Firstly, I only do direct-drive. I dont use driver chips. Instead, I use either NPN or NMOS discrete devices, because I can ensure constant-current thru the cathode. Using a dropping-resistor will result in some current-variation due to the tube's characteristics, or supply voltage variation.
For larger tubes, you may want to tweak the cathode-current. For example, a '1' numeral will require less current than an '8'. This is possible with per-cathode drivers. Basically, the current-limiting resistor is placed between source and ground for an NMOS driver, or emitter and ground for an NPN driver. The resistor provides current-feedback. NPN Driver. I use this on my neon watch because the supply voltage of 3.7V is too low for high-voltage NMOS devices. To calculate the emitter resistor value, R=(Vsupply-0.7)/current. If current is in mA, R is in k-ohms. If you want 335uA for a 3.7V supply, then use 8.9K (the recommended current for an SP-151 display). Be aware that most of the heat will be dissipated by the NPN device, not the resistor. NMOS driver. The topology is similar, but the gate-source threshold voltage has more variation than Vbe for an NPN transistor. I use 12V for my big-clock, and Vgs is around 3.4V. The driver logic runs at 12V, so variations in Vgs have relatively small effect on the cathode current. -- 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/1d08be2e-5ca2-4378-9599-4d31ec13a93e%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
