Marsy Electronic

Link Articolo Originale: http://hem.passagen.se/communication/vfd.html

Vacuum Fluorescent Display (VFD) 

On this page I will introduce you to a type of display called Vacuum Fluorescent Display (VFD).
This display is commonly used in VCR, car radios and 
in other electronic equipment.
This type of display gives a bright light with great contrast.
Many people think this display is difficult to control, but no, it is NOT!
I will explain how you can re-use this display in your homebrewed projects.
All contribution to this page are most welcome! 

Background
A nice looking display always impresses and gives a nice light from your homebrewed project. I believe one can always have the use of a display. Most common displays are LED which gives a bright light, but can most often only show numbers, not characters. Then we have the LCD type with backlight. This type is less attractive and the contrast and light is not good. The VFD-display is much better. They give a bright green light. Almost every VCR uses a VFD to show time and other info. Many home stereos also use VFD. The latest trend is to have as many effects as possible on those VFDs. The bars and text are flashing and moving.

Many late evenings, I usually visit the local dump station for for electronic parts. There, I have found many VCR and broken stereos. All of them had nice VFDs. As you will see, the VFD display is custom made for each purpose, so there is lot of preprinted text on the VFD like "PLAY, REC, AM, FM, VCR, START, END......" and there are also some digits. If you are lucky, you may find a VFD with lots of digits. It is not difficult to get such a display working. All you need is some power and a few parts. Most often you will find lots of pins connected to the glass of the VFD. 
I will now go on explaining how they work, then how you can build a unit to make the display come alive.

1. VFD Operation
The VFD is composed of three basic electrodes; the Cathode (Filaments),  Anodes (Phosphor) and Grids under a high vacuum condition in a glass envelope. The Cathode consists of fine tungsten wires which are coated with alkaline earth metal oxides which emit electrons.  The Grids are a thin metal mesh which control and diffuse electrons emitted from the Cathode.  The Anodes are conductive electrodes on which the phosphor is printed to indicate characters, icons or symbols. Electrons emitted from the Cathode are accelerated with positive potential applied to both Grid and Anode, which upon collision with the Anode excites the phosphor to emit light. The desired illuminated patterns can be achieved by controlling the positive or negative potentials on each Grid and Anode. This voltage can be as low as 10VDC.
Fig.1 Basic VFD Structure	1.	Glass Substrate (Anode Plate)	10.	Getter
	2.	Conductive Layer	11.	Face Glass (Cover Glass)
	3.	Anode (Base)	12.	Spacer Glass
	4.	Insulation Layer	13.	Evacuation Tube
	5.	Phosphor (Display Pattern)	14.	NESA (or ITO) coating
	6.	Conductive Paste	15.	Lead Pin
	7.	Grid Mesh	16.	Mold Resin
	8.	Conductive Frit Glass	17.	Solder
	9.	Filament (cathode)	18.	Frit Glass

Disassembled VFD:

The VFD is composed of a vacuum envelope with a front glass and the base plate, in which cathode (filament), grid and anode are formed as the basic electrodes

Cross section of VFD :

Cross section of VFD:

The filament consists of tungsten, coated with the oxidized Ba, Sr and Ca. The power coated filament generates heat and emits thermal electrons which are dispersed and selected by the grid electrode and reach the anode electrode. On the anode electrode, display pattern is formed by the phosphor which emits light.

This display was salvaged from a VCR. The two large terminals at each side is the connection to the filament. The voltage across the filament should be about 2-3V the current consumption is about 100mA. The display has 11 grids. Each of them has a connection out from the glass substrate. The remaining nine pins are connection to the segments in the display.
The VFD is controlled like this:
The first grid will be raised to +14V and the rest held at 0V. Then apply +14V to the segment you want to be lighted in the first grid. Then it will put the first grid to 0V and got to the next grid and apply +14V. This scan will continue untill the last grid was activated and the process will start all over again. The scanntime is fast so you will not see that only one grid is activated at one time. You will see them all shining.

If you want to test a VFD just connect 3V over across filament. Then you apply +14V to one grid. The minus should be connected to the filament. Now take a wire from the +14V and touch the segment pins from the display except the filament. Touching the filament will burn it up! 
You will now see that the segment in the active grid will light up. If you touch another grid nothing will happen, so don't worry. 

How to drive the filament

AC Drive
Most popular method for the audio system and large-size VFDs.
[ Fig. 4 Connection of AC Drive ]	[Fig. 5 Potential of AC Drive ]
DC Drive
Mainly used for small-size VFDs driven by the car batteries. In this case, there are differences in the grid and anode voltages at the ends of the display pattern in the value of filament voltage, which requires correction of the filament structure. Therefore, DC drive is not available for large-size VFDs.
[ Fig. 6 Connection of DC Drive ]	[ Fig. 7 Potential of DC Drive ]
Pulse Drive
Used for relatively small size VFDs which are driven by the car battery not by DC drive. When setting the filament voltage, use the instrument to measure the effective value to obtain the optimum filament temperature.