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Unexpected VFD behaviour

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Vacuum Fluorescent Display, not the other kind of VFD!
Anyway, I was experimenting with VFD filament drive based on this article: http://www.kerrywong.com/2013/06/05/vfd-filament-driver-using-555/comment-page-1/ and trying out different sizes of ferrite core (yes, very tedious), I'd found one where it was coming on but still needing a bigger core.

Most unexpected was the segments being lit more brightly at the ends of the display than in the middle! I suppose this makes interesting evidence of the filament resistance affecting brightness, even with AC drive.

I suppose the effect just disappears when adequate filament drive is used, but I thought it worth commenting on :)
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