What extends life of incandescent tubes: Variable voltage or variable duty cycle - Page 1
Forum section: electronics / projects

author: buck converter
author_rank: Regular Contributor
timestamp: July 19, 2017, 02:59:20 pm
content: So i'm building a numitron clock wanted a way to dim the brightness. That would be easy with a 555, but if it extends the life of these rare tubes, should I use a LM317?

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 19, 2017, 03:24:07 pm
content: As long as your PWM frequency is significantly higher than the Numitron's element thermal constant, you should be OK.To find that time constant, I would use an IR photodiode with its respective amplifier. Connect the amp's output with a scope.Now feed the Numitron with a low frequency signal (say, 20 Hz) while monitoring the scope. You'll definitively see ripple.Mow start increasing the frequency until the ripple becomes negligible.That's it!

author: buck converter
author_rank: Regular Contributor
timestamp: July 19, 2017, 03:55:55 pm
content: I am a electronics noob, would this circuit work?

author: SeanB
author_rank: Super Contributor
timestamp: July 19, 2017, 04:54:47 pm
content: Simply use 3 1N4001 diodes in series with the common lead, to drop 1V8 from the display voltage. That will make them dimmer, and as you likely are not multiplexing them this will do it easily. In a clock you will have low enough current that 1N4001 diodes will handle the current, and will dim them down enough.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 19, 2017, 07:15:57 pm
content: If the dimming is always constant, then Sean's suggestion would be by far the simplest.OTOH, variable dimming requires some sort of control device, either linear or PWM.

author: floobydust
author_rank: Super Contributor
timestamp: July 20, 2017, 10:06:04 pm
content: Incandescent lamps last the longest on AC, on DC ion migration makes the tungsten filament tapered and a bit shorter life.I would not use low frequency PWM as the filament can vibrate and have mechanical fatigue failures, not to mention annoying noise.For longest life, I would use ballast resistors for each segment to level out the inrush current.But your segment driver IC's might have some on-resistance to help.Numitron looks like 24mA at 4.5VDC and incandescent inrush, well you can measure the cold resistance. I think lower than textbook 10x inrush current for tiny lamps.It's less efficient but I would vary the tube's voltage.

author: calexanian
author_rank: Super Contributor
timestamp: July 23, 2017, 04:03:31 am
content: Numeritron tube segments are typically all the same length so overall reducing the brightness by reducing voltage, and therefore current will help extend life. They are actually quite resilient. They have filaments that are alloyed with rhenium so they do not bend and sag like regular incandescent filaments. They are also typically mounted on a ceramic substrate that helps thermally regulate temperature change. If you run them at half voltage they will still glow nicely and last practically for ever.

author: floobydust
author_rank: Super Contributor
timestamp: September 16, 2017, 11:59:35 pm
content: Stumbled onto old RCA datasheet for CD4511B and it shows a 400pre-heat resistor used with the DR2000 numitron tubes.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 18, 2017, 12:55:25 pm
content: Floby;very clever idea!It will serve the dual purpose of pre-heating the elements and limiting the inrush current that the IC has to supply.