Hi Gideon,
How many of these clocks are you planning to build? If it is just a
few, then I wouldn't sweat the costs too much. Both of those companies
(used to be - I assume still) are very generous with samples. I agree
that the Supertex and Maxim parts can be difficult to obtain for a low
price in individual quantities from normal vendors. I've got a drawer
full of Supertex and Maxim samples, I'd be happy to mail you the parts.
FYI, requests for samples get a much more positive response if you use a
company or university email and describe a plausible project in the
request. Any time that I can't find some obscure part that is listed for
a project, I move to the obscure part manufacturer's website and request
a sample. No problem.
Contact me off-list with your address & parts request and I'll see what
I can do. Of course, if you're designing a clock for mass production,
then all bets are off. :)
-Adam
On 11/19/2013 11:39 AM, Gideon Wackers wrote:
I know the difference between a nixie and a VFD tube but I have to say
that your explanation is very nice to summarize all the loose bits of
information in my head.
So If I have a 6 volt supply for my four filaments (in series) I would
not need a resistor. I think the 7806 won't get that warm with roughly
400ma going through it but I can always take a simple switching psu
for it or attach it to my (metal) enclosure. I will simply use a DC
filament supply then to keep things simple (this project is going to
be complicated enough).
My only remaining concern is the driving method, I would like to order
all my parts off ebay and/or tayda electronics. Places like mouser etc
have (in my opinion) ridiculous shipping rates so I would like to
avoid them. Another point is that the max6921 costs around 6-7 dollar
a piece +1 euro for a plcc socket, I am trying to keep the costs down
a bit. So a method to avoid using these drivers is preferred.
HV5812: cheap but shipping costs are 40-50 dollar
MAX6921 expensive and/or high shipping costs
Op dinsdag 19 november 2013 20:06:50 UTC+1 schreef Adam Jacobs:
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
<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
<http://i.imgur.com/D4FGaV1.jpg> and the other being this one
http://i.imgur.com/5fMc7ty.png <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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