Well, it's reasonably easy to drive anode and grid, but not so trivial to drive the filament (or maybe it's just me being a noob
)
My goal is to drive filament efficiently (I want to create battery powered device). So filament in my VFD is 10 Ohm, it needs around 30mA, so power dissipation should be P = 0.03^2 * 10 = 9 mW. My trouble is how to get this 30 mA without wasting too much power.
I'm using 9v battery atm, so first guess was just to : 9 / 0.03 = 300 Ohm resistor ... minus 10 for filament. So I just placed 290 Ohm there. But it's no good because we dissipate 0.03^2 * 290 = 0.261 W of energy for nothing.
Then I checked
https://www.noritake-elec.com/display/vfd_operation.html this document says that it's the best to drive filament with AC. If you drive it with DC you will get luminosity gradient because of gradient of potentials between filament and grid/anode.
So I though about it a while, it's not trivial to make DC-AC conversion efficiently. So ...
Let's PWM this !
I did really simple circuit which is driven by 1KHz PWM with low duty cycle (<10%).
So it works just fine, I have 180 mA through mosfet and filament, but only for 0.1 ms. So power dissipation here (maybe I'm wrong in calculations) = I^2 * (Rf + Rds) * 0.1 = 0.18^2 * (10 + 2) * 0.1 = 38.88 mW. Which is not bad compare to perfect 9 mW. And because of low duty cycle, most of the time filament should have ground potential, so luminosity should be consistent. If we calculate Vrms = sqrt(0.03888 * (Rf + Rds)) = 0.218 V. Which will give us average current = Vrms / R = 18 mA.
Two questions :
1) Is it bad mode for driving filament ?
2) Are there more efficient solutions ?