Assuming you have high-enough HV power-supply headroom for the nixies (180V or more), the choice for the resistor begins with the isolated power supply; I chose 12V because there are a lot of small, efficient DC-DC converters that provide 12V. You can use other values, but be careful to stay below the rated Vgs(max).
Next, you want some adjustment range for the anode current. I set the nominal gate voltage at 10V. That gives +/- 2V of range to adjust the anode current without changing resistors. Next, from the datasheet, you need to find Vgs(on) for the MOSFET. It's usually plotted on a curve. For the FQD7P20TM that I use, it's about 4V (see the transfer characteristics curve). Next, you need the target anode current. For a Burroughs 5092, it's 2.2mA. The voltage-drop across the resistor is 10V - Vgs(on); in this case, it's 6V. Now you can solve for the resistor value: R=V/I = 6/0.0022 = 2727 ohms. The nearest standard value = 2.7K. Sanity-check the power in the resistor: P=I^2R; in this case, P=13mW, so even with tiny surface-mount devices you are fine. You should also sanity-check the PMOS transistor worst-case power dissipation. Assume a ridiculously low nixie voltage of 125V. The voltage across the PMOS is Supply_voltage - Nixie_voltage - Resistor_voltage = 200-125-6. PMOS power = Max voltage * typical current = 69*0.0022 = 150mW. This device has a Theta j-a of 110C/W, so the temp-rise = 110*0.15 = 17 degrees. Plenty of margin here. -- 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/42998d43-12fe-48f9-8e3f-e3b98a7a7b6a%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
