I've built quite a few Numitron-based clocks, as well as other displays 
using the technology.  Personally, I love them: they're easy to drive, last 
a *long* time, and are, to my eyes anyway, very attractive.  They provide a 
wonderful glow, and because they're broad-spectrum, they can be filtered to 
any color.  However I usually leave them "naked", and the non-monochromatic 
light they emit seems to give a richer, warmer look than LEDs, or even 
Nixies, to some extent.  (Of course, they don't have the fully-formed 
character set of the Nixies.)

I've used a variety of schemes to power the devices.  At first, I used a 
dedicated microprocessor (an AT89C2051 - about $1 each) for each tube, with 
the segments direct-driven from the processor outputs.  I implemented a 
serial communication scheme that allowed the displays to be daisy-chained 
to any length, and controlled with a single 2400-baud serial signal.  These 
were driven using the RCA-recommended strategy of keeping the filaments 
just below a visible glow whenever they were "off" for maximum lifespan, I 
did this using PWM, and implemented brightness controls also, since I 
already had the code to do the PWM.  These worked very well, and the only 
failure I've had, even in extremely rough service (a mutant vehicle roaming 
the desert at Burning Man) was due to mechanical damage to the tube.  
(Amazingly, it even kept working for a few minutes without any glass 
envelope at all!)

A scheme that I've never used, but which is easy and should work well, is 
to use a single fixed resistor from each segment to the supply (or ground, 
of course, depending upon your circuit topology) to provide the keep-warm 
current for the filaments

For most of my clocks, I've just ignored the keep-warm function, and have 
seen no ill effects as a result.  I've been using 74HCT595 serial-in 
parallel-out latching shift registers to direct-drive the tubes; their 
outputs are rated to sink 35 mA (74LS595 are even higher, at 130mA), and I 
PWM the enable input to provide dimming.  I've had clocks running for a 
decade now with no tube or other part failures (except for wall-wart power 
supplies - they're often notoriously poor in quality).  The highest stress 
is probably on the colon lamps, which are two 3.5-volt grain-of-wheat bulbs 
that I'm running in series to match the luminosity of the Numitrons; even 
though they've been blinking on and off every second for years, I haven't 
had any failures there either.  Note that since my clocks are often placed 
in bedrooms, they're usually run at less than full brightness - PWM ratio 
of around 50% or so; however, even the one I gave to my 94-year-old mother, 
which runs at full power, has also had no failures.

A note about using the Numitron clocks (or Nixies, for that matter) in 
bedrooms: they're excellent for this application!  A reasonable amount of 
research shows that ambient light containing a significant blue-spectrum 
component can disturb sleep, reducing the amount of REM sleep and overall 
sleep time.  This is true even when the eyes are closed!  The warm glow of 
Numitrons, especially when dimmed, contains almost no blue light, and so is 
the ideal bedroom clock display.  I'm lucky enough to have acquired most of 
the remaining supply of DTF104B tubes (1" digit height, end-view), and 
clocks made with these can be seen easily across a room, even if the viewer 
normally wears glasses.

I always use direct-drive in my designs, because multiplexing pretty much 
obviates the big advantage of Numitrons, which is the ability to drive them 
directly from the +5-volt logic supply - there isn't sufficient brightness 
without a higher-voltage supply.  I did use multiplexing once, to retrofit 
an existing 3-digit LED display on a musical keyboard I rebuilt into a 
steampunk enclosure.  In this case, I simply replaced the LEDs outright 
with the Numitrons, and removed the current-limiting resistors.  This 
actually worked better than I thought it would; although the displays were 
a bit on the dim side, they were still completely legible in normal room 
lighting.

Other comments about the lifespan of Numitrons are very accurate - at 
reduced voltages, the tubes will last basically forever.  RCA data sheets 
show a mean life expectancy at full voltage of 100,000 hours (longer than 
LEDs are specified for!), and their lifespan curves are basically 
asymptotic to infinity at 70% voltage.  Also of note is the specified 
ruggedness of these devices: rated impact acceleration (while operating!) 
of 200G, vibration 5-200 Hz 20G, etc.  The Russian tubes are not specified 
as highly (15,000 hrs operational life - I have no idea about vibration), 
but I use them all the time, and again have never had a failure.
~~
Mark Moulding



On Wednesday, February 21, 2018 at 5:26:39 AM UTC-8, Tomasz Kowalczyk wrote:
>
> Hi, 
> I'm recently getting more and more interested in making an IV-16 clock. 
> I'm planning to make 6 tubes one, so burning filament can be an issue - 
> while the datasheets can state 5000+hrs, I'd like to be extra careful.
> One idea is to use a LED driver with programmable current, for 
> example STP16CP05XTTR - current is set by an external resistor, so it would 
> be possible to introduce dimming the display without PWM. There are 
> numerous chips which allow current sinking, however I am concerned with 
> power dissipation on these - that's why my choice is a package with thermal 
> pad. As far as I know, most critical moment in incandescent bulbs is turn 
> on due to inrush current - with a CCS this problem would simply not exist.
> However I've noticed a small mention of numitron driving in this CD4511 
> datasheet:
> https://www.ti.com/lit/ds/symlink/cd4511b.pdf 
> <https://www.google.com/url?q=https%3A%2F%2Fwww.ti.com%2Flit%2Fds%2Fsymlink%2Fcd4511b.pdf&sa=D&sntz=1&usg=AFQjCNFkxbN5R5Smzeurxtf3m7gNcRqeOA>
> On last page there is a small schematic. It shows another method of 
> dealing with inrush current - all filaments are prebiased with small 
> current, which causes them to be already on edge of glowing - kind of how 
> VFD filaments are biased.
>
> What are your experiences with numitrons? What methods of prolonging their 
> life did you use? Or did you just ignore the problem, as IV-9 and IV-16s 
> are still quite cheap?
> Please share your experiences and thoughts.
>

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