Current-regulators are very simple to implement: NPN transistor with emitter resistor. Though you can use NMOS devices, they have a Vgs(on) of several volts, so they need to be driven from a higher voltage, around +10V, otherwise minor variations in Vgs (it's temperature-dependent, and varies device-to-device) will cause unacceptable variations in current.
The current causes a voltage-drop across the emitter resistor, which gives the required negative feedback. The current is set by (Vsupply-0.7)/Isegment . This assumes the beta of the transistor is "large", say > 40 and Vbe is 0.7V . For example, if you have +3.3V driving logic, and you want 4mA segment current, the resistor would be about 650 ohms. I use a spreadsheet and factor-in resistor-tolerance, voltage-supply variation, and normalize to a standard resistor value to fine-tune the current. In this example, if the supply were to increase 3% from 3.3V to 3.4V, the current would increase 4% to 4.15mA. The current is independent of the tube's voltage drop (unless your anode-supply voltage is very close to the tube's voltage-drop). Now, compare this to an unregulated driver that runs at +180V. If your tube-voltage-drop is nominally 140V, a 10K resistor will give you 4mA. But if the tube ages, or you replace it with another one, and the voltage drop increases 4% to 145V, your current will drop 13% to 3.5mA which may be an undesirable reduction in brightness. -- 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 post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/91a8142d-043e-4c08-8f56-70031fb6ca08%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
