Hi:I am designing a few vacuum fluorescent display clocks using Russiansingle digit VFD tubes. One thing I spent some time thinking about howto do in my own way was the filament drive. I wanted AC drive, but notwith magnetics. They are either too big, too expensive, too muchtrouble to design to a cost optimum, or too time consuming to windmyself in quantities of more than 1-2.So I settled on a capacitively coupled half-bridge.The only drawback, is that at present it is open-loop. It takes itsbridge power from either the +5V main logic supply, or an optionalalternate +12VDC bus. The PWM source is designed to be flexible. ThePCB can be configured (by not populating some parts, and settingappropriate jumpers) to take alternating PWM pulses from a uC capable ofthis. Or a single PWM signal from a uC, with pulse steering logic onboard. Or by using an on-board 555 oscillator with an inverteravailable to allow configuring for nearly any duty cycle to feed to thepulse steering.The AC coupling, as would be true with a transformer, removes the riskof blowing the filaments if the PWM locks up, which is a significantrisk if a uC is the PWM source.So the remaining hazard is that the power supply voltage might rise toohigh and fry the filaments.Oh, a secondary hazard is what happens if one tube filament opens. Thenthe voltage will rise on all the other tubes a little bit. Perhaps theuser would notice a blank tube and seek repair before this leads torunaway, so this is probably a negligible concern. It's more of aconcern on a socketed tube design where one can easily pull out tubes.This is a rather remote danger but it does disturb me a bit. If it isjust for my personal hobby clock, and I blew a set of tubes while havinga lot of spares, it wouldn't be a big deal.But I plan to sell a few of these, probably when I retire. So I'd hateto have one go poof where my reputation is involved (well, honoring awarranty and being generous when out of warranty can go pretty far...

That is a good deal. Although, for a Russian this is a lot of money.-- Regards, Joerg"gmail" domain blocked because of excessive spam.Use another domain or send PM.

Not according to our Rottweiler :-)-- Regards, Joerg"gmail" domain blocked because of excessive spam.Use another domain or send PM.

Is this a hazardous design?

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Mr.CRC 15 years ago

The NL840s have presumably all been tested. I double check them, testing so far about 10-12% of the bunch. I've yet to find any bad ones. Since they were probably made

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Mr.CRC 15 years ago

Fortunately, the Russian VFDs are actually quite inexpensive. About $1.50 per tube.

_____________________ Mr.CRC crobcBOGUS@REMOVETHISsbcglobal.net SuSE 10.3 Linux 2.6.22.17

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Joerg 15 years ago

That is a good deal. Although, for a Russian this is a lot of money.

Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.

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Warren 15 years ago

Joerg expounded in news: snipped-for-privacy@mid.individual.net:

Yes- don't forget cats. Our one cat likes to knock things off the dresser until someone finally gets out of bed to feed him and give attention.

Scolding doesn't work. As soon as you climb back into bed he's back up there again. We almost had the clock radio go sliding off one morning.. it's just a matter of time.

Warren

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John Larkin 15 years ago

We have two cats. Ajax and Comet. Very clean cats. Comet can open drawers, so almost every night she opens my sock drawer, unpacks/scatters about half the socks, and sleeps on the rest.

The kitchen table is glass, which is hard and cold. Newspapers are relatively warm and soft. So we don't get to read much of the paper in the morning.

Cats are the top of the food chain.

John

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Joerg 15 years ago

Not according to our Rottweiler :-)

Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.

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Warren 15 years ago

John Larkin expounded in news: snipped-for-privacy@4ax.com:

It's funny how they like to interfere with reading the paper!

My other cat problem is keeping the sister cat from sleeping on my cable PVR (DVR). She likes those "always warm" air vents to sleep on. The problem is that her hair gets inside. It's just a matter of time, which was one deciding factor in renting the unit.

When we first got our HD plasma TV, the first HDMI cable was sacrificed to cats. Now the cables have that plastic automotive harness covering over them. The little chewers can't get at 'em now.

(Don't get me started-) Years ago a cat was responsible for knocking over something on my workbench. It shorted out and started a fire (in the basement). We caught it in time fortunately, thanks to smoke detectors.

Cats are fuzzy walking hazzards. Don't underestimate!

Warren

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Jim Thompson 15 years ago

Your Rottweiler never met my Burmese.

One of our Dachshunds decided it was fun the T-bone the cat, until the cat bounced up from such a hit and bit the end of the Dachshund's tail off :-)

My wife was seeing these funny red semi-circular red marks on the kitchen tile... it was the Dachshund wagging his tail :-) ...Jim Thompson

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John Larkin 15 years ago

There's something about the white Apple computer cables that's irresistible to cats.

John

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krw 15 years ago

Expensive tastes.

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josephkk 15 years ago

Like

can

and

do

digit.

nice

bring

Except NIixie(tm) tubes are neon filled not vacuum. Different voltage line up as well, about 180 V instead of 10 to 20 for many VFD.

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Is this a hazardous design?

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