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Alternate VFD powering scheme.
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Alternate VFD powering scheme.

by kraftphysics on Fri Oct 09, 2009 10:41 am

I'm curious as to why so many people chose to power VFDs with such high voltages. I've been able to power mine two ways:

Method 1) (As typically used in "Kit" form electronics) is to power the filament with +2-5v and have the segments powered with 60-90v, depending on tube spec. (I'm not considering the grids at this point)

Method 2) the tubes themselves are designed to be operated with a negative bias on the filament, i.e. the two filament lines get ground and -2-5v. Then, the segments only need about +12-16v to light up. It has been my experience that the negative bias option makes the tubes last longer and shine brighter.

The theory is based on the fact that when you heat the filament, electrons are excited (This is the principle behind the electron gun in the back of a CRT as well as an electron microscope) and given off, thereby being freed from the tungsten of the filament. If the filament is biased with positive voltage, then the electrons are attracted to it, requiring a greater force to pull them the distance to the segments. (The force is proportional to the distance squared- the physics is based on the electric force between the particles, and is calculated by F=kQq/r^2 where F is the force, k is a constant, Q and q are the charges of the particles, and r is the distance between them) By biasing the filament negatively, you impart a "Push" to the electrons to get them going, as once they are liberated, they are repelled by the negative bias, and head towards the segments.

Another advantage is that you don't get brightness shift from one side of the display to the other, as often happens in positively biased tubes.

As for powering it, I've found that using an AC adapter as opposed to a DC adapter, do the rectification on the board and pull the negative side of the bridge rectifier. It's a few extra components, but reduces the need for the step-up circuit.

--Justin.

Disclaimer: I am not an electronics engineer by any stretch of the imagination, I am a physicist. I base my statements on my own experience and research, as well as the physics involved, not on electronics literature.
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Re: Alternate VFD powering scheme.

by adafruit on Fri Oct 09, 2009 11:51 am

you can light it with 12V just fine with static drive, but you cant multiplex them at that voltage
http://www.noritake-elec.com/vfd_technology_an.htm

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Re: Alternate VFD powering scheme.

by kraftphysics on Fri Oct 09, 2009 12:02 pm

I've read that document, and it only says that in static mode, you can power it with 10-12v, but it doesn't say that you can't multiplex at lower voltages. According to the physics (And my basic setup here in my lab- not high speed multiplexing, it blinks visibly...), you should be able to maintain a +12v on all grids except the one you are iluminating, and then ground the one you want to see. Just inverse the multiplexing signal.

I don't want to seem difficult, I'm just trying to bridge theory and practice here... :)

--Justin.
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Please be positive and constructive with your questions and comments.