Hi Gideon,
I think that you have some confusion regarding how VFDs are driven. VFDs
are not nixie tubes or anything even similar. Nixies are not vacuum
tubes, they are cold-cathode (neon) tubes. With nixies, we place ~180vdc
across the anode & cathode via a current-limiting resistor. The reason
for current-limiting in a nixie is because as current increases, nixie
impedance decreases, causing the neon tube to rapidly begin dissipating
a catastrophic amount of heat. Nixies, being neons, regulate voltage to
their maintenance voltage. Any reasonable voltage above the striking
voltage will work fine.
VFDs are triodes: Hot-cathode vacuum tubes. Ideally, the filament is
driven with an AC supply of the designed voltage (via the two filament
pins, usually a couple of volts) and the anode segments are driven with
a voltage regulated DC supply at the designed voltage (usually 20-30v
for direct and ~60v for multiplex). The grid is driven exactly the same
as an anode segment and is used to turn the display 'on'. There are no
current-limiting resistors used for driving the anode segments or the grid!
In my VFD clocks, I used the trick (which I learned from here) of
driving the filaments with DC. This works fine if you are using
individual numeral VFD tubes, I wouldn't try it if you are using the big
multi-numeral VFD display tubes. The (known) current draw of the
filament is used for calculating a resistor-divider. One leg of the
filament is tied to ground, the other leg to +5vdc via the resistor. If
you go with this approach be sure to do your math on the front end.
These filaments draw a lot of current, which means a lot of heat
dissipation in the voltage-divider resistor. Driving this setup via a
linear regulator supply would need a very large heat-sink. I would
recommend using a switching supply like the LM2575. Obviously, if you
have an AC supply that is the correct voltage for the filament then no
resistor is needed.
For a schematic: http://elbastl.sweb.cz/6-digit-VFD.zip
I liked Maxim IC's 6921 driver IC. I know you specifically called it out
as not an option, but it worked well for me. Specifically, I used the
PLCC package because I work exclusively in protoboard and there are easy
DIP PLCC sockets. There are numerous advantages in my opinion to the
dedicated VFD driver chip. Instead of 8 dedicated GPIO pins (7-segments
+ grid), the 6921 uses a standard SPI interface. This would be even more
advantageous on 16-segment VFDs.
One thing you mentioned: Yes, 60v applied to the anodes will be VERY
bright, even if multiplexed. I had to move my VFD clocks to bright
places like my desk at work. a LOT brighter than equivalent nixie
designs. I spent some time dialing back the supply voltages before
finally giving up and accepting that VFDs are just designed to be bright.
-Adam
On 11/19/2013 5:09 AM, Gideon Wackers wrote:
I want to build a four digit VFD clock (three of them actually)
without using drivers like the max6921. I have two possibilities at
this moment, one is this http://i.imgur.com/D4FGaV1.jpg and the other
being this one http://i.imgur.com/5fMc7ty.png . Which would be best?
Another question is the resistor that I will need:
0.0035/25*3 = 0.00042
60/0.00042 = 140k = 130K or 150K ohm for R1 and 100k for R2
For the grids the same circuit applies but with a slightly different
value for R1 due to the different current.
But this implies that I use 60 volt. A few people said that the tubes
are very bright at the recommended 50-70 volt for multiplexed tubes so
if I lower the voltage to 40 volts I would suddenly need only 100K for
R1. Will I bump into problems if I lower the voltage to lets say 40
volts but when my resistors are calculated for 60 volt?
I want to make the filament supply with a 7806 and wires them in
series, AC will be generated by four bs170 mosfets. Do I still need a
filament resistor if I match my voltages this exact?
Suggestions are welcome, my PCB can be roughly 9 by 5 cm so maybe
there are other (cheap) alternatives to using resistors as drivers? I
first got this idea because everything had to be stuffed onto a
smaller pcb so maybe now that I have more space I can now use some
IC's instead?
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