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Author Topic: Vacuum Fluorescent Display driving.  (Read 455 times)
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xeontory
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« on: October 07, 2015, 01:32:34 13:32 »

Dear friends,
I have a bunch of Vacuum Fluorescent Display (VFD). I have attached an image from the front side. In the back side, there is a text:
NEC
LD8117A
TUBE NO. D4 (some are D5)
MADE IN SINGAPORE (some are JAPAN)

All of them are 8 digit seven segment with dots and having 18 pins. The pins at both end are showing 56 Ohms (most probably the filament). I have searched that model no in internet and found nothing about how to find out if they are good or bad, light it up, voltage needed to drive, which pin is ground etc. Can anybody help me to enlighten them?
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Magnox
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« Reply #1 on: October 07, 2015, 07:29:40 19:29 »

I have a load of those too; exactly the same I think except mine aren't marked on the back. You should be able to figure out just by looking closely at it which are the grid connections (the mesh in front of each digit, used for turning on/off that digit when multiplexing) and the segments which are all connected in parallel. It's connected simlarly to a multiplexed LED display but with a grid instead of a common for each digit.

It's been a very long time since I played with a bare VFD, but in essence a VFD is a triode: A heated cathode (filament) which emits electrons, a grid which controls the flow of electrons through it, and an anode (segments) which can attract the electrons if made more postive than the cathode.

You need to drive the filament with a low potential across it (maybe a couple of volts or less) to warm it up (it should not glow at all), and pull a grid and segment more positive (10V to 60V or so, higher is needed when multiplexing bigger displays). The electrons from the warm filament can then strike the segments (the anodes) and excite the phosphor.

I know it was simple enough to get mine lighting up for a functionality test. Search the internet for general VFD display circuits; there is a lot of information. If you want to improvise a test like I said above, wire up the segments and a grid to one PSU at maybe +12V to +20V, then slowly increase the potential on the filament until you see something. If that something is the filament glowing instead of a segment lighting, stop!

In practice, a large negative supply is usually employed that is transistor switched to the segments/grids between the controller's logic supply level and the negative floor to turn on and off. The filament is run from AC to give even illumination across the display (with DC one end of the filament is at a different voltage of course) and is floated a little above the negative floor to prevent 'ghosting'.
« Last Edit: October 07, 2015, 07:46:00 19:46 by Magnox » Logged
titi
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« Reply #2 on: October 07, 2015, 07:52:39 19:52 »

Hi xeontory, Magnox,

generally VFD Display is quite simple to drive,
you have to powered the filament, one side of filament: GND
Other side of filament: 2.5-3V DC
Anode and grids: 12-36V DC depend on the model.
These are some examples to determine what pin is use for and how to drive.
http://www.instructables.com/id/Arduino-VFD-Display-Clock-Tutorial-A-Guide-To-VFD-/

How to control a salvaged VFD (Vacuum fluorescent display)
https://www.youtube.com/watch?v=oxMdLEGoQgU

EEVblog #717 - How To Hack Vacuum Fluorescent Displays
https://www.youtube.com/watch?v=clUVEyi_YNM

Remark: mht file can be open directly with IE by double click on the file or can be open with Word (mht contain html and images)
May be this helps you.
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germanium
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« Reply #3 on: December 26, 2015, 10:19:02 22:19 »

A method I've used to figure out the filament drive level is to use a variable power supply, start at 0 and increase the voltage *slowly* till you see the filament start to glow, then back off until the glow is no longer visible.   Go slow, if you overdo it the filament will burn through and the display is destroyed...
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