Numitron clock driving question

[Tomasz Kowalczyk]

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

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.

[Terry Kennedy]

On Wednesday, February 21, 2018 at 8:26:39 AM UTC-5, Tomasz Kowalczyk wrote:

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.

I have a pair of Numimini clocks (from Jon Ellis here) with IV-9 tubes which have been running continously here for over 8 years with no issues. They are powered by the host PCs' USB ports, so there is some magic involved in staying under the available 500mA available when driving the filaments.

[David Forbes]

The inrush current performs the important job of heating up the filament so that it will glow. Don't expect an LED driver to work well. Experiment with some low ohmage series resistors first.

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[jf...@my-deja.com]

On Wednesday, February 21, 2018 at 7:43:27 AM UTC-8, nixiebunny wrote:

The inrush current performs the important job of heating up the filament so that it will glow. Don't expect an LED driver to work well. Experiment with some low ohmage series resistors first.t.

It has been a long time and, technically, this applies to RCA numitrons and not necessarily the Russian clones.  ISTR that they were designed to be directly driven by a 7447 and a 5V power supply without needing any series current-limiting resistors.  One of the advertised advantages was that they could be read in direct sunlight, which was not true for the early LEDs.

I also recall that GE claimed that the life of an incandescent lamp was inversely proportional to the twelfth power of the applied voltage.  If you can extrapolate from household lighting running on 110VAC, you can expect the life of your numitrons to be increased by about 4X by adding a series diode to drop the applied voltage by about  0.6V.

[gregebert]

Bottom line: Keep the current as low as possible, and limit the inrush current.

I've done a fair amount of research on extending filament life for a project I'm working on. After a lot of tradeoffs and analysis, the best solution for me was to use a *higher* operating voltage and add series resistance. This reduced the peak inrush current, was the simplest design, and the added energy cost vs a "perfect" constant current source was less than the circuit cost for the regulator. This project uses 6 tubes, and each tube has it's own filament fuse, and an FPGA to monitor the condition of the fuse, filament, and the dropping resistor. I can also measure the filament current on 2 tubes the an onboard A/D converter. 

Here's a technical paper on extending the filament life of expensive transmitter tubes, and inrush current was the #1 killer. Reducing the filament current even a few percent resulted in much longer lifetime. See https://www.photonis.com/uploads/literature/pt/Voice-of-America-Paper.pdf

From my own measurements, a filament's resistance varies substantially when cold vs. hot

The tube I'm working with varies from 2.8ohms (cold) to 7 ohms (hot). So there's roughly 2.5X the surge current on this particular tube if you drive it from a constant voltage.

The technical paper above reported variations of 10:1 for transmitter tubes, and 12:1 for incandescent bulbs.

On another project, I have a #47 bulb driven by a current regulator. It goes on/off 24 times per day, and has been running for 2 years now. And, it was a used bulb so I have no idea how many times it has been cycled. But it's still working. The current regulator definitely cause the bulb to light more slowly.

If you want to read about a really long-lived filament, check out the story of the lightbulb in a fire station in Livermore, CA:   http://www.centennialbulb.org/

[Tomasz Kowalczyk]

I didn't purchase any numitrons yet, however my tests with a CCS and a small 15V 30mA bulb show that with current limit of 20mA, the filament turns on with a fade in effect, which takes about 200ms, in this case. I enjoy it, so I hope I'll be able to reproduce it with numitrons, as it would both help their life and add a nice effect.

 With 30mA the turn on time drops to under 100ms, and with brightness of this bulb is hardly noticable. 

I think the inrush current is needed for bigger filaments and/or faster switching - small bulbs (and, I hope, numitrons) have short pieces of wire with low thermal capacity. 

gregebert, thanks alot for the link to the paper. I've read it, definitely learned alot from it.
I was prepared to lower the current of segments to help them live longer, and with a LED current sink this will be very easy. If only producers told how the resistor is sensed by the chip, so I could replace it with a transistor and a DAC/lowpass filtered PWM... That's why I didn't want to use limiting resistor and higher voltage. On second thought, maybe a regulated boost converter would do the job - I could dim the clock by changing whole tube power supply voltage.

W dniu środa, 21 lutego 2018 16:43:27 UTC+1 użytkownik nixiebunny napisał:

The inrush current performs the important job of heating up the filament so that it will glow. Don't expect an LED driver to work well. Experiment with some low ohmage series resistors first.

On Feb 21, 2018 6:39 AM, "Terry Kennedy" <terry-...@glaver.org> wrote:

On Wednesday, February 21, 2018 at 8:26:39 AM UTC-5, Tomasz Kowalczyk wrote:

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.

I have a pair of Numimini clocks (from Jon Ellis here) with IV-9 tubes which have been running continously here for over 8 years with no issues. They are powered by the host PCs' USB ports, so there is some magic involved in staying under the available 500mA available when driving the filaments.

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[gregebert]

Instead of the CD4511, have you considered multiplexing ? It should work very well for incandescent displays because of their inherent storage time and you wont have any visible flicker. And best of all, no ghosting !!

You can also add PWM to keep the dark segments warm, which should help extend their lifetime.

My father used to design aircraft instruments, and it was common practice (until LEDs came along) for incandescent bulbs in lighted switches to be kept always-on, either bright or dim. Those bulbs were rated for 100,000 hours of operation. You'd be amazed how much heat was generated in the cockpit of a 747.

[Mark Moulding]

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

[GastonP]

Just one correction: inrush current of a filament lamp (or heater) is a physical characteristic of the component and not something one designs to happen. Every filament or heater be it huge or tiny has an inrush current, and the amount of it and the time that takes between this maximum current and the nominal one is a function of many physical characteristics.

The suggestion of using a current source is to limit this inrush current and thus avoid the possible damage caused by it.

[Tomasz Kowalczyk]

Thanks! That's alot of positive information - it seems that numitrons are indeed sturdy devices and they do not need that much protection as I thought.

What is the frequency of the dimming PWM? I wonder how high it has to achieve steady temperature due to thermal inertia of filaments. 

PWM dimming would end up much cheaper than LED drivers, I was afraid that it could affect the filaments life length, but now that I know that the idea is well tested... I'll still think about a way to change the brightness without directly PWMing the filaments, but it is really tempting.

I'll definitely use direct driving, for same reasons as you mentioned - ability to use any old phone charger as power supply, to keep things cheaper and more portable.

By the way - I stumbled upon this graph taken from DA-2300 datasheet:

Taken from this site: http://www.dos4ever.com/numitron/numitron.html

Note the logarithmic scale on tube life!

[Jon]

On Wednesday, February 21, 2018 at 1:39:24 PM UTC, Terry Kennedy wrote:

I have a pair of Numimini clocks (from Jon Ellis here) with IV-9 tubes which have been running continously here for over 8 years with no issues. They are powered by the host PCs' USB ports, so there is some magic involved in staying under the available 500mA available when driving the filaments.

Nice to hear they're still running well!  Having liberated 50-odd Numiminis into the wild over the years, I'd say that your experience seems to be widely shared. I don't recall any reports of either IV-9 or IV-16 failing in use, though a few tubes have suffered terminal mechanical trauma (cats, office cleaners etc). As we've discussed here a few times, the lifetime figures quoted on the datasheets for the Russian indicator tubes (nixie and numitron at least) seem to be pretty conservative.

Not sure I'd claim much magic in the design - though might confess to having given it a little thought :) It uses a pair of A6279 LED drivers which have constant current sink outputs to help with the filament inrush current issue. Total current management is achieved by having the clock operate in direct drive mode if it's being powered from a wall wart USB charger or as a 2x2 multiplex when being powered from a standard USB port. The PIC that runs the show can tell what the power source is and configures the display mode automatically (can also be fixed manually if you like). It also actively controls the sink current setting on the A6279s, and that's used to govern overall display brightness in response to user preference or ambient light.

In practice I think the design results in the tubes almost always being significantly under-driven, except maybe if you deliberately max out brightness with a wall wart power supply.

Jon.

[Larry]

Hi Mark,

It's nice to find another burner on the list.

Larry

On Friday, February 23, 2018 at 8:10:24 AM UTC-6, Mark Moulding wrote:

[Bill Notfaded]

I found an old datasheet for a CD4115B BCD-7 segment latch decoder driver that shows using using 400 Ohm R to keep DR2000 series segments slightly on and warm.  I've been also bitten by the numitron bug.  I appreciate your comment and knowledge Mark.  It sounds like your original design was a sort of smartsocket for numitrons!

Bill

[gregebert]

Series resistor on filaments will also reduce the surge-current when the segment is turned-on, greatly extending the lifetime. 

From what I've researched on tube filaments, reducing the surge-current during turn-on (ie, adding series resistance) is the best way to extend lifetime.

There wasn't clear evidence that keeping the filament warm at a lower current, thereby reducing the thermal-cycling effects, played a significant role in extending lifetime.