Hey could you explain how a circuit switches the numbers on a nixie tube? I can't find anywhere that gives a straightforward answer. I mean I know (well, think) it's changed by switching to the appropriate pin, but how does a circuit accomplish that switch and what kinda voltage and amps are required?
Nixie tubes are basically neon lamps with a common anode(+) and multiple cathode(-) elements, one for each digit.
To turn on a digit, a switch of some sort connects the cathode pin of the desired digit to ground; assuming there's sufficient anode voltage, the glow discharge illuminates the digit. If you replace the switch with a transistor or MOSFET, you can then use a signal to do the switching rather than physically flipping a switch.
In this example, the 1 and 2 digits are being controlled by SW1 and SW2. SW1 controls its element directly, but SW2 generates a 5v logic level signal that controls a MOSFET gate. In something like a clock, the gate control signal would be generated by a decoder chip or microcontroller rather than just a switch, but the principle is exactly the same.
As far as the voltage and current requirements, tubes vary but usually need an anode voltage of around +170v, around 1 milliamp of drive current, and have a "turn off" voltage of around 110v.
Once the tube turns on, it'll attempt to conduct as much current as physically possible, so R1 is needed to limit it to a safe level.
When the Nixie is on, it drops voltage equivalent to the turn off voltage, so to calculate the required current limiting resistor value, subtract the turn off voltage from the anode voltage and use Ohm's law to solve for the desired current:
Vcathode = 170v - 110v = 60vIwant = .001A (1ma) Ranode = Vcathode / Iwant= 60k ohm
There's quite a bit of latitude - if you use a 47k resistor as in my example schematic, the current will come out to 1.2ma, which is totally acceptable. It's also good practice to ensure your current limiting resistor can handle the power dissipation, so multiply Vcathode by Iwant and make sure your resistor is rated for more watts than that. In the example this is .06W, so a typical 1/4W resistor is more than capable.
Since there's the voltage drop from the tube, the transistor only needs to be able to handle Vcathode, in this case 60v. The BS108 in the schematic is good to around 200v, so there's a huge margin for error in case the high voltage supply floats above 170v.
I used a Supertex driver array in my clock, but it's just 32 n-channel MOSFETs and a shift register in a single package. The reasoning and calculations are the same as if I implemented it with 32 discrete MOSFETs. The chip's drive outputs are all MOSFET drain terminals, and the source terminals are just internally tied to ground.
There are lots of ways to drive these things, including some vintage 74 series chips that are explicitly designed for driving Nixies. Much more info here and here.
