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Is this a hazardous design?

Hi:

I am designing a few vacuum fluorescent display clocks using Russian single digit VFD tubes. One thing I spent some time thinking about how to do in my own way was the filament drive. I wanted AC drive, but not with magnetics. They are either too big, too expensive, too much trouble to design to a cost optimum, or too time consuming to wind myself 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 its bridge power from either the +5V main logic supply, or an optional alternate +12VDC bus. The PWM source is designed to be flexible. The PCB can be configured (by not populating some parts, and setting appropriate jumpers) to take alternating PWM pulses from a uC capable of this. Or a single PWM signal from a uC, with pulse steering logic on board. Or by using an on-board 555 oscillator with an inverter available to allow configuring for nearly any duty cycle to feed to the pulse steering.

The AC coupling, as would be true with a transformer, removes the risk of blowing the filaments if the PWM locks up, which is a significant risk if a uC is the PWM source.

So the remaining hazard is that the power supply voltage might rise too high and fry the filaments.

Oh, a secondary hazard is what happens if one tube filament opens. Then the voltage will rise on all the other tubes a little bit. Perhaps the user would notice a blank tube and seek repair before this leads to runaway, so this is probably a negligible concern. It's more of a concern 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 is just for my personal hobby clock, and I blew a set of tubes while having a 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 hate to have one go poof where my reputation is involved (well, honoring a warranty and being generous when out of warranty can go pretty far as well). But the costs of going closed loop are not desirable.

I know many VFDs are driven open loop from AC lines (via transformers) and the like. So am I being paranoid?

This is actually an interesting exercise for me because I don't normally have to think about costs in my work. So having to deal with cost vs. reliability trade-offs is an interesting new challenge.

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Zener diodes?

Good Luck! Rich

Reply to
Rich Grise

Make sure that capacitor can handle the AC current over the long haul.

Operating the thing with some tubes pulled would fall under the category "user stupidity", or maybe even "daft". That's like roaring off in a car with numerous lug nuts missing.

Cost? Huh? A few resistors and caps, that's all. If you want it simple you could consider hysteretic operation which even a uC should be able to do.

You have to get the datasheet for those tubes and check the filament voltage tolerance. Usually quite high. I don't know about Russian tubes but figure 10 in this Noritake document shows that the luminance remains relatively flat from 60% to 100% filament voltage:

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Often people "revive" tired VFDs by increasing the filament voltage to where you can see the wires emit a faint red glow in the dark and they still live on for years. If your 5V is reasonably stable the risks seem to be a grossly wrong PWM ratio in the uC case or maybe a blown capacitor. Doesn't sound too unusual or unreasonable to me. I mean, everything is going to go kaputt some day, nothing lives forever. Even you and I won't.

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Reply to
Joerg

You could drive the filaments pulsed, from +12 to ground, at a low duty cycle. Pulse the filaments when the display segments are off, which eliminates the filament voltage interaction. The filament driver can be a p-fet, with capacitive coupling from a cmos (port pin?) level up to the gate. That's fail-safe.

John

Reply to
John Larkin

Until that P-channel fails and shorts out. 12V is a bit much for most VFDs, I'd drive it from the 5V rail if available.

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Joerg

Or the power supply catches fire. Or until a supernova goes off and melts the Earth. Hey, life is risky.

I wonder what those Russian VF tubes are like.

John

Reply to
John Larkin

Where you guys are I'd be more worried about this clock falling off the shelf because the shelf fell over, because the wall behind the shelf fell onto it after a major rumbler ...

Right now there is an amazing flurry of NOS tube stuff showing up. Like the PL81, a flyback sweep tube from the early 50's. A guy on a German forum had an interesting problem where he needs to swing over 4kV in nanoseconds and I was musing aloud that, yeah, if this old PL81 was still around it would be sweet. Then someone posted a link where you can buy those off the shelf for a couple of bucks. Blew me away. It's essentially like a 6KD6 but much skinnier and can do 7kV.

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Joerg

I'll have 0.6A RMS current with either of two tube configurations, one with 1.5VAC, the other at 2.4VAC. My PCB has two locations for parallel caps. Pana FM series, either a 150uF 35V or 220uF 25V can handle 0.95A with 0.056ohms ESR. These should suffice.

Alternatively, "within the reach of children." I certainly used to abuse the TV set and other household electronics when I was a kid in this manner ;-)

What is "hysteretic operation ?"

The IV-12 (7-seg 21mm high digit with DP) seems to allow a range of

90-110mA fil. current, at a nominal 1.5V . The IV-17 (16-segment 16mm character) has poorer translations, with roughly 50mA current. We can assume a similar +-10% tolerance.

With the 2.4V tubes, it will actually be barely possible to reach this with 5V supply and accounting for losses. So no way to burn these out.

The 1.5V tubes are possible to fry with a 5V supply.

I agree it's not much worry.

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Mr.CRC

I hate it when that happens.

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Paul Hovnanian P.E.

[...]

It trips off the conduction phase when the voltage has reached target but won't come back on until it has dropped a few ten millivolts below target:

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The comparator has a hysteresis and thus isn't as "nervous" as a regular comparator. This does cause ripple on the output but for a filament drive that shouldn't matter and those converters are simple, IME very stable.

Think of it as the accelerator pedal of a Ferrari Testarossa on ice versus that of a John Deere tractor in a muddy field :-)

For a VFD that would be a very narrow tolerance.

You might want to place two 1.5V tubes each in series.

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Joerg

Or MOVs. Anything to clamp transients that caps like a whole lot less than transformers.

There are lots of LED nightlight and other low power products that use cap dividers of various topologies right off the line.

Problem: a clock is a rather complex device, with lots of other voltage sensitive goodies in there. If the VFD supply jumps one way due to a power line spike but the uP stays put on another supply, what gives up first?

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Presumably with the other end of the filament biased at +3 to +5V with a zener diode so that anode and grid current keeps the cathode positive w/respect to the grid, right?

I would have to do some experiments to see what the time constant is for the decay of temperature. Those wires are awfully thin, so I have my doubts.

Oh, come to think of it I am planning to have the 7-seg. tubes on direct drive, for maximum brightness, rather than multiplexed. So this doesn't really work with continuously on tubes.

The 16-seg. tubes OTOH, will be a set of 12 multiplexed as six pairs. This might work there. But I am trying to create a universal driver board with filament drive, boost voltage for anodes+grids, and up to 60 bits of serial-to-parallel HV output drivers. So whatever scheme I use has to work in both situations.

I'll probably stick with the 1/2 bridge, since it's all laid out already.

Thanks for the input.

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Mr.CRC

Hmm. This is an interesting idea, since it is way simpler than my bridge/2, and thus cheaper. But it does create a new risk.

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Mr.CRC

Here's some examples of the VFDs:

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I have a bunch of Russian 40mm digit Nixies, as well as German 30mm, and US and German 15mm. The smaller Russian tubes are not to my liking, do to crude construction or silly upside down '2' characters for the '5' digit.

The US (National NL840) and Telefunken ZM1210 15mm tubes have very nice aesthetics, so I have bought up much of the available surplus supply. Once the Russian tubes run down in 10-15 years, I will retire and bring out the nicer looking western made ones, which at that point should be extremely rare.

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Mr.CRC

I'm not sure there's much to worry about with regard to differentials between the VFDs and the other electronics. There will probably be a regulated 12VDC power supply from the line, a wall-wart with UL rating so the safety issue is outsourced.

5V for logic would be derived from that. The VFD filament driver can be set up to work off the 5V or the 12V. The worst thing that can happen is the 12V supply goes bonkers. So perhaps something to protect against that is sufficient. Some MOV or TVS, and a fuse. 
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Mr.CRC

Dunno. The VFs that I've used didn't seem to care. They worked fine with filament near ground. But you could return the low ide of the filament to the +5 supply, if it needs it.

Hey, try it.

Sure. Tubes are fun. I like VFs, but they are getting hard to find. Noritake EOLd one 8-char, 16-segment tube that we used in a bunch of products. We made a PCB with a uP and an LCD, that accepted the VF display scan, converted it back into ASCII, and drove the LCD.

John

Reply to
John Larkin

Nice. Looks like the filaments aren't driven to the point where they glow so there should be quite some reserves.

Let's hope they'll keep their vacuum :-)

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Joerg

Well, if the tubes are super expensive and need to be babied a lot, why not place a fuse in series with the filament? Or a Polyfuse, but make sure that can't cause anything to catch fire.

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Joerg

Since I assume there's some electronics hanging off of the 12V that might go *POOF* before the tubes do I'd consider a crowbar. MOV and TVS have a large tolerance. If you build it using a TL431 plus SCR it'll be very precise and still cheap. In the end it all comes down to the angst factor and how expensive the stuff is that could blow, and whether it can cause consequential damage.

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Joerg

You can always get more vacuum, from Sloman's head.

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Michael A. Terrell

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