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 VGS and ID, there will be a voltage drop across drain-source:

Vds vs Id graph

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


1 Answer 1


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

Drain curve with load line

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.

  • \$\begingroup\$ 1) Where do you get the red line from - it seems like it's drawn completely arbitrarily, is is not? 2) When Id is 0, Vds is 5V. When Id is 60A, Vds is 0 - huh? where does that come from? \$\endgroup\$ Sep 25 at 14:40
  • \$\begingroup\$ "This would be a 5V supply and 5/60 = 82mΩ resistor." \$\endgroup\$ Sep 25 at 14:43
  • \$\begingroup\$ What do you mean? \$\endgroup\$ Sep 25 at 14:45
  • \$\begingroup\$ Suppose V1 = 5V and Load = 82mΩ. What would Vds be while varying Id? Plot them. \$\endgroup\$ Sep 25 at 14:48
  • \$\begingroup\$ Ah I see. I understand that much, but what I'm confused about is how can I "vary Id" if the current through the circuit depends on the voltage drop across the load, which depends on the supply voltage minus the Vds drop? \$\endgroup\$ Sep 25 at 14:49

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