Turned off VFD elements still have some residual brightness and flicker? - Page 1
Forum section: electronics / beginners

author: julian1
author_rank: Frequent Contributor
timestamp: July 30, 2016, 12:20:20 am
content: I have a VFD unit from an Agilent 34970a.When the elements are turned on they are nice and bright.When the elements are turned off, there is a small amount of residual brightness and flicker - especially towards the ends of the VDU. It's sufficient to be distracting to the eye.Is there a known cause (or solution) for this kind of VFD behavior?

author: helius
author_rank: Super Contributor
timestamp: July 30, 2016, 03:30:31 am
content: VFDs are normally multiplexed, with the grid electrodes (one per character) cycling through from one end to the other, while the anodes (phosphor-coated segments, typically bussed together across all characters of the display) must change synchronously with the grids. If the anode drivers are desynchronized with the grid drivers, you will see flicker. Possibly a problem in anode drivers or their supply.

author: Ian.M
author_rank: Super Contributor
timestamp: July 30, 2016, 04:31:03 am
content: Is it still in the original Agilent 34970A circuit?More flicker at the ends of the display is indicative of insufficient negative grid bias when cutoff.As the cathode is the surface of the directly heated filament, its normal to drive the filament with AC (to reduce the brightness variation due to the voltage drop along the filament that would occur if DC was used). If, on the peaks of the drive waveform Vgk goes more positive than the cutoff voltage, whichever segment anodes are active at that instant in the multiplexing sequence will glow. The center of the filament is where the antiphase filament drive waveforms cancel out so it doesn't get the peaks superimposed on the standing Vgk bias so doesn't suffer from glow/flicker on inactive digits.

author: mike_drz
author_rank: Newbie
timestamp: May 24, 2017, 03:59:24 pm
content: I have the exact same problem with a VCR that I just recapped, it's driving me nuts. Did you find a solution to your issue? Here is a video of what my VFD does. I know this is an old thread, and you may not even remember, but it's worth a shot. hahahttps://youtu.be/68BuT-MloW0

author: janoc
author_rank: Super Contributor
timestamp: May 24, 2017, 04:38:07 pm
content: One way to solve the turned off elements not being completely off is to to raise the filament voltage above the regular ground (thus your GND will have negative potential relative to the filament/cathode) using a simple circuit like this:http://www.kerrywong.com/2013/06/05/vfd-filament-driver-using-555/(tested working, just check the filament voltage of your VFD - may not be the same!)That will ensure that every segment that you turn off really will be off and also drives the filament with AC, ensuring uniform brightness.VFD is kinda like a depletion FET - it needs negative voltage on the grid relative to the cathode to fully make it stop conducting, so only turning a driver off or grounding one of the grids will not be enough to turn the segments off due to leakage from nearby segments.

author: mike_drz
author_rank: Newbie
timestamp: May 24, 2017, 04:46:47 pm
content: Thanks, I don't have the voltages due to no manual, but I think it uses 32 volts, through some sort of step up converter using a coil. I wonder if it's a power supply issue, or the chip driver itself.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 24, 2017, 05:18:28 pm
content: It looks like the heater voltage is not symmetrical - i.e. one side is biased at a different level to the other. The heater supply should be AC and equal at both sides.Isn't the "oscillation" simply due to the fact that the clock has not been set?

author: mike_drz
author_rank: Newbie
timestamp: May 24, 2017, 07:09:56 pm
content: When the clock is set and not blinking 12, the grids that are supposed to be off do that constantly, at least from the left side of the VFD, haven't noticed the other side doing it. Since not all grids are doing that, it leads me to believe it might be some sort of driver chip issues, and the timing of the multiplexer etc.There are a lot of diodes and some smd components on the board, maybe one of them is off. The diodes measure okay, but the surface mount stuff is really old and hard to measure.