After some experimenting, I ended-up using 5.1 ohms for a single resistor. I could have used 2 resistors, one on each side, to maintain DC balance but instead I chose to use a single resistor so I could have my FPGA monitor the filament status, along with it's associated fuse and resistor. Each filament has it's own 250mA fuse for protection (this is a 6-tube NIMO clock). It wasn't necessary it was to go to that extreme, and it burned-up a lot of FPGA pins, but this is a project/experiment where I really wanted to go overboard with the instrumentation and self-checking. DC-balance really isn't necessary because everything averages-out over time, and I just adjust the DC offset (with a DAC under software control).
Regarding the filament, it's actually 2 parallel-connected filaments inside the tube. So when 1 of them blows, only half of the digits will work. My original intention was that I would monitor the anode-current for all 6 tubes, and I could detect a partial filament burnout by checking for lower-than-expected current. Software could isolate the failing tube, then shut down the clock. Unfortunately there is a source of noise in the op-amp summer circuit I use that gives me erratic current readings. Eventually I'll figure out what's wrong. -- 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 [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/80ce6285-b50a-40af-8554-2c1fe75de18bo%40googlegroups.com.
