Thank you all for your input.
Currently (!) I am looking at a small A/C driver - one per display - I
should be able to report back once tests have been performed - waiting on
board fabrication - I would also only have the whole thing 'on' once
triggered (that includes both filament and segment
Another thing to consider is when to turn filaments on and off. Having a
PIR motion sensor to turn-off the segment supply is important, because
long-term you will see dark regions on the phosphor closest to the filament.
Turning filaments on and off too many times will wear them out from
Arghh - I accidentally hit send on that last one. So anyway, if you don't
plan to use the grid to turn the tube off, you can wire it to the segment
positive voltage and ignore the need for a bias. You still might want to
put the series resistor in though. The exact value you use is going to
I don't know the specs for these specific tubes, but a resistor in series
with the filament servers a couple of purposes:
1. It reduces the inrush current (i.e. when the filaments are cold). The
resistance of the filaments is low when they are cold, so if you put the
steady-state
I spent a long time trying to figure out if an AC filament drive was worth
it. I tried multiple AC circuits. In the end, they all suffered from the
same problem - the voltage varied with the load. This was a killer for me,
as I wanted the circuit to work for multiple VFD types. In the end it
It can indeed.
For single digit tubes the brightness gradient isn't usually noticeable but
for the multidigit ones eg the one from the adafruit iceclock, it certainly
is.
David
On Wed, 14 Sept 2022, 10:42 Adrian Godwin, wrote:
> Using DC for the filament means there is more accelerating
Using DC for the filament means there is more accelerating voltage at one
end than the other and results in a noticeable difference in brightness.
I have wondered if it could be solved without a transformer by driving the
filament with a h-bridge.
On Wed, Sep 14, 2022 at 9:02 AM Richard Scales
I think I already see blunders in my original post - the filament is the
cathode (I think) at 5V, the segments (individual anodes) at 25V giving a
19V difference over the grid which is at 5V.
I am unsure as to the correct value of the series resistor for the heaters-
10R has been used by others
Hello everyone,
I have just managed to acquire a set of these and would ideally like to
drive them the best possible way.
I had initially though about a 5V for the filament and then 24V for the
cathodes and grid.
The clock would only have segments activated when someone is nearby - the
segments
Yes! That's exactly what happens. The current through the center-tap is the
sum of the anode (segment) current and the grid current. And the peak
current through the filament wires is actually *greater* than just the
current to heat the filaments.
*Tomasz* - As you found out, those VFDs draw
VFDs are basically directly heated triodes. To reach cutoff, the grid has
to be at some negative potential in respect to the cathode. It is just
easier to elevate the cathode above ground and swing the grids to ground
than creating yet another voltage rail.
The question is - if the filaments
NIMO tubes operate similarly to VFD's that have a grid; I drive the
center-tap of the filament transformer from a DAC ( + amplifier) to give me
software control. You can clearly see that if the grid is not sufficiently
negative (ie, grid=0 volts with filament biased positively), other numerals
That would be what I would infer too (that the anode voltage would have to be
suitably higher to maintain the same brightness).
This reference design shows an interesting way to produce an oscillating
filament voltage - basically using an additional secondary winding of the
transformer used in
I am trying to get my head around that, but I see the diagram you mention:
"If the filament potential is lower than the anode and grid cut off
voltage, thermionic electrons can reach the anode and cause illumination of
the phosphor. The filament
bias voltage should be increased to prevent this
Papers I have read also emphasize the need to bias the filament above the
cutoff voltage of the grid or anode, so I assume if the center tap is attached
to ground, that the cutoff voltage of the anode or grid is significantly below
that? In fact it would have to be below the negative swing of
I agree with Greg, the voltage WRT Gnd is important for brightness (as its
what gives the electrons the incentive to move in the first place). Series
connected filaments on individual tube VFDs will give you a nice
demonstration of the same aformentioned problems with linearity of
brightness
I'm pretty sure the the reason for progressive dimness is because of the DC
bias is different on each tube. The brightest one is closest to GND; as the
tubes get closer to the + filament supply, they get dimmer. I suggest
running them in-parallel from an AC source, or a +/- square-wave that
I took a look at the LM9022 datasheet and it's drive method seems similar
to my H-bridge + oscillator. I've built and tested it but haven't ran it
long term with my ILC1-1/7s. I'm still looking for anything to improve on
before I finalize the driver for my clock.
[image: 3.PNG]
--
You
Those are very cool-looking tubes !!
Are you connecting filaments in-series ? If so, when you run them on DC
there will n=be a net bias and that will cause non-uniform brightness
Even if the filaments are in-parallel, there will be some DC bias that
makes some segments dimmer.
A center-tapped
I thought this was worthy of a separate discussion.
Small VFDs, and even NIMO tubes, use the filament as the cathode and in
doing so the filament is the sole source of electrons to produce the
glowing segments. So, some of the filament current is going towards heating
of the filament and some
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