Timely subject....my 7971 PC boards just cleared US customs and are on the plane to Oregon....
I have independent current-regulators on each cathode. 4 tubes * 15 cathodes = 60 current regulators. The driver transistor is a dual NPN in a surface-mount package to save area. The cathode (segment) currents are not adjustable, but there are SO many available resistor values it's easy to tweak to fine resolution. I purposely chose 1% tolerance SMT (0805) resistors. The NPNs are controlled by a TTL-level shift register (74HC595) running at 3.3V. For the anodes, each tube has an overall current limit (PMOS), set to 22mA (datasheet max). Total of 4 per board. The boards are end-to-end abuttable, so my first (and only) project will have 7 tubes to start with. Of my seven b7971 tubes, I gathered I-V data for each segment. This gave me an overall idea how much current it takes to get decent illumination. Next, I wrote a spreadsheet that calculates the total current for a variety of ASCII characters (0-9, A-Z, _ ^ - * ). By setting the individual segment currents in the spreadsheet so that most characters are at/below the tubes max-rated value, I get an idea where to start. A few characters go over the 22mA spec value, and that's where the anode regulator kicks-in. Hopefully it will result in segments fair-sharing their current. From there, it's easy to calculate the cathode resistor and select from available values. -- 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/4b2db638-ce71-4aa8-a71d-918168648d24%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
