> So now I only have to figure out wether my two transistor driver is a good > way to drive my IV-11 tubes :)
I was hoping someone more versed in the intricacies of transistors would address this, but they haven't, so I'll take a stab at it. > 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 This one has Q1 in a "common base" configuration, which does not provide current gain. It seems to me like it would put Q1 into its linear region (due to the drop across R1), so you might have to fiddle with the R values to get Q2 to turn on and off reliably. Since you're dealing with low currents, it should work fine. > and the other being this one http://i.imgur.com/5fMc7ty.png . This is a more traditional "common emitter" configuration, where both transistors can give current gain. Instead of the usual series base resistor to limit current, there's an emitter resistor which does double duty of limiting both the base and collector currents. > Which would be best? I'd slightly prefer the second over the first (although there are reasons you might want to use the first one, involving current limiting and the like). They both work essentially the same way: the PNP transistor is kept turned off by its base bias resistor to V+, until the NPN transistor turns on, and pulls the base down toward ground, thereby turning on the PNP transistor, which then provides V+ out of its collector to drive the VFD anodes/grids. The NPN transistor's emitter resistor limits the current flowing through both transistors (it does not control the amount of current available to the VFD, except in a sort of indirect fashion - normally the transistors are fully off or fully on). > 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. As I said, those resistors don't really control the current into the VFD. However, the VFD doesn't need to be current limited, so all the resistors do is protect the transistors and microcontroller. > 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? Again, the resistors are only there for current limiting, and they're not terribly critical. If the transistors turn on and off reliably, and there isn't too much current through them, you're fine. - John -- 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. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/503D6874-D978-43FF-9203-45EBB30472AA%40mac.com. For more options, visit https://groups.google.com/groups/opt_out.
