Numitron Lifetime And The Miller-Larson Effect, Is It A Consideration? - Page 1
Forum section: electronics / projects

author: The Doktor
author_rank: Regular Contributor
timestamp: September 01, 2018, 03:40:14 am
content: I'm planning to build a clock using the very nice Russian IV-13 numitron tubes. As these tubes are both rather expensive and long out of production, I have a strong desire to obtain maximum lifetime from them.I will be driving the tubes with the TLC5916, which is an 8-bit shift register with constant current sink outputs designed for driving LEDs. A single resistor sets the current for all 8 outputs to 3-120ma, and this can be further reduced by sending a digital code to each chip. The tubed will be direct driven, no multiplex and no PWM. They will also be operated well below their maximum current rating, which both extends life and gives a nice orange glow instead of bright yellow. The constant current drive should be much easier on the filaments, it also gives a pleasant "fading in" effect when a segment lights. I'm sure of all these design features so far...Now the part I'm not sure about. Being both a Ham Radio operator and a big fan of tube gear, I am fairly familiar with the effects of things like inrush current, the Miller-Larson effect, and thermal cycling in general on the lifetime of large transmitting tubes with thoriated tungsten filaments. By using constant current drivers, inrush current should not be an effect. The numitron filaments will be running much cooler than a transmitting tube, is Miller-Larson and thermal cycling something I'm wise to consider, or just a waste of time?I have devised a plan to keep the filaments always warm, if deemed a good idea. I would use 2 TLC5916 per tube, with their outputs in parallel (they're sink only, so no problem here). 1 would be the "lit" driver, the other the "dark" driver. The "lit" driver would have a resistor that operated the filament at the maximum brightness desired, the "dark" driver would be set to just barely visible. Both could be digitally adjusted down. In operation, the "lit" driver would drive the segments being displayed, the "dark" driver would receive inverse bit pattern and drive the unused segments to a temperature just below visible. Both levels could be automatically varied based on an input from a ambient light sensor if desired.What is your opinion? The driver chips are very cheap compared to IV-13s, and it wouldn't be much extra code for the µcontroller to make it work. Question is, would I actually get longer life, or am I just adding useless parts?Ed

author: Gyro
author_rank: Super Contributor
timestamp: September 01, 2018, 09:43:33 am
content: IIRC, the Miller-Larson effectonlyapplies to Thorated Tungsten filaments. The filaments in Numitron tubes don't need to produce electrons, just light so will be pure Tungsten.The main use for Numitron displays was for sunlight visible displays - Petrol pumps etc. The best way to extend their lives is to drop their filament voltages a bit - as with other Tungsten filament lamp, life expectancy will increase exponentially.Hopefully you can find the right series resistor value that gives you a dimmer, nicer colour with a bit of fade effect and a little bit of inrush protection too (good for the drivers).Going further than that probablyisoverkill.P.S. Lower operating voltage should also minimise filament sag.

author: floobydust
author_rank: Super Contributor
timestamp: September 02, 2018, 01:23:42 am
content: I think inrush current is the main lifetime issue you can address before things get too complicated.some discussion here:https://www.eevblog.com/forum/projects/what-extends-life-of-incandescent-tubes-variable-voltage-or-variable-duty-cycle/The IV-13 datasheet says avoid 100-500Hz vibration and PWM.I'd use a dimmer at night time, based on hours like my old VCR 10pm -6am or a phototransistor.