A 5K-ohm load with 2mA thru it only drops 10V; with a 200V supply, you would have 190V across the mosfet. At 2mA load current, that translates to 380mW dissipation in the mosfet. From the datasheet, the max Theta ja I saw listed was 170 C/W. That would translate to a 65C rise, which is fine for room-temp even in a hot climate.
At 4mA, though, you will get into trouble as you noted. Typically you wont see anywhere near that much voltage across a nixie current-limiter. Assuming 150V typical nixie voltage, and 10V for the resistor, you will get 40V across the transistor. That translates to 80mW, and a temp-rise of about 15C, which is plenty of margin. A current limiter wont reduce the overall power dissipation, but it will reduce the variation in nixie current over supply-voltage variations and nixie aging. -- 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/820f82b0-8b70-4c70-b589-d224f70db2f3%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
