How Do I know what V(ds) will be?

Question

I am trying to understand how to use a MOSFET as a switch. I want to use a microcontroller to control a load using an AOI508 MOSFET as in this schematic:

^{simulate this circuit – Schematic created using CircuitLab}

Now, if this were using a mechanical switch instead of a MOSFET, we could determine the current that would flow through the load using I=V/R: I=12/1k=12mA. However, the datasheet for my MOSFET shows that, depending on V_GS and I_D, there will be a voltage drop across drain-source:

So, the true current through the load would be given by I_load=(12-V_DS)/R_load. But in order to calculate this we need to know what V_DS is, which depends on I_D, which depends on V_load, which recursively depends on V_DS. Obviously I'm missing something in my understanding but I have no clue what.

Answer 1

Suppose we plotted what Vds will be, as a function of Id:

When Id is 0, Vds is 5V. When Id is 60A, Vds is 0. This would be a 5V supply and 5/60 = 82mΩ resistor.

Note that, for any given point between min and max, Id or Vds depends steeply upon Vgs; and we don't really know Vgs, because it's relative to Vgs(th) which has a parameter spread. We also need to respect device power dissipation or temperature ratings.

For saturated switching operation, you want to be as far left as possible when "on", so simply applying some reasonably high Vgs will do. These are documented in the Vgs(on) for Rds(on) parameters, and Rds(on) vs. Vgs or Id graphs. Pay attention to the min/max values, so you can be sure it operates as intended despite that parameter spread.

This transistor is rather overrated for a 12mA load, but I'm guessing that's just a placeholder, not a real load.