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 > > 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? > -- > You received this message because you are subscribed to the Google Groups > "neonixie-l" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to neonixie-l+...@googlegroups.com <javascript:>. > To post to this group, send an email to neoni...@googlegroups.com<javascript:> > . > To view this discussion on the web, visit > https://groups.google.com/d/msgid/neonixie-l/62997dbb-f1ac-4b11-8e24-42f3eec74ad6%40googlegroups.com > . > For more options, visit https://groups.google.com/groups/opt_out. > > > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. 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