The Real Size and Function of an E-Gun

[vatly]

Hi ,

I have a wonder about the real size of a tivi (sony, toshiba...) e-gun, how it works and what's the maximum e-current of it ? Please tell me something about it.

 

[berkeman]

Google can give you lots of hits to read through. Here's one:

from http://hypertextbook.com/facts/2000/MichelleHong.shtml

"TVs and monitors may have up to 35 KV on the CRT but the current is low -- a couple of milliamps."

Basically, the electron gun has a heated element at the back end called the cathode. The cathode is coated with a material that boils off electrons when heated. The electron gun is put in the back end of the vacuum TV tube, and the cathode is held at a negative potential and the CRT screen is held at a high positive potential to attract the electrons. There are several metal plates of various shapes in the gun itself, and these are held at several intermediate voltages to form electron lenses that contain and shape the beam as it flies forward toward the CRT face. Two magnetic coils are placed at the neck of the CRT (where the tube for the electron gun expands out to mate with the CRT faceplate). These magnetic coils are driven with the horizontal and vertical deflection currents, to cause the electron beam to sweep in the raster pattern across the CRT face. When the electron beam hits the phospurs on the inside of the CRT face, that's what causes the light that we see as the picture.

TVs and computer monitors involve some pretty interesting and fun technologies. And the manufacturing techniques used to make the color shadow mask CRT faceplates is pretty amazing.

Here's a link to a discussion about TVs at HowStuffWorks.com for further reading:

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

 

[vatly]

Google can give you lots of hits to read through. Here's one:

from http://hypertextbook.com/facts/2000/MichelleHong.shtml

"TVs and monitors may have up to 35 KV on the CRT but the current is low -- a couple of milliamps."

Basically, the electron gun has a heated element at the back end called the cathode. The cathode is coated with a material that boils off electrons when heated. The electron gun is put in the back end of the vacuum TV tube, and the cathode is held at a negative potential and the CRT screen is held at a high positive potential to attract the electrons. There are several metal plates of various shapes in the gun itself, and these are held at several intermediate voltages to form electron lenses that contain and shape the beam as it flies forward toward the CRT face. Two magnetic coils are placed at the neck of the CRT (where the tube for the electron gun expands out to mate with the CRT faceplate). These magnetic coils are driven with the horizontal and vertical deflection currents, to cause the electron beam to sweep in the raster pattern across the CRT face. When the electron beam hits the phospurs on the inside of the CRT face, that's what causes the light that we see as the picture.

TVs and computer monitors involve some pretty interesting and fun technologies. And the manufacturing techniques used to make the color shadow mask CRT faceplates is pretty amazing.

Here's a link to a discussion about TVs at HowStuffWorks.com for further reading:

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

Thanks ,
But I still have a doubt about the number of few mA current. Since this few mA current is actually can produce a considerable bunch of bremstralung rays when the hit the phosphorus coated screen, so harful to the audiences. So if you or anyone has some specifications of a tv from the manufactures ? Some even says the current is just few pA. Please help this out

 

[berkeman]

Thanks ,
But I still have a doubt about the number of few mA current. Since this few mA current is actually can produce a considerable bunch of bremstralung rays when the hit the phosphorus coated screen, so harful to the audiences. So if you or anyone has some specifications of a tv from the manufactures ? Some even says the current is just few pA. Please help this out

That's why CRTs have lead in their glass envelopes, and that's why you can't just throw out a TV or monitor (lead = hazardous substance).

Here's another link with more info on TV's HV section. I just googled flyback transformer television, or something like that -- you get lots of hits:

http://www.powerlabs.org/flybackdriver.htm

This site also talks about typical TV flyback transformers being sized at 1-2mA output. Your cathode drive circuit (video amp) pulls that current out of the cathode connection of the electron gun, which pushes the electrons in and out into the boil, and the high voltage flyback transformer attracts the electrons to the +12kV to +30kV CRT face coating.

 

[vatly]

Thanks berkeman,

Here is a link mentioned the pA of the currents: http://www.coursework.info/i/172.html
I agree with you that a CRT needs lead mixed glass envelope to shield the EM radiation. I went through the link you gave but it seems that the mentioned few mA current is the current run through the cathode and this current boils the es in the cathode off then those es are speeded up by the electric field btw the cathode and anode, then heading towards the phosphorus screen.

My woder was about those es current ( or e beam). How much is this e beam ?

Thanks.

 

[berkeman]

Thanks berkeman,

Here is a link mentioned the pA of the currents: http://www.coursework.info/i/172.html
I agree with you that a CRT needs lead mixed glass envelope to shield the EM radiation. I went through the link you gave but it seems that the mentioned few mA current is the current run through the cathode and this current boils the es in the cathode off then those es are speeded up by the electric field btw the cathode and anode, then heading towards the phosphorus screen.

My woder was about those es current ( or e beam). How much is this e beam ?

Thanks.

The link you post is questionable. The way he calculates the beam current is very roundabout. The reason I posted the link to the flyback transformer info is that the flyback current is the current that results from the electron beam that hits the anode material on the faceplate of the CRT. This is the same as the cathode current back at the electron gun. The extra energy to boil off the electrons comes from the filament circuit, not from the cathode drive circuit itself.

I did some more googling, and got a very good hit when I googled "electron gun" "cathode current" +pinout:

http://repairfaq.ece.drexel.edu/sam/crtfaq.htm

This is an extensive discussion of TV technology, including the operation of the electron gun and high voltage circuit. Here is the pinout of a typical CRT (the pins that stick out of the glass at the back end of the CRT):

http://repairfaq.ece.drexel.edu/sam/crtfaq.htm#crtpin

You can see that the filimant circuit is separate from the cathode drive, and there are separate connections for the intermediate-voltage shaping "grids".

With a similar google search, you should be able to get to the datasheets for typical electron guns used in CRTs. It's been over 15 years since I worked designing monochrome computer monitors, so I don't have any of my old datasheets or manufacturer names handy. But you should be able to find them with google. Good luck! -Mike-

 

[vatly]

I just got an equation which governs the electron emission of a hot body. That is the so call Richardson equation : I = A T^2 exp(-b/T), where A and b are the two constants. = ampere/cm^2. I used this formula and assumed that the emission area of a tungsten cathode in a television is around 1 mm^2 to 1 cm^2, and the temperature of this cathode within a range of 100 K to 200 K, then the currents were from the orders of 10^(-9) (A) to few mA.

The energy of this electron beam is the order of KeV.

But i am not sure about the real working current and energy for a spectific television since I don't have the whole specific engineering information.

 

[Antiphon]

I just got an equation which governs the electron emission of a hot body. That is the so call Richardson equation : I = A T^2 exp(-b/T), where A and b are the two constants. = ampere/cm^2. I used this formula and assumed that the emission area of a tungsten cathode in a television is around 1 mm^2 to 1 cm^2, and the temperature of this cathode within a range of 100 K to 200 K, then the currents were from the orders of 10^(-9) (A) to few mA.

The energy of this electron beam is the order of KeV.

But i am not sure about the real working current and energy for a spectific television since I don't have the whole specific engineering information.

Real TV cathodes use complex electron emitters and are only heated from
beneath by tungsten. Temperatures are not very high. Matierials like
thorium and cesium are commonly employed because they have a low
work function for emitting electrons compared to tungsten.

 

[vatly]

For tungsten, the melting point is 3600 (K). It seems to mee that the normal temprature of a tungsten cathode is around 2700 (K).