I have six NEO-5000 and two SP-352 displays that I'm planning to use to build a clock. I'll use four of the NEO-5000s as hours and minutes, and one of the SP-352s for the seconds. It'll all be driven by a microprocessor, maybe an ESP8266. The micro part is easy but I don't have a lot of experience with these Panaplex type displays and I don't want to damage them. BTW, I'll keep the extra displays as spares just in case one in the clock fails...
My plan is to multiplex the two SP-352 digits using a single DM8880 as the cathode driver and two TLP627s as the high side switch. I have a couple of the DM8880s; they have a built in constant current sink so I don't have to mess with separate cathode resistors, and I'm fine with the decimal only font for the seconds display. The four NEO-5000s will be multiplexed using a SN75468 with the outputs clamped to 90V as the cathode driver. The high side driver will be four more TLP627s. Since the 75468 doesn't have a constant current sink I'll use seven discrete resistors to limit the segment current. This arrangement will allow me to generate any pattern I want on the NEO-5000 segments. Three 74HC595 shift registers should provide enough output bits to control all the displays and only need three I/O bits on the ESP8266. All told, this is a pretty simple, low part/chip count design. Questions - is this a reasonable plan? I found a datasheet for the SP-35x parts and it recommends 350uA per segment. Is it reasonable to double this for a 50% duty cycle? Also, I'm a little puzzled about what I'm supposed to do with the "Keep Alive" cathodes on these things. How should they be connected? The NEO-5000s are a bigger problem. The only data I could find on them was on tubeclockdb, and it recommends an average cathode current of 0.6 to 0.9mA. It also recommends a duty cycle of 1/16th, segment cathode resistors of 4K and an anode supply of 220V. Oh, and an ON time of not more than 500uS. is 4K ohms really correct, even for a 1/16 duty cycle? That seems way too low based on my math. Anyway, as long as I keep the average cathode current to, say, 0.75mA does the duty cycle really matter? With a 4 digit display I'd nominally have a 25% duty cycle. I could insert dead time to artificially limit it to 1/16, but it seems like that'd waste a lot of brightness. I guess the question is, is it better to run them at very high current for very short pulses, or much lower current for longer pulses? Thanks for the advice - I want these guys to last a long time! Bob Armstrong -- 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/6f8b3c05-88f2-4444-a00a-dd7099ddcb5e%40googlegroups.com.
