Vacuum Fluorescent Display Filament Power Question - Page 1
Forum section: electronics / projects

author: rfdes
author_rank: Regular Contributor
timestamp: October 26, 2022, 10:51:11 pm
content: I own a '95 era Fluke signal generator with a VFD display. I use the generator infrequently. maybe 4 times a year, I'm guessing. The generator has a standby mode when OFF in order to keep the reference oscillator powered. Examining the schematic I discovered that the filament power (6VAC) remains on in standby mode also. Does anyone have any technical knowledge as to whether this harmful to the display over time? Since the generator is powered off 99% of the time, what is the effect (good or bad) of the filament power remaining ON? What is the *reason* for the filament power remaining ON?Thanks for your inputJim

author: james_s
author_rank: Super Contributor
timestamp: October 27, 2022, 12:48:56 am
content: I don't know how much of an effect it will have, but it certainly does gradually boil off the emissive coating on the filament. I can see no good reason for it to be powered on all the time, just the power consumption is going to add up, probably only a few watts but it will be tens of kWh per year.

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2022, 01:57:24 am
content: In a vacuum tube it would cause interface resistance of the cathode if it was not designed for that, but I do not know if that applies to the directly heated cathode of a vacuum fluorescent display.If the cathode of a vacuum fluorescent display even coated?

author: james_s
author_rank: Super Contributor
timestamp: October 27, 2022, 06:20:14 am
content: Quote from: David Hess on October 27, 2022, 01:57:24 amI'm not positive, but I think so. It is my understanding that the coating is why boosting the voltage for a few seconds can clean up the filament of a tired old VFD and give it a boost.

author: Gyro
author_rank: Super Contributor
timestamp: October 27, 2022, 08:38:40 am
content: Yes, the cathodes are coated (they are white when examined cold). They run at too low a temperature to operate without an emissive coating.My feeling, based on other tube technology is that running the displays with the segments gated off (no cathode current) will shorten their life.

author: strawberry
author_rank: Super Contributor
timestamp: October 27, 2022, 04:44:16 pm
content: I think, sunlight degrade anode coating so that it gets dim . Usage will degrade more unevenly.filament power for 112x25mm VFD = 0.4W

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2022, 11:07:49 pm
content: Quote from: Gyro on October 27, 2022, 08:38:40 amThat would be consistent with interface resistance.This subject comes up in connection with test instruments with CRTs where people turn the intensity off to preserve the phosphor screen, but operating the cathode in full cutoff degrades the cathode with "interface resistance", so it is better to turn the intensity to low rather than off.Vacuum tubes intended to be used in digital applications where they would spend considerable time in full cutoff used a different cathode to prevent this problem.

author: james_s
author_rank: Super Contributor
timestamp: October 28, 2022, 11:49:09 pm
content: Regardless how serious the effect is, I can think of no possible advantage to keeping the displays powered at all when they are not in use.

author: David Hess
author_rank: Super Contributor
timestamp: October 29, 2022, 12:20:01 am
content: Quote from: james_s on October 28, 2022, 11:49:09 pmThere is no advantage unless there is a requirement that they power on instantly, which which case the filament needs to be kept hot. But whatever intensity control or blanking is available leaves the filament powered.

author: Gyro
author_rank: Super Contributor
timestamp: October 29, 2022, 10:14:15 am
content: I think it is often just laziness. I don't know about the OP's unit but on a lot of gear, the filament supply is a low voltage winding on the mains transformer. With the transformer constantly powered in standby, it's easier to just leave the filament running than to switch it.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 14, 2022, 04:32:32 pm
content: I'd add a "warming resistor" in series when its in standby and use a tiny relay to take it out of circuit when you're going to make a measurement.This will keep your filament on tick over- the nore full on-off cycles the filament gets, the shorter it's life. That why a burn off boost off can leave you embarassed.Before the days of leds, warming resistors were common in high rel applications- Ah, lamp test buttons.....mmmm