That's the driver topology I use, except I omit the zener diode (good idea, though, for NMOS) and the base resistor because I dont see a hazard without them. If there is a power-on transient that occurs when the HV supply is energized (very unlikely, because the HV DC filter cap would need to be missing/open) , it would be a positive voltage-spike and the worst it could do is cause the driver NPN to turn-on briefly. More-likely, any leakage from collector-to-base would bleed-away thru the ESD network on the driving logic; the charge would be very small, perhaps negligible, because it would be whatever leaked from the collector of the driver NPN to the base (this is a reverse-biased junction). Regardless, there still is current-limiting because of the emitter resistor.
I'm not aware of any mechanism that would cause a current-spike thru the tube at turn-on. When the logic goes to '1', the transistor will turn on rapidly, and it may actually saturate because the tube itself requires several microseconds to ionize. During this time the base-current will be at it's maximum (the target tube current). As the tube ionizes, it's current will go thru the transistor and the negative feedback at the emitter will gradually reduce the base-current from the logic. There's definitely a current-spike from the driving logic as the tube stabilizes, but it's only a few mA. You could add a series base-resistor to reduce the peak base-current, but if you check the datasheet for the transistor it's very likely the max base-current spec is much higher, making the resistor unnecessary. Unless I'm really missing something, I dont see where current thru the tube will spike at turnon or turnoff. As far as a current-spike at tube turnon/turnoff, yes there will be some Ldi/dt and 1/2LI^2 effects, but they will be small because we're dealing with milliamps. For example, if you are switching 5mA off in 10nsec, and there is 1uH of wiring inductance, the voltage spike (Ldi/dt) is 0.5 volts. [ Imagine if this was a motor controller with a 50 amp load.....ouch!... but there are circuit techniques to deal with that ]. -- 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/7d85cfa2-34b9-40e5-af8e-8b7bf286eccc%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
