot of electronic design we don't care about cycles, we care about electrons going from one node, through some components, to another node, and we have a device that recycles them back to the first node which is called a "power supply", and we don't really care about it very much as long as it recycles the electrons correctly. – user253751 Apr 19 '22 at 16:53

2 Answers2

1

One thing that may be missing from the simple explanations is that the phosphors typically used in CRTs are not electrically conductive, so the electrons could have trouble getting to the anode connection- that would lead to charge build-up on the phosphor.

To prevent that, a thin (100nm) layer of aluminum is deposited on the back side of the phosphor and connected to the anode. So the electrons go in a loop, as expected.

The cathode is kept near ground because the control electrode (the grid) which has to be driven by circuitry is then near ground. Having it at -kV would greatly and unnecessarily complicate the circuitry.

Spehro Pefhany
  • 397,265
  • 22
  • 337
  • 893
0

why is the fluorescent screen connected to the anode?

from what I've understood...some of the cathode electrons are released, [and are] accelerated using the anode

You almost answered your own question. What accelerates the electrons is the voltage difference between the anode and the cathode. Opposite charges attract: The negatively charged electrons are attracted to the relatively positive voltage of the anode. That attraction is what accelerates them.

shouldn't the screen just be connected to the ground?

Just to be clear, "Ground" is an arbitrary circuit node that we use as a reference compared to which, other voltages in the circuit are measured. The voltage of the ground node is zero by definition.

By convention, the "ground" node in a circuit with a single-ended power supply usually is the most negative voltage. (I.e., ground is zero, and all other voltages in the circuit are more positive than ground—they're all greater than zero.)

The electrons can only accelerate if the anode is more positive than the cathode. But in a circuit that follows the usual convention, ground can't be more positive than any other node. Therefore, in the usual convention, the anode can't be ground.

Solomon Slow
  • 3,103
  • 16
  • 20