# Transistor switch for VFD tube grid

I am designing a on/off switch for the grid of a VFD tube, and I would like to use a NPN transistor. The requirements are:

• The Grid is on when 12V are applied. At this voltage, 0.85mA flow.
• I want to use a LCU with 3.3V logic.

I already have a solution that uses a high side switch (see my other question here), but now I'd like to see if I can spare a transistor.

I ended up with this design:

simulate this circuit – Schematic created using CircuitLab

And here is my reasoning:

1. If 0.85mA flow in the grid when 12V are applied, I can think it as a 14.1 kOhm resistor.
2. When Q1 is cut-off, R1 and R GRID form a voltage partitor. If I assume that 12V is the voltage applied to R GRID, then 0.85mA flow through it (as per datasheet). This mean 0.85 flow through R1 too. So R1 = (15-12)/0.00085 Ohm, and the next standard 5% resistor value is 3.6k So, when Q1 is cut-off, the grid should be on with 12V on it.
3. When the LCU pin is logic HIGH the Q1 enters its saturation region, current flows through it cutting R GRID out (actually is not completely grounded, the grid will see the collector-emitter saturation voltage, that for the 2N3904 should be 0.2V in this situation, but it's anyway too weak to have any effect.).

Now my question: is there any weak point in this design?

• The current through R_GRID will never be 0. Jul 6, 2016 at 8:10
• @IgnacioVazquez-Abrams you're right, but this shouldn't be a big problem I guess. I think that at 0.2V, the current will be extremely small. Jul 6, 2016 at 8:12
• Also, saturation is usually "guaranteed 0.2V or less" and in practice, much less : maybe 0.05V at such low currents
– user16324
Jul 6, 2016 at 11:15