Newbie with a question on how to drive VFDs

[ZY]

Hello. I'm pretty new to nixies/vfds, and I have some questions on how to drive a VFD tube before I pick some up on ebay:

1) If the filament needs 1.5v at 0.02A, and I'm driving with 5v, would I just do (5-1.5)/0.02 to get the resistor value?

2) For the same filament, if I have a 1.5v source, am I safe to drive it without the resistor?

3) I read that the grid needs a negative voltage to make sure all the segments are off. Is this true? I've seen some circuits where they just bias the filament voltage a bit higher with a diode or a resistor before the ground. Does that work? Is this what the max6931 filament bias does? As in I supply -7v to the IC, and drive my filament between 0-1.5V, and the IC would switch between something like 30V when things are on and -7V when things are off so that it is negative of the cathode voltage?

4) Why do some circuits bias the filament at a negative voltage like -30v?

5) Do I need current limiting resistors at the grid and anodes?

6) If I'm not using a driver IC, and I want to control the gate/anodes using some mosfets, what would be the best way to do it? Would I do something like this http://cnx.org/resources/8fb47c89fd6fbc28ff5e2699eb54c529ee279f9c/4.35.png

or would that resistor impact performance?

7) Is it safe to leave grid/anode pins floating?

8) If I drive one segment more than the other segments, would it "wear out"? Kinda like cathode poisoning with nixies?

9) What contributes more to tube life? Less filament current, or lower grid/anode voltages? Is it both?

10) What is the mechanism in which VFDs fail? If I don't drive them out of spec, are they "immortal" like LEDs are, or does turning them on consume some sort of material inside that puts a timer on their lifespan? I'm just curious that if I leave a VFD clock on, and it's rated for 2000h or something would it fail in a few months?

11) If I want to dim the VFD, do I pulse the grid or do I pulse the filament?

Thanks for the help!

[John Rehwinkel]

> Hello. I'm pretty new to nixies/vfds, and I have some questions on how to drive a VFD tube before I pick some up on ebay:
>
> 1) If the filament needs 1.5v at 0.02A, and I'm driving with 5v, would I just do (5-1.5)/0.02 to get the resistor value?
>
> 2) For the same filament, if I have a 1.5v source, am I safe to drive it without the resistor?
>
> 3) I read that the grid needs a negative voltage to make sure all the segments are off. Is this true?

Yup, it's true. However, it's easy if you're running your filament from 5V: just put the resistor between filament and ground and hook the filament directly to 5V. Then when you ground the grids/anodes, they'll be more negative than the filament.

> 4) Why do some circuits bias the filament at a negative voltage like -30v?

So they can drive the anodes and grids by simply grounding them (which makes them 30V more positive than the filament).

> 5) Do I need current limiting resistors at the grid and anodes?

Nope.

> 7) Is it safe to leave grid/anode pins floating?

Floating anodes is okay, but you should probably hook unused grids to your most negative voltage (usually ground), so they don't float high and turn on unwanted digits.

> 8) If I drive one segment more than the other segments, would it "wear out"? Kinda like cathode poisoning with nixies?

Very slowly. The phosphor used in VFDs is much more durable than the one used in eye tubes, so it lasts a long time. However, a segment that's on most of the time will eventually be
dimmer than one that's rarely lit.

> 9) What contributes more to tube life? Less filament current, or lower grid/anode voltages? Is it both?

For optimum life, run the filament at its rated voltage, no higher and no lower. Lower brightness (which comes from lower anode voltages) will make the phosphors last longer, however VFDs generally have very long lifetimes.

> 10) What is the mechanism in which VFDs fail? If I don't drive them out of spec, are they "immortal" like LEDs are, or does turning them on consume some sort of material inside that puts a timer on their lifespan? I'm just curious that if I leave a VFD clock on, and it's rated for 2000h or something would it fail in a few months?

While they're not immortal, lifetimes from 30,000 to 100,000h are usual. They're very long-lived devices. Generally they get dimmer and dimmer as they get older, but filaments very rarely burn out (and even if one does, most VFDs have multiple filaments and will still function). 2000h is a VERY short lifetime for a VFD.

> 11) If I want to dim the VFD, do I pulse the grid or do I pulse the filament?

Pulse the grid, you don't want to undervolt the filament (this damages its emissive properties).

- John

[Charles MacDonald]

On 16-02-20 04:03 PM, ZY wrote:
> Hello. I'm pretty new to nixies/vfds, and I have some questions on how
> to drive a VFD tube before I pick some up on ebay:

>? I've seen some circuits where they just
> bias the filament voltage a bit higher with a diode or a resistor before
> the ground. Does that work? Is this what the max6931 filament bias does?
> As in I supply -7v to the IC,

> 4) Why do some circuits bias the filament at a negative voltage like -30v?

> 8) If I drive one segment more than the other segments, would it "wear
> out"?

> 9) What contributes more to tube life? Less filament current, or lower
> grid/anode voltages? Is it both?
>
> 10) What is the mechanism in which VFDs fail?

VFD displays are little triode Vacuum tubes. with some phosphor on the
anodes which glow when their is electron current. (*)

The anode needs to be typicaly 30V more positive than the
Filament/Cathode. SO you can make the cathode negative and keep the
anode near ground or make the anode positive and have the cathode near
ground. Or even a mixture! as long as the DIFFERENCE is whatever the
display is expecting - Ie 30V.

If their is a grid, it is generaly there to allow things like
multiplexing. The data sheet should show the range of voltage it is
expecting. It can be used to turn on or turn off the entire digit, or
even one way of dimming the display.

the filament is the source of electrons so you generally want it to have
an AC voltage at the rated voltage and current. If it has DC and goes
across a multi-digit display the DC levels between the filament and the
grid will vary and so with the light output.

The Phosphor will gradually loose output over use, and that loss tends
to be more pronounced during the first part of the Tubes life. Not
using a segment for a long time then lighting it up MAY result in the
"fresh Segment" being brighter.

The other burn-out mechanism is the filament burning out like a light
bulb, or losing emission over time. In some cases leving the filament
lit up but the tubes biased off may lead to cathode contamination, and
reduced emission.

(*) some enterprising folks have made Phonograph pre-amplifiers using a
VFD. it is not a great triode, but the glow does change with the current.

--
Charles MacDonald Stittsville Ontario
cm...@zeusprune.ca Just Beyond the Fringe
No Microsoft Products were used in sending this e-mail.

[ZY]

Thanks for the quick response and help!