Negative voltage, contrast and display

[pozz]

Hi all,

can someone tell me what is the technique used in the display
to adjust the contrast? Normally there's a negative voltage
to make that but what happens when that voltage changes?
Why a negative voltage is necessary?

I want to study the display technology. Can someone tell me
some tutorial or web site with a lot of informations about
this subject?

 

[Rich Grise]

On Mon, 09 May 2005 18:06:23 +0000, pozz wrote:

Hi all,

can someone tell me what is the technique used in the display
to adjust the contrast? Normally there's a negative voltage
to make that but what happens when that voltage changes?
Why a negative voltage is necessary?

I want to study the display technology. Can someone tell me
some tutorial or web site with a lot of informations about
this subject?

Well, here's a couple of places that you can start at...

Ah, Schtool! After a search at
http://www.howstuffworks.com/search.php , they leave the URL
in the window, unlike google. I did a search on "cathode-ray
tube" (without the quotes) and on "CRT display" (also without
the quotes) and a lot of the links that showed up sound like
a very good introduction to CRT principles.

The short answer is that they use a negative voltage because
it's a "tube". The electrons boil off of the cathode, and are
attracted to the anode(s), just like any tube, but they have
focusing (focussing?) electrodes to make a beam; but generally,
the control grid will be more negative than the cathode, to
regulate the rate of flow of electrons. This will have a DC
bias, which sets the "brightness", or "intensity" level, and
a control signal (the "video") which changes the intensity
very quickly as the electron beam is scanned across the screen.

The "contrast" control actually controls the gain of the video
amplifier - more variations in control grid voltage give more
variations in intensity of the "flying spot".

Hope This Helps!
Rich